Loudspeaker unit

Abstract

A loudspeaker unit for producing sound at bass frequencies an array of two or more diaphragms. The first radiating surface and the second radiating surface are located on opposite faces of the diaphragm, and one or more of the diaphragms are included in a first subset of the diaphragms and one or more of the diaphragms are included in a second subset of the diaphragms; a plurality of drive units.

Claims

1. A loudspeaker unit for producing sound at bass frequencies including: an array of two or more diaphragms, each diaphragm in the array having a first radiating surface and a second radiating surface, wherein the first radiating surface and the second radiating surface are located on opposite faces of the diaphragm; a plurality of drive units, wherein each drive unit is configured to move a respective one of the diaphragms in the array based on a respective electrical signal; a frame, wherein each diaphragm in the array is suspended from the frame via one or more suspension elements, wherein the frame is configured to allow sound produced by the first radiating surfaces to propagate out from the loudspeaker unit; at least one enclosure configured to receive sound produced by the second radiating surfaces, wherein the enclosure includes a plurality of vents, wherein each vent is configured to allow sound produced by the second radiating surfaces to propagate out from the loudspeaker unit; drive circuitry configured to provide each drive unit with a respective electrical signal derived from the same audio source such that the sound produced by the second radiating surfaces is out of phase with respect to the sound produced by the first radiating surfaces, wherein the loudspeaker unit is configured for use with a first ear of a user located at a first listening position that is 40 cm or less from the first radiating surface of one of the diaphragms whilst a second ear of the user is located at a second listening position that is 40 cm or less from the first radiating surface of one of the diaphragms; the loudspeaker unit includes at least one pair of diaphragms; the diaphragms in the/each pair is oriented with one of the diaphragms included in the/each pair having a first radiating surface that faces in a first direction and with the other one of the diaphragms included in the/each pair having a first radiating surface that faces in a second direction that is opposite to the first direction; the plurality of vents include a first vent configured to allow sound to propagate out from the loudspeaker unit in a third direction that is transverse with respect to the first direction, and a second vent configured to allow sound to propagate out from the loudspeaker unit in a fourth direction that is opposite to the third direction.

2. A loudspeaker unit according to claim 1, wherein the enclosure includes one or more partitions configured to direct sound produced by the second radiating surface of each diaphragm out of a respective one of the vents.

3. A loudspeaker unit according to claim 1, wherein the loudspeaker unit is a subwoofer configured to produce sound at bass frequencies, wherein the bass frequencies includes frequencies across the range 50-100 Hz.

4. A loudspeaker unit according to 1, wherein the frame from which each diaphragm is suspended is a secondary frame, wherein the diaphragms are suspended from one or more primary frames via one or more primary suspension elements, wherein the/each primary frame is suspended from the secondary frame via one or more secondary suspension elements.

5. A loudspeaker unit according to 1, wherein the frame from which each diaphragm is suspended is part of or configured to fixedly attach to a rigid supporting structure.

6. A loudspeaker unit according to 1, wherein the loudspeaker unit is configured for use in performing noise cancelation at bass frequencies.

7. A loudspeaker unit according to claim 1, wherein the drive circuitry is configured to apply a predetermined delay to one or more of the electrical signals provided to the drive units.

Description

SUMMARY OF THE FIGURES

(1) Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

(2) FIGS. 1(a) and 1(b) illustrate the farfield polar response of a “dipole” loudspeaker unit including a single diaphragm acting as a dipole loudspeaker.

(3) FIGS. 2(a) and 2(b) illustrate the polar response of a “quadrupole” loudspeaker unit including an array of two diaphragms, wherein each of the two diaphragms in the array provides a respective dipole loudspeaker, and wherein drive circuitry is configured to provide one of the diaphragms with an electrical signal that is out of phase with respect to an electrical signal that is provided to the other of the diaphragms.

(4) FIGS. 3(a) and 3(b) illustrate the polar response of an “octopole” loudspeaker unit including an array of four diaphragms, wherein each of the four diaphragms in the array provides a respective dipole loudspeaker, and wherein drive circuitry is configured to provide two of the diaphragms with an electrical signal that is out of phase with respect to an electrical signal that is provided to the other two of the diaphragms.

(5) FIGS. 4(a)-(e) illustrates various loudspeaker arrangements for use in a simulation to demonstrate the proximity effect.

(6) FIGS. 5(a)-(d) respectively show the results of a simulation to demonstrate the proximity effect, using the loudspeaker units of FIGS. 4(a)-(e), with sound being produced at a frequency of 50 Hz (FIG. 5(a)), 100 Hz (FIG. 5(b)), 200 Hz (FIG. 5(c)), 400 Hz (FIG. 5(d)) respectively, relative to the SPL of the 2π monopole loudspeaker unit.

(7) FIGS. 6(a)-(d) show the same simulation results as FIGS. 5(a)-(d) (respectively), but with SPL shown in absolute form and with distance from the loudspeaker unit (r) being shown with a linear (rather than a log) scale.

(8) FIGS. 7(a) and 7(b) illustrate that it is favourable for a listening position to be located in front of a centre of a radiating surface, rather than a centre of an array of a multipole loudspeaker unit.

(9) FIG. 8 is a schematic view of a loudspeaker unit 101 for producing sound at bass frequencies according to the first aspect of the invention.

(10) FIGS. 9(a) and 9(b) each show an example of drive circuitry 150, 150′ which may be included in the loudspeaker 101 of FIG. 8.

(11) FIG. 10 shows the polar response in the y-z, x-y and x-z planes for a dipole, a quadrupole and an octopole loudspeaker unit as described with respect to FIGS. 1(a), 2(a) and 3(a) respectively.

(12) FIGS. 11(a)-(c) illustrate some preferred listening positions for use with (a) a dipole loudspeaker unit, (b) a quadrupole loudspeaker unit and (c) an octopole loudspeaker.

(13) FIGS. 12(a)-(b) illustrate some other possible listening positions for use with (a) a quadrupole loudspeaker and (c) an octopole loudspeaker.

(14) FIGS. 13(a)-(d) show how an octopole loudspeaker unit including four dipole loudspeakers arranged in a square array could be configured for use in a car headrest.

(15) FIGS. 14(a)-(f) show various implementations of a multipole loudspeaker unit incorporating various numbers of diaphragms implemented in a car headrest, wherein each diaphragm provides a respective dipole loudspeaker.

(16) FIG. 15 illustrates various ways in which an octopole loudspeaker unit including four diaphragms arranged in a square array could be configured to alter its performance.

(17) FIG. 16 shows how a multipole loudspeaker unit, in this example a quadrupole loudspeaker unit, could multiple operational modes

(18) FIGS. 17(a)-(h) show various further implementations of a loudspeaker unit incorporating various numbers of diaphragms implemented in a car headrest, wherein each diaphragm provides a respective dipole loudspeaker.

(19) FIGS. 18(a) and 18(b) show a first example loudspeaker unit 101a which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(20) FIGS. 19(a) and 19(b) show a second example loudspeaker unit 101b which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(21) FIGS. 20(a) and 20(b) show a third example loudspeaker unit 101c which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(22) FIGS. 21(a) and 21(b) show a fourth example loudspeaker unit 101d which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(23) FIGS. 22(a)-(c) show a fifth example loudspeaker unit 101e which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(24) FIGS. 23(a) and 23(b) show a fifth example loudspeaker unit 101f which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(25) FIG. 24 is a schematic view of a loudspeaker unit 201 for producing sound at bass frequencies according to the second aspect of the invention.

(26) FIG. 25 shows the polar response in the y-z, x-y and x-z planes for a monopole loudspeaker unit including a single diaphragm (wherein an enclosure is configured to receive sound produced by a second radiating surface of this diaphragm), a dipole loudspeaker unit including a two diaphragms (wherein an enclosure is configured to receive sound produced by the second radiating surfaces of these diaphragms) and a quadrupole loudspeaker unit including four diaphragms (wherein an enclosure is configured to receive sound produced by the second radiating surfaces of these diaphragms).

(27) FIGS. 26(a)-(b) illustrate some preferred listening positions for use with a quadrupole loudspeaker unit formed of four monopole loudspeakers arranged in a 2×2 array, where the electrical signals provided to the drive units configured to move the first subset of diaphragms are out of phase with respect to the electrical signals provided to the one or more drive units configured to move the second subset of diaphragms.

(28) FIGS. 27(a)-(c) show the diaphragms arranged as shown in FIG. 26(b) from various angles.

(29) FIGS. 28(a)-(b) illustrate some less preferred listening positions for use with a quadrupole loudspeaker unit formed of four monopole loudspeakers arranged in a 2×2 array, where the electrical signals provided to the drive units configured to move the first subset of diaphragms are out of phase with respect to the electrical signals provided to the one or more drive units configured to move the second subset of diaphragms.

(30) FIGS. 29(a)-(c) show the diaphragms arranged as shown in FIG. 28(b) from various angles.

(31) FIGS. 30(a)-(d) show a first example loudspeaker unit 201a which implements the loudspeaker unit 201 of FIG. 24 in a car headrest.

(32) FIG. 31 illustrates effects of applying a delay Δt to a signal from a selected electrical signal supplied to one of the drive units.

(33) FIG. 32 shows a second example loudspeaker unit 201b which implements the loudspeaker unit 201 of FIG. 24 in a car headrest.

(34) FIG. 33 is a schematic view of a loudspeaker unit 301 for producing sound at bass frequencies according to the third aspect of the invention.

(35) FIGS. 34(a) and 34(b) each show an example of drive circuitry 350, 350′ which may be included in the loudspeaker 301 of FIG. 33.

(36) FIGS. 35(a)-(d) illustrate a preferred listening position for use with a headrest that incorporates loudspeaker unit formed of two monopole loudspeakers arranged back to back.

(37) FIGS. 36(a)-(d) show a first example loudspeaker unit 301a which implements the loudspeaker unit 301 of FIG. 33 in a car headrest.

(38) FIGS. 37(a)-(c) show a second example loudspeaker unit 301b which implements the loudspeaker unit 301 of FIG. 33 in a car headrest.

(39) FIGS. 38(a)-(b) show a third example loudspeaker unit 301c which implements the loudspeaker unit 301 of FIG. 33 in a car headrest.

(40) FIGS. 39(a)-(b) illustrate an experimental set up used to obtain experimental data 1.

(41) FIGS. 40(a)-(b) illustrate experimental data 1 obtained using the experimental set up of FIGS. 33(a)-(b).

DETAILED DESCRIPTION OF THE INVENTION

(42) Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

(43) Herein, loudspeaker units incorporating one or more diaphragms acting as a dipole loudspeaker are referred to as “multipole” loudspeaker units, with loudspeaker units incorporating one diaphragm acting as a dipole loudspeaker being referred to as “dipole” loudspeaker units, with loudspeaker units incorporating two diaphragms acting as dipole loudspeakers being referred to as “quadrupole” loudspeaker units, and with loudspeaker units incorporating four diaphragms acting as dipole loudspeakers being referred to as “octopole” loudspeaker units.

(44) FIGS. 1(a) and 1(b) illustrate the farfield polar response of a “dipole” loudspeaker unit including a single diaphragm acting as a dipole loudspeaker.

(45) In FIG. 1(a), in-phase sound is indicated by a plus sign (+) whereas out-of-phase sound is indicated by a negative sign (−). Note that sound produced by opposite surfaces of the diaphragm are necessarily out of phase with each other.

(46) The relationship between pressure pa, produced by the dipole loudspeaker unit of FIG. 1(a) at bass frequencies in the farfield, k and D can theoretically be represented by the following relation:
p.sub.di∂k.Math.D.Math.cos(α)  (1)

(47) Where k=2π/λ, and D is a “path length”.

(48) For an ideal dipole loudspeaker formed of two out of phase monopole point sources (which is only achievable in theory), path length can be understood as the distance between the two out of phase monopole point sources.

(49) For a real dipole loudspeaker, the path length can be understood as a distance between two out of phase monopole point sources which causes the two point monopole point sources to approximate the behaviour of the real dipole loudspeaker, i.e. the distance D as shown in FIG. 1(a).

(50) FIGS. 2(a) and 2(b) illustrate the polar response of a “quadrupole” loudspeaker unit including an array of two diaphragms, wherein each of the two diaphragms in the array provides a respective dipole loudspeaker, and wherein drive circuitry is configured to provide one of the diaphragms with an electrical signal that is out of phase with respect to an electrical signal that is provided to the other of the diaphragms.

(51) In FIG. 2(a), in-phase sound is indicated by a plus sign (+) whereas out-of-phase sound is indicated by a negative sign (−).

(52) The relationship between pressure p.sub.qu produced by the quadrupole loudspeaker unit of FIG. 2(a) at bass frequencies in the farfield, k, D and d can theoretically be represented by the following relation:
p.sub.qu∝k.sup.2.Math.D.Math.d.Math.cos(α).Math.sin(α)  (2)

(53) Where d is a distance in the between the geometrical centres of the radiating surfaces on the same side of the quadrupole loudspeaker unit as measured along the y-axis.

(54) FIG. 3(a) and FIG. 3(b) illustrate the polar response of an “octopole” loudspeaker unit including an array of four diaphragms, wherein each of the four diaphragms in the array provides a respective dipole loudspeaker, and wherein drive circuitry is configured to provide two of the diaphragms with an electrical signal that is out of phase with respect to an electrical signal that is provided to the other two of the diaphragms.

(55) In FIG. 3(a), in-phase sound is indicated by a plus sign (+) whereas out-of-phase sound is indicated by a negative sign (−).

(56) The relationship between pressure p.sub.oc produced by the quadrupole loudspeaker unit of FIG. 3(a) at bass frequencies in the farfield, k, D, d and d′ can theoretically be represented by the following relation:
p.sub.oc∝k.sup.3.Math.D.Math.d.Math.d′.Math.cos(α).Math.sin(α).Math.cos(β)  (3)

(57) Where d′ is a distance in the between the geometrical centres of the radiating surfaces on the same side of the octopole loudspeaker unit as measured along the x-axis.

(58) From relations (1), (2) and (3) above, it can be seen that: Increasing D, d, or d′ will increase the far-field pressure response of the multipole loudspeaker unit, i.e. will worsen the cocooning effect at bass frequencies. Due to the k, k.sup.2, k.sup.3 terms, the far field pressure response drops off more rapidly with frequency at bass frequencies as the number of dipole loudspeakers included in the array is increased, i.e. as the order of multipole is increased (e.g. 6 dB/octave for a dipole, 12 dB/octave for a quadrupole and 18 dB/octave for an octopole)

(59) In general, reference herein to a “cocooning” effect refers to reduced SPL at large distances, compared with an equivalent monopole loudspeaker.

(60) FIGS. 4(a)-(e) illustrates various loudspeaker arrangements for use in a simulation to demonstrate the proximity effect.

(61) The loudspeaker arrangements shown in FIGS. 4(a)-(e) include: (a) A 2π monopole loudspeaker unit (a dipole loudspeaker mounted in an infinite baffle such that only one radiating surface of the diaphragm radiates into 2π space) (b) A 4π monopole loudspeaker unit (a diaphragm mounted in an infinite tube such that only one radiating surface of the diaphragm radiates into 4π space) (c) A dipole loudspeaker unit (as explained with reference to FIG. 1(a) above) (d) A quadrupole loudspeaker unit (as explained with reference to FIG. 2(a) above) (e) An octopole loudspeaker unit (as explained with reference to FIG. 3(a) above)

(62) FIGS. 5(a)-(d) respectively show the results of a simulation to demonstrate the proximity effect, using the loudspeaker units of FIGS. 4(a)-(e), with sound being produced at a frequency of 50 Hz (FIG. 5(a)), 100 Hz (FIG. 5(b)), 200 Hz (FIG. 5(c)), 400 Hz (FIG. 5(d)) respectively, relative to the SPL of the 2π monopole loudspeaker unit.

(63) For the simulation results shown in FIGS. 5(a)-(d), sound pressure level (SPL) was simulated on the basis of the diaphragms having radiating surfaces of area S=78.5 cm.sup.2 (equivalent to a disc of area 100 mm diameter), D=5.5 cm, d=11.0 cm, d′=11.0 cm.

(64) For the purposes of these simulation results, SPL was simulated for the 2π monopole and 47 monopole loudspeaker units along the z-axis (α=0°). Since measurement of sound pressure level (SPL) along the z-axis would result in a null for the quadrupole and octopole loudspeaker units, SPL for these units was simulated along α=45° for the quadrupole loudspeaker unit and α=45° and β=45° for the octopole loudspeaker unit.

(65) FIGS. 5(a)-(d) show that at small distances, an SPL level comparable to equivalent monopole loudspeaker units can be achieved with all of the multipole loudspeaker units. This effect is referred to herein as the “proximity effect”.

(66) FIGS. 5(a)-(d) also show that increasing the number of dipole loudspeakers included in the array (i.e. increasing the order of multipole used) results in a better cocooning effect at bass frequencies, and that the higher the number of dipole loudspeakers used, the higher the frequency at which a reasonable cocooning effect can be achieved. However, even with the octopole loudspeaker unit, the cocooning effect is not really strong enough to permit the creation of a personal sound cocoon at frequencies exceeding ˜500 Hz.

(67) FIGS. 6(a)-(d) show the same simulation results as FIGS. 5(a)-(d) (respectively), but with SPL shown in absolute form and with distance from the loudspeaker unit (r) being shown with a linear (rather than a log) scale.

(68) FIGS. 7(a) and 7(b) illustrate that it is favourable for a listening position to be located in front of a centre of a radiating surface, rather than a centre of an array of a multipole loudspeaker unit.

(69) In FIG. 7(a), distance r from the centre of a quadrupole loudspeaker unit is shown by a solid line in FIG. 7(a), and the corresponding SPL as measured at 100 Hz with the same parameters as FIG. 5(b) is shown in FIG. 7(b). As can be seen from the solid line in FIG. 5(b), there is a dip in the SPL at small distances from the centre of the loudspeaker quadrupole loudspeaker unit, since there is a null (SPL=0) along the z-axis.

(70) By modifying the path along which r is measured extend towards the centre of a radiating surface of a diaphragm in the quadrupole loudspeaker unit rather than towards the centre of the loudspeaker unit itself, as shown by the dotted line which branches from the solid line in FIG. 7(a), the SPL can continue to increase towards that of an equivalent 2π monopole as r reduces towards zero. This demonstrates that it is favourable for a listening position to be located in front of a centre of a radiating surface, rather than a centre of an array of a multipole loudspeaker unit.

(71) Some interim conclusions may be drawn from the discussion so far: As the number of diaphragms is increased, there is an improvement in the drop in SPL with increasing distance, whilst a comparable SPL is maintained at small distances. E.g. at 200 Hz there is an additional 14 dB sound reduction at 1 m for a quadrupole compared to a dipole, while at 10 cm the levels are equal. Increasing the number of diaphragms may allow the upper bound of the low frequency range across which a useful sound cocoon can be maintained to increase. The graphs where the observation distance r is plotted on a logarithmic scale clearly shows the distance where the proximity effect kicks in. Those graphs show the distance up to 10 m and are referenced to a 2 pi monopole equivalent which has a 6 dB per octave SPL reduction for every double distance in the far field.

Examples Implementing First Aspect of the Invention

(72) FIG. 8 is a schematic view of a loudspeaker unit 101 for producing sound at bass frequencies according to the first aspect of the invention.

(73) The loudspeaker unit 101 includes an array of n diaphragms 110 (features relating to an individual diaphragm are labelled with the suffix “−1”, “−2”, “−3” . . . “−n”). Each diaphragm has a first radiating surface 112, and a second radiating surface 114, wherein the first radiating surface 112 and the second radiating surface 114 are located on opposite faces of the diaphragm.

(74) The loudspeaker unit 101 also includes a frame 130, wherein each diaphragm 110 in the array is suspended from the frame 130 via one or more suspension elements 132 such that the first radiating surfaces 112 are facing in a first (“forwards”) direction F and the second radiating surfaces 114 are facing in an opposite (“backwards”) second direction B, wherein the frame 130 is configured to allow sound produced by the first radiating surfaces 112 to propagate out from a first side 104 of the loudspeaker unit 101 in the first direction F and to allow sound produced by the second radiating surfaces 114 to propagate out from a second side 106 of the loudspeaker unit in the second direction B.

(75) The loudspeaker unit 101 also includes a plurality of drive units 140, wherein each drive unit 140 is configured to move a respective one of the diaphragms 110 in the array based on a respective electric signal.

(76) One or more of the diaphragms 110 are included in a first subset of the diaphragms 110 and one or more of the diaphragms 110 are included in a second subset of the diaphragms 110.

(77) The loudspeaker unit 101 also includes drive circuitry (not shown in FIG. 8) configured to provide each drive unit 140 with a respective electrical signal derived from the same audio source such that the electrical signal(s) provided to the one or more drive units 140 configured to move the first subset of diaphragms 110 is/are out of phase with respect to the electrical signal(s) provided to the one or more drive units 140 configured to move the second subset of diaphragms 110.

(78) FIGS. 9(a) and 9(b) each show an example of drive circuitry 150, 150′ which may be included in the loudspeaker 101 of FIG. 8 and be configured to provide each drive unit 140 of the loudspeaker unit 101 of FIG. 8 with a respective electrical signal derived from the same audio source such that the electrical signal(s) provided to the one or more drive units 140 configured to move the first subset of diaphragms 110 are out of phase with respect to the electrical signal(s) provided to the one or more drive units 140 configured to move the second subset of diaphragms 110.

(79) For brevity, sound produced by a first radiating surface of a diaphragm in the first subset of diaphragms may be referred to as “in-phase” and/or marked with a ‘+’ in drawings shown herein. Similarly, and also for brevity, sound produced by a first radiating surface of a diaphragm in the second subset of diaphragms may be referred to as “out-of-phase” and/or marked with a ‘−’ in drawings shown herein. However, for avoidance of any doubt, the terms “in-phase” and “out-of-phase” and the symbols ‘+’ and ‘−’ are used in this way merely as a convention in order to indicate out of phase sound produced by different radiating surfaces.

(80) The example drive circuitry 150 of FIG. 9(a) includes a digital signal processor (“DSP”) 152 configured to provide each drive unit 140 with a respective electrical signal via a respective amplifier 154, wherein the respective electrical signal is derived from an audio signal (in this case a digital audio signal) provided by the audio source at node 156. It is straight forward for such a unit to provide manipulate the electrical signals provided to each drive unit 140 so that each drive unit 140 is provided with a respective electrical signal derived from the same audio source such that the electrical signal(s) provided to the one or more drive units 140 configured to move the first subset of diaphragms 110 (marked with a ‘+’) is/are out of phase with respect to the electrical signal(s) provided to the one or more drive units 140 configured to move the second subset of diaphragms 110 (marked with a ‘−’). As described in more detail below, the DSP 152 may additionally be used to manipulate the electrical signal respectively provided to each drive unit 140, e.g. to modify the phase, delay or amplitude of the electrical signal respectively provided to each drive unit 140 so as to optimise the sound provided to a user (e.g. in a manner described below).

(81) The example drive circuitry 150′ of FIG. 9(b) includes an amplifier 154′ and wiring 155′ configured to reverse the polarity of the electrical signal(s) provided to the/each drive unit 140 configured to move a diaphragm 110 in the second subset of diaphragms (marked with a ‘−’) compared to the electrical signal(s) provided to the/each drive unit 140 configured to move a diaphragm in the first subset of diaphragms 110 (marked with a ‘+’), e.g. with + and − wires supplying an audio signal provided by the audio source 156′ via the amplifier 154′ being connected to the/each drive unit 140 configured to move the second subset of diaphragms the other way around compared with the way + and − wires are connected to the/each drive unit 140 configured to move the first subset of diaphragms 110.

(82) The drive circuitry 150, 150′ of FIGS. 9(a) and 9(b) is preferably configured to provide each drive unit 140 with a respective electrical signal that includes frequencies across the range 60-80 Hz, preferably frequencies across the range 40-100 Hz, and may include frequencies across the range 40-160 Hz, and with frequencies that do not exceed 400 Hz, more preferably 200 Hz. If the frequencies do not exceed 200 Hz, the loudspeaker unit 101 may be understood as a subwoofer.

(83) The following drawings and corresponding discussion sets out some guiding principles for how the loudspeaker unit 101 of FIG. 8 could be implemented in a car headrest. In some cases, a dipole loudspeaker unit containing only one diaphragm is depicted for comparative purposes.

(84) FIG. 10 shows the polar response in the y-z, x-y and x-z planes for a dipole, a quadrupole and an octopole loudspeaker unit as described with respect to FIGS. 1(a), 2(a) and 3(a) respectively.

(85) Knowing these polar responses can help with deciding on a preferred implementation of a multipole loudspeaker unit.

(86) FIGS. 11(a)-(c) illustrate some preferred listening positions for use with (a) a dipole loudspeaker unit, (b) a quadrupole loudspeaker unit and (c) an octopole loudspeaker.

(87) FIGS. 12(a)-(b) illustrate some other possible listening positions for use with (a) a quadrupole loudspeaker and (c) an octopole loudspeaker.

(88) In the octopole loudspeaker units of FIG. 11(c) and FIG. 12(b), there are three diaphragms arranged in a linear array, with a central diaphragm having radiating surfaces with twice the area of the other two diaphragms. Although there are only three diaphragms (and so technically this is a hexapole loudspeaker), this is referred to as a linear octopole loudspeaker unit because it is directly equivalent to a linear array of four diaphragms of equal size in which the two central diaphragms are driven with the same polarity as each other, and the two outer diaphragms are driven with the opposite polarity.

(89) In each of FIGS. 11(a)-(c), the ears of a user are located at first and second listening positions which are in front of a radiating surface of the same diaphragm of the loudspeaker unit. This is preferred, since this helps to maximise the SPL at those listening positions by placing both ears well within one lobe.

(90) The arrangement of FIG. 12(a) is not preferred because the ears of a user are located at first and second listening positions which are in front of radiating surfaces of the loudspeaker unit driven out of phase with each other. In experiments conducted by the present inventor, it was found that using this configuration at frequencies up to 150 Hz could be fatiguing/unpleasant for a user, though SPL levels were acceptable. By lowering the frequency to 100 Hz, more preferably 80 Hz, this arrangement could provide acceptable performance (i.e. without over-fatiguing a listener), though performance was not as good as with “in phase” reproduction for both ears.

(91) The arrangement of FIG. 12(b) is not preferred because the ears of a user are located at first and second listening positions which are close to SPL nulls.

(92) FIGS. 13(a)-(d) show how an octopole loudspeaker unit including four dipole loudspeakers arranged in a square array could be configured for use in a car headrest.

(93) As shown in FIG. 13(a), orienting the array of diaphragms in two vertically stacked pairs within a car headrest could lead to the ears of a user being located at first and second listening positions which are in front of radiating surfaces of the loudspeaker unit driven out of phase with each other (or at nulls).

(94) By flipping the orientation of the diaphragms by 45° as shown in FIG. 13(b), a car headrest can be obtained as shown in FIGS. 13(c) and 13(d) in which the ears of a user are located at first and second listening positions, wherein both listening positions are located in front of a geometric centre of a respective radiating surface, with those radiating surfaces being driven in-phase with each other. This helps to avoid the fatiguing of a listener as described with respect to FIG. 12(a).

(95) FIGS. 14(a)-(f) show various implementations of a multipole loudspeaker unit incorporating various numbers of diaphragms implemented in a car headrest, wherein each diaphragm provides a respective dipole loudspeaker.

(96) In FIG. 14(a), the loudspeaker unit is a dipole loudspeaker unit mounted within the headrest so that the ears of a user are located at first and second listening positions in front of the same radiating surface.

(97) In FIG. 14(b), the loudspeaker unit is mounted within the headrest so that the ears of a user are located at first and second listening positions which are in front of radiating surfaces of the loudspeaker unit driven out of phase with each other. This is not preferred for reasons discussed above.

(98) In FIGS. 14(c)-(d), the loudspeaker unit is mounted within the headrest so that the ears of a user are located at first and second listening positions which are in front of a radiating surface of the same diaphragm of the loudspeaker unit, which is preferred for reasons discussed above.

(99) In FIGS. 14(e)-(f), the loudspeaker unit is mounted within the headrest so that the ears of a user are located at first and second listening positions, wherein both listening positions are located in front of a geometric centre of a respective radiating surface, with those radiating surfaces being driven in-phase with each other. In FIG. 14(f), the shapes of the diaphragms are also configured to maximise the surface area of the radiating surfaces.

(100) FIG. 15 illustrates various ways in which an octopole loudspeaker unit including four diaphragms arranged in a square array could be configured to alter its performance.

(101) As explained above with reference to FIGS. 3(a) and 3(b), the relationship between the pressure p.sub.oc, k, D, d and d′ produced by the quadrupole loudspeaker unit of FIG. 3(a) at bass frequencies and in the farfield can theoretically be represented by the following relation:
p.sub.oc∝k.sup.3.Math.D.Math.d.Math.d′.Math.cos(α).Math.sin(α).Math.cos(β)  (3)

(102) As shown in FIG. 15, each of D, d and d′ can be altered by adding a baffle (which changes D) or by changing the separation of the diaphragms (which changes d and/or d′), which in turn can be used to alter the performance of (e.g. level of cocooning provided by) the loudspeaker unit.

(103) FIG. 16 shows how a multipole loudspeaker unit, in this example a quadrupole loudspeaker unit, could have multiple operational modes, wherein: in a first operational mode (shown on the right-hand side of the figure), the drive circuitry is configured to provide the drive unit(s) configured to move the first subset of diaphragms with an electrical signal that is out of phase with respect to an electrical signal that is provided to the drive unit(s) configured to move the second subset of diaphragms; and in a second operational mode (shown on the left-hand side of the figure), the drive circuitry is configured to provide the drive unit(s) configured to move the first subset of diaphragms with an electrical signal that is in phase with respect to an electrical signal that is provided to the second subset of the diaphragms.

(104) In the second operational mode, it can be seen that the quadrupole loudspeaker unit is in effect operating as a dipole loudspeaker unit. This may be useful e.g. to allow the loudspeaker unit to produce higher sound pressure levels in situations in which creating a personal sound cocoon is not needed or not as important (e.g. where all passengers in a car are listening to the same audio).

(105) FIGS. 17(a)-(h) show various further implementations of a loudspeaker unit incorporating various numbers of diaphragms implemented in a car headrest, wherein each diaphragm provides a respective dipole loudspeaker.

(106) In FIGS. 17(a)-(c), an example is shown in which there are eight diaphragms which provide eight dipole loudspeakers. FIG. 17(a) shows one operating mode for this loudspeaker unit in which the drive units configured to move a first subset of diaphragms (‘+’) are provided with an electrical signal that is out of phase with respect to an electrical signal provided to the drive units configured to move a second subset of diaphragms (‘−’). FIG. 17(b) shows another operating mode for this loudspeaker unit in which all drive units are provided with an electrical signal having the same phase, such that the loudspeaker unit is in effect operating as a dipole loudspeaker unit. Yet further operating modes, e.g. in which the first and second subsets are changed, may also be implemented with the loudspeaker unit of FIGS. 17(a)-(c).

(107) In FIGS. 17(d)-(h), there are sixteen diaphragms which provide sixteen dipole loudspeakers, to potentially provide an even better cocooning effect.

(108) FIGS. 17(f)-(h) show that whilst first radiating surfaces of each diaphragm in the array all face in a first direction (in this case a “forwards” direction F) so that sound produced by the first radiating surfaces can propagate out from a first side of the loudspeaker unit in the first direction and the second radiating surfaces of each diaphragm in the array all face in an opposite second direction (in this case a “backwards” direction B) so that sound produced by the second radiating surfaces to propagate out from a second side of the loudspeaker unit in the second direction, the principal radiating axes of the first and second radiating surfaces need not be parallel to each other, and may be arranged e.g. with the principal radiating axes of the first radiating surfaces being arranged to converge (as in FIG. 17(h)) or diverge (as in FIG. 17(g)).

(109) Examples which implement the loudspeaker unit 101 of FIG. 8 in a car headrest will now be described, with alike reference numerals indicating corresponding features that do not need to be described further, except where further explanation is provided.

(110) FIGS. 18(a) and 18(b) show a first example loudspeaker unit 101a which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(111) In this example, there are four diaphragms 110a arranged in a 2×2 array.

(112) In this example, the diaphragms 110a take the form of paper cones, wherein the concave surface of the cones provide the first radiating surfaces 112a.

(113) In this example, the loudspeaker unit 101a is implemented with a single frame configuration, wherein the frame 130a of the loudspeaker unit includes an outer frame 134a as well as a number of subframes.

(114) The outer frame 134a is open at both the first and second sides of the loudspeaker 101a in order to allow sound produced by the first radiating surfaces 112a to propagate out from the first side 104 of the loudspeaker unit 101a in the first direction F and to allow sound produced by the second radiating surfaces 114a to propagate out from a second side 106a of the loudspeaker unit 101a in the second direction B, with only an acoustically transparent grill 135a of the outer frame 134a being provided in front of the first radiating surfaces 112a and second radiating surfaces 114a of the diaphragms 110a. The outer frame 134a may be covered by an acoustically transparent covering (not shown).

(115) Each subframe includes one or more rigid supporting elements (e.g. arms) 136a configured to hold a magnet unit of each drive unit 140a in front of the second radiating surface 114a of a respective diaphragm 110a. Each drive unit 140a may be an electromagnetic drive unit that includes a magnet unit configured to produce a magnetic field, and a voice coil attached to the diaphragm (that the drive unit is configured to move). Such drive units are well known and do not need to be described further.

(116) The diaphragms 110a are suspended from the frame 130a via suspension elements 132a which in this example include roll suspensions, as can most clearly be seen in FIG. 18(a).

(117) The loudspeaker unit 101a is configured to be fixedly mounted to a car seat frame via mounting pins 182a.

(118) In this example, there are four diaphragms 110a arranged in a square array and mounted within the headrest 180a similarly to FIG. 13, such that the ears of a user are located at first and second listening positions, wherein both listening positions are located in front of a geometric centre of a respective first radiating surface 112a, with those radiating surfaces being driven in-phase with each other (indicated by a ‘+’).

(119) Note that since the diaphragms are being moved out of phase with each other, the forces on the frame 130a due to movement of the diaphragms 110a will cancel out with each other, at least in a first operational mode of the loudspeaker unit 101a as described above. However, if the loudspeaker unit 101a is configured to also operate in a second operational mode in which all the diaphragms are moved in phase with each other, then the forces on the frame 130a due to movement of the diaphragms 110a will add to each other, and it may be desirable to suspend the frame 130a from another frame, e.g. as described below with reference to FIG. 20.

(120) FIGS. 19(a) and 19(b) show a second example loudspeaker unit 101b which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(121) In this example, there are four diaphragms 110b arranged in a 2×2 array, where the shape of the diaphragms 110b is intended to maximise the surface area of the radiating surfaces 112b, 114b.

(122) In this example, the diaphragms 110b take the form of single pieces of lightweight material, such as extruded polystyrene, wherein opposite faces of the lightweight material provide the first radiating surfaces 112b and second radiating surfaces 114b.

(123) Each diaphragm 110b is suspended from the frame 130b via suspension elements 132a which in this example include roll suspensions, as can most clearly be seen in FIG. 19(b). The roll suspensions include “front” roll suspensions attached between the first radiating surfaces 112b of the diaphragms 110b and the frame 130b and “back” roll suspensions attached between the second radiating surfaces 114b of the diaphragms 110b and the frame 130b. For each diaphragm 110b, the position, number and length of the “front” and “back” roll suspension are matched to help eliminate any asymmetries in the performance of the roll suspensions.

(124) Preferably, the one or more suspension elements (e.g. one or more roll suspensions) attached between the first radiating surface of the diaphragm and the frame correspond to (e.g. match, e.g. match in position, number and length) the one or more suspension elements (e.g. one or more roll suspensions) attached between the second radiating surface of the diaphragm and the frame.

(125) Similarly to the example of FIGS. 18(a) and 18(b), in this example the loudspeaker unit 101b is implemented with a single frame configuration, with one or more rigid supporting elements 136b (e.g. arms) configured to hold a magnet unit of each drive unit 140b in front of the second radiating surface 114b of a respective diaphragm 110b.

(126) In this example, each diaphragm 110b includes cavities in the second radiating surface 114b, wherein each cavity is configured to have a respective rigid supporting element 136b extend through it when the loudspeaker unit 101b is in use. This may allow the loudspeaker unit 101b to have a lower profile in the thickness direction of the diaphragms.

(127) FIGS. 20(a) and 20(b) show a third example loudspeaker unit 101c which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(128) In this example, there are four diaphragms 110c arranged in a 2×2 array, where again the shape of the diaphragms 110b is intended to maximise the surface area of the radiating surfaces 112b, 114b.

(129) In this example, the loudspeaker unit 101c is implemented with a dual frame configuration, and includes a primary frame 130c and a secondary frame 131c, wherein each diaphragm 110c is suspended from the primary frame 130c via primary suspension elements 132c, and wherein the primary frame 130c is suspended from the secondary frame 131c via one or more secondary suspension elements 133c.

(130) This dual frame configuration may be useful to reduce vibrations passing from the loudspeaker unit 101c into the environment.

(131) The mounting of just one diaphragm 110c in the loudspeaker unit 101c is illustrated in FIG. 20.

(132) FIGS. 21(a) and 21(b) show a fourth example loudspeaker unit 101d which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(133) In this example, there are two diaphragms 110d arranged in a linear array.

(134) This example also implements a dual frame configuration, and includes a primary frame 130d and a secondary frame 131d, wherein each diaphragm 110d is suspended from the primary frame 130d via primary suspension elements 132d, and wherein the primary frame 130d is suspended from the secondary frame 131d via one or more secondary suspension elements 133d.

(135) In this example, there are only two diaphragms 110d configured such that the ears of a user are located at first and second listening positions which are in front of radiating surfaces 112d of the loudspeaker unit driven out of phase with each other. This is not preferred for reasons discussed above.

(136) FIGS. 22(a)-(c) show a fifth example loudspeaker unit 101e which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(137) In this example, there are two diaphragms 110e arranged in a linear array.

(138) In this example, the loudspeaker unit 101e is implemented with a single frame configuration, each diaphragm 110e being suspended from the frame 130e via suspension elements 132e.

(139) In this example, the drive unit 140e is shown in more detail in FIG. 22(c), and includes a magnet unit 142e and a voice coil (not shown).

(140) In this example, the voice coil is attached (e.g. glued) to the diaphragm 110e via a voice coil coupler 144e (described in more detail below).

(141) In this example, the magnet unit 142e is suspended from the diaphragm 110e via two magnet unit suspension elements 143e-1, 143e-2 and the voice coil coupler 144e. In this example, the two magnet unit suspension elements 145e-1, 145e-2 take the form of spiders which may be made from an impregnated textile (metal springs may be used in other examples). A spider may be understood as a textile ring having circumferentially extending corrugations (which may facilitate movement along the longitudinal axis whilst substantially preventing movement perpendicular to this axis), as is known in the art. The spiders may be made of impregnated textile. The magnet unit 142e includes a permanent magnet 142e-1, and magnetic field guiding elements 142e-1. The permanent magnet 142e-1 and the magnetic field guiding elements 142e-2 of the magnet unit 142e are configured to define an airgap 146e and to provide a magnetic field having concentrated flux in the air gap 146e. The voice coil is configured to sit in the airgap 146e when the diaphragm 110e is at rest.

(142) In this example, the voice coil coupler 144e takes the form of a housing provided with surfaces 208-1, 208-2 configured to allow the two magnet unit suspension elements 147e-1, 147e-2 to be attached (e.g. glued) to the voice coil coupler 144e. In this example, the housing of the voice coil coupler 144e also includes a cylindrical guiding surface 147e-3 onto which the voice coil may be mounted (e.g. glued) in place, though the voice coil is not shown in FIG. 20.

(143) When a current is passed through the voice coil, it will produces a magnetic field which interacts with the magnetic field produced by the magnet unit 142e which will cause the diaphragm 110e to move relative to the magnet unit 142e, with this movement being accommodated by the magnet unit suspension elements 145e-1, 145e-2.

(144) This example therefore shows how a magnet unit 142e can be suspended from the diaphragm 110e, rather than mounted to the frame 130e, as in the previous examples.

(145) In this example, the voice coil coupler 144e is an element which attaches the voice coil to the second radiating surface 114e of the diaphragm 101. In this example, the voice coil coupler 144e is glued to both the voice coil and the diaphragm 110e, thereby attaching the diaphragm 110e to the voice coil, and may therefore include lots of holes to facilitate gluing. The voice coil coupler 144e may provide a safety element (located between the magnet unit and second radiating surface) which is configured to prevent the magnet unit 142e from passing through diaphragm 110e in the event of a crash. Because the voice coil coupler 144e attaches the voice coil to the second radiating surface 114e of the diaphragm 110e, the diaphragm 110e does not require a dustcap on the first radiating surface 110e in this example (unlike the example shown in FIGS. 16(a)-(b), for example).

(146) The voice coil coupler 144e could be made of plastic, e.g. ABS, PC, or PVC, and may be filled with (e.g. 20%) glass fibres to improve structural strength. The voice coil coupler 144e could also be perforated to facilitate gluing and/or to allow visual inspection of the amount and curing of glue used. The size of the voice coil coupler 144e could be extended as needed for crash impact protection.

(147) FIGS. 23(a) and 23(b) show a fifth example loudspeaker unit 101f which implements the loudspeaker unit 101 of FIG. 8 in a car headrest.

(148) In this example, there are three diaphragms 110d arranged in a linear array.

(149) This example implements a dual frame configuration, and includes a primary frame 130f and a secondary frame 131f, wherein each diaphragm 110f is suspended from the primary frame 130f via primary suspension elements 132f which are provided in this example as roll suspensions, and wherein the primary frame 130f is suspended from the secondary frame 131f via one or more secondary suspension elements 133f which are provided in this example as roll suspensions.

(150) In this example, each diaphragm 110f is provided by a first cone 110f-1 and a second cone 110f-2 which are glued back to back and which respectively provide the first and second radiating surfaces 112f, 114f.

(151) In this example, each diaphragm 110f and the frames 130f, 131f are curved.

(152) In this example, the magnet unit of each drive unit 140f is held in front of a respective second radiating surface 114f by rigid supporting elements (e.g. arms) 136f. For each diaphragm 110f, a rigid safety element 144f located between the magnet unit and second radiating surface 114f is configured to prevent the magnet unit of the drive unit 140f from passing through diaphragm 110e in the event of a crash. The safety element 144f can be viewed as a voice coil coupler configured to attach the voice coil to the second radiating surface 112f of the diaphragm 110f, and gluing a voice coil former 148f In this case, that attachment is provided by gluing the rigid safety element 144f to a voice coil former 148f on which the voice coil (not shown) is mounted.

Examples Implementing Second Aspect of the Invention

(153) FIG. 24 is a schematic view of a loudspeaker unit 201 for producing sound at bass frequencies according to the second aspect of the invention.

(154) The loudspeaker unit 201 includes an array of n diaphragms 210 (features relating to an individual diaphragm are labelled with the suffix “−1”, “−2”, “−3” . . . “−n”). Each diaphragm has a first radiating surface 212, and a second radiating surface 214, wherein the first radiating surface 212 and the second radiating surface 214 are located on opposite faces of the diaphragm.

(155) The loudspeaker unit 201 also includes a frame 230, wherein each diaphragm 210 in the array is suspended from the frame 230 via one or more suspension elements 232 such that sound produced by the first radiating surfaces 212 is allowed to propagate out from the loudspeaker unit 201.

(156) As depicted in FIG. 24, that the first radiating surfaces 112 are facing in a first (“forwards”) direction F and the second radiating surfaces 114 are facing in an opposite (“backwards”) second direction B with the frame 130 being configured to allow sound produced by the first radiating surfaces 212 to propagate out from a first side 204 of the loudspeaker unit 201 in the first direction F. However, this is only schematic, and for reasons that can be understood from explanations elsewhere in this disclosure, other orientations of the diaphragms are possible (and indeed preferred).

(157) The loudspeaker unit 201 also includes a plurality of drive units 240, wherein each drive unit 240 is configured to move a respective one of the diaphragms 210 in the array based on a respective electric signal.

(158) The loudspeaker unit 201 also includes at least one enclosure 235 configured to receive sound produced by the second radiating surfaces 214. As depicted in FIG. 24, there is a single sealed enclosure 235 configured to receive sound produced by all the second radiating surfaces 214, thereby inhibiting sound produced by the second radiating surfaces 114 from propagating out from a second side 106 of the loudspeaker unit 201 in the second direction B. However, other enclosure arrangements are possible. For example, each of the second radiating surfaces 214 may face towards a central space which is enclosed by a single enclosure configured to receive sound produced by each one of the second radiating surfaces. It would also be possible for each second radiating surface to be provided with its own (respective) enclosure, for example.

(159) One or more of the diaphragms 210 are included in a first subset of the diaphragms 210 and one or more of the diaphragms 210 are included in a second subset of the diaphragms 210.

(160) The loudspeaker unit 201 also includes drive circuitry 250 configured to provide each drive unit 240 with a respective electrical signal derived from the same audio source such that the electrical signal(s) provided to the one or more drive units 240 configured to move the first subset of diaphragms 210 is/are out of phase with respect to the electrical signal(s) provided to the one or more drive units 240 configured to move the second subset of diaphragms 210.

(161) Such drive circuitry may be implemented in a similar manner to the drive circuitry 150, 150′ shown in FIG. 9(a) or 9(b), for example.

(162) The following drawings and corresponding discussion sets out some guiding principles for how the loudspeaker unit 201 of FIG. 24 could be implemented in a car headrest. In the following examples, at least one enclosure is configured to receive sound produced by each diaphragm, such that a single diaphragm can be viewed as a monopole loudspeaker, two diaphragms can be viewed as a dipole loudspeaker, and four diaphragms can be viewed as a quadrupole loudspeaker.

(163) In some cases, a monopole loudspeaker unit containing only one diaphragm is depicted for comparative purposes.

(164) FIG. 25 shows the polar response in the y-z, x-y and x-z planes for a monopole loudspeaker unit including a single diaphragm (wherein an enclosure is configured to receive sound produced by a second radiating surface of this diaphragm), a dipole loudspeaker unit including a two diaphragms (wherein an enclosure is configured to receive sound produced by the second radiating surfaces of these diaphragms) and a quadrupole loudspeaker unit including four diaphragms (wherein an enclosure is configured to receive sound produced by the second radiating surfaces of these diaphragms).

(165) Knowing these polar responses can help with deciding on a preferred implementation of a multipole loudspeaker unit.

(166) A particular point to note from FIG. 25 is that a monopole loudspeaker has a spherical polar response at bass frequencies, meaning it can be oriented in any direction according to design requirements, without changing the performance of the loudspeaker unit.

(167) FIGS. 26(a)-(b) illustrate some preferred listening positions for use with a quadrupole loudspeaker unit formed of four monopole loudspeakers arranged in a 2×2 array, where the electrical signals provided to the drive units configured to move the first subset of diaphragms are out of phase with respect to the electrical signals provided to the one or more drive units configured to move the second subset of diaphragms.

(168) As above, sound produced by a first radiating surface of a diaphragm in the first subset of diaphragms is marked with a ‘+’ and sound produced by a first radiating surface of a diaphragm in the second subset of diaphragms is marked with a ‘−’.

(169) Since the polar response of an individual monopole loudspeaker is spherical, it is to be noted that the arrangement of FIG. 26(a) and that of FIG. 26(b) are directly equivalent, though the arrangement shown in FIG. 26(b) is preferred because it could more easily be incorporated into a car headrest.

(170) In the arrangements of FIGS. 26(a) and 26(b), a principal radiating axis of each first radiating surface lies in the same vertical plane when the loudspeaker unit is in use.

(171) FIGS. 27(a)-(c) show the diaphragms arranged as shown in FIG. 26(b) from various angles.

(172) FIGS. 28(a)-(b) illustrate some less preferred listening positions for use with a quadrupole loudspeaker unit formed of four monopole loudspeakers arranged in a 2×2 array, where the electrical signals provided to the drive units configured to move the first subset of diaphragms are out of phase with respect to the electrical signals provided to the one or more drive units configured to move the second subset of diaphragms.

(173) As above, sound produced by a first radiating surface of a diaphragm in the first subset of diaphragms is marked with a ‘+’ and sound produced by a first radiating surface of a diaphragm in the second subset of diaphragms is marked with a ‘−’.

(174) Again, since the polar response of an individual monopole loudspeaker is spherical, it is to be noted that the arrangement of FIG. 28(a) and that of FIG. 28(b) are directly equivalent, though the arrangement shown in FIG. 28(b) is preferred because it could more easily be incorporated into a car headrest.

(175) However, the arrangement shown in FIG. 28(b) is nonetheless less preferred to that shown in FIG. 26(b), since the ears of a user are closer to nulls in the arrangement of FIG. 28(b) compared with the arrangement of FIG. 26(b).

(176) In the arrangements of FIGS. 26(a) and 26(b), a principal radiating axis of each first radiating surface lies in the same horizontal plane when the loudspeaker unit is in use.

(177) FIGS. 29(a)-(c) show the diaphragms arranged as shown in FIG. 28(b) from various angles.

(178) Examples which implement the principles of the loudspeaker unit 201 of FIG. 24 will now be described, with alike reference numerals indicating corresponding features that do not need to be described further, except where further explanation is provided.

(179) FIGS. 30(a)-(d) show a first example loudspeaker unit 201a which implements the loudspeaker unit 201 of FIG. 24 in a car headrest.

(180) In this example, there are four diaphragms 210a arranged in the preferred manner depicted in FIG. 26(b), i.e. with a principal radiating axis of each first radiating surface 212a lying in the same vertical plane when the loudspeaker unit is in use. A principal radiating axis of each first radiating surface 212a further points outwardly from a central space 239a.

(181) A sealed enclosure is provided by walls of the frame 230a and which encloses the central space 239a is configured to receive sound produced by the second radiating surfaces 214a.

(182) In this example, each diaphragm 210a is a cone diaphragm, wherein a concave surface of each cone provides a respective first radiating surface 212a. Each diaphragm is suspended from the frame 230a via respective suspension elements which include for each loudspeaker a roll suspension 232a-1 and a spider 232a-2.

(183) Each drive unit 240a configured to move a respective diaphragm 210a is a conventional electromagnetic drive unit.

(184) An acoustically transparent grill 249a fixedly attached the frame 234a, in order to provide support for an acoustically transparent covering material.

(185) The headrest is covered by an acoustically transparent material, which has been omitted from FIGS. 30(a)-(d) so that the diaphragms can be viewed on the front of the headrest (FIG. 30(b)) and the top of the headrest (FIG. 30(d)).

(186) The loudspeaker unit 201a is configured to be fixedly mounted to a car seat frame via mounting pins 282a.

(187) FIG. 31 illustrates how, by applying a delay Δt to a signal from a selected electrical signal supplied to one of the drive units (these signals are referred to as channels CH1-CH4 in FIG. 31) causes the selected diaphragm 210a to be virtually moved by a distance Δd further away from a reference diaphragm 210a having no delay (Δt=0).

(188) The distance by which a diaphragm is virtually moved can theoretically be represented by the following relation:
Δd=Δt.Math.c  (4)

(189) Where c is the speed of sound.

(190) However, it is to be noted that applying such a delay Δt will in general worsen the level of cocooning provided by the loudspeaker unit 201a and may also diminish force cancelling and therefore cause vibrations to propagate out into the environment via the frame 230a.

(191) FIG. 32 shows a second example loudspeaker unit 201b which implements the loudspeaker unit 201 of FIG. 24 in a car headrest.

(192) In this example, the frame 234b is suspended from the acoustically transparent grill 249b by suspension elements 239b provided in this case in the form of an elastic suspension.

(193) So in this example, the transparent grill 249b provides a secondary frame and the frame 234b provides a primary frame, wherein the diaphragms 210b are suspended from the primary frame 234b by primary suspension elements 232b-1, 232b-2, and the primary frame 23b is suspended from the secondary frame 249b by secondary suspension elements 239b.

(194) This dual frame configuration may be useful to reduce vibrations passing from the loudspeaker unit 201b into the environment. This may be useful e.g. if adding a delay between channels of equal polarity as proposed with reference to FIG. 31 causes diminished force cancelling.

(195) In view of the above discussion, some advantages of the monopole type implementations described with reference to FIGS. 24-31 can be understood: With 4 equal diaphragms 210a the complete assembly is “vibration free” since the inertial forces from the mass of the diaphragms 210a cancel each other. There is also no pressure build-up inside the enclosure. Whereas dipole loudspeakers in quadrupole configuration (as described e.g. with reference to FIGS. 21-22 will not completely cancel their forces and will instead create a momentum based on the distance the diaphragms are located from each other. The back of the loudspeakers are sealed so that motor noises (e.g. blowing noises from compressed air thru the magnet gap) are better sealed compared to the dipole type implementation implementations described with reference to FIGS. 8-23 Delay flexibility: with individual monopole loudspeakers being used, the dimensions D, d of our dipole-pair and quadrupole-pair as depicted in FIG. 31 are easily adjusted both mechanically (by moving the monopole loudspeakers) and by using a delay as described above with reference to FIG. 31. Whereas with a quadrupole loudspeaker unit that uses dipole loudspeakers as described with reference to FIGS. 8-23, the dimension D, is defined by the dimensions of the diaphragm and cannot be altered using a delay. Only on the quadrupole pair (distance d) and on the octopole pair (distance d′) can we usefully apply delay. A dipole path length D of 10 cm using a dipole loudspeaker would imply a diaphragm with a 20 cm diameter and may be much too large for practical implementation in a slim headrest (especially if two such diaphragms are required), while with a loudspeaker unit that incorporates monopole loudspeakers a distance of 10 cm for the first dipole pair can easily be achieved whilst maintaining a compact headrest size. Note that we have seen previously that the pressure of our quadrupole is directly proportional with pathlength D and pathlength d. Vibration can easily be introduced on purpose, e.g. for signaling features.

Examples Implementing Third Aspect of the Invention

(196) FIG. 33 is a schematic view of a loudspeaker unit 301 for producing sound at bass frequencies according to the third aspect of the invention.

(197) The loudspeaker unit 301 includes an array of n diaphragms 310 (features relating to an individual diaphragm are labelled with the suffix “−1”, . . . “−n”). Each diaphragm has a first radiating surface 312, and a second radiating surface 314, wherein the first radiating surface 312 and the second radiating surface 314 are located on opposite faces of the diaphragm.

(198) The loudspeaker unit 301 also includes a frame 330, wherein each diaphragm 310 in the array is suspended from the frame 330 via one or more suspension elements 332 such that sound produced by the first radiating surfaces 312 is allowed to propagate out from the loudspeaker unit 301.

(199) As depicted in FIG. 33, that the first radiating surfaces 312 are facing in a first (“forwards”) direction F and the second radiating surfaces 314 are facing in an opposite (“backwards”) second direction B with the frame 130 being configured to allow sound produced by the first radiating surfaces 212 to propagate out from a first side 304 of the loudspeaker unit 301 in the first direction F. However, this is only schematic, and for reasons that can be understood from explanations elsewhere in this disclosure, other orientations are possible (and indeed preferred).

(200) The loudspeaker unit 301 also includes a plurality of drive units 340, wherein each drive unit 340 is configured to move a respective one of the diaphragms 310 in the array based on a respective electric signal.

(201) The loudspeaker unit 301 also includes at least one enclosure 335 configured to receive sound produced by the second radiating surfaces 314. As depicted in FIG. 33, there is a single enclosure 335 configured to receive sound produced by all the second radiating surfaces 314. The enclosure includes a plurality of vents 337, wherein each vent is configured to allow sound produced by the second radiating surface to propagate out from the loudspeaker unit in a different direction. Other enclosure/vent arrangements are possible.

(202) It is important to note that the vents 337 do not serve as traditional “bass reflex” vents to extend the low frequency performance of the loudspeaker unit 301 based on creating a Helmholtz resonator tuned at a low frequency for increasing the bass output at that tuning frequency. Here, since the volume is small and the vent 337 opening large, the tuning frequency of those openings will be high compared to the low frequencies we are addressing in this application. Basically, it is neither intended nor desirable to use the Helmholtz resonance phenomenon. The vents 337 are instead used to provide a means by which air can be emitted from the enclosure whilst being out of phase and thus creating the other pole at the exit of the vent 337.

(203) Thus, each vent 337 is preferably open enough such that any Helmholtz resonator provided by the enclosure has a tuning frequency that is above 200 Hz, more preferably above 400 Hz. The size of each vent required to achieve this will depend on various factors such as the size of the enclosure, and neck length leading to each vent. The principles of Helmholtz resonators are well known by the skilled person and do not require further description herein.

(204) The loudspeaker unit 301 also includes drive circuitry 350 configured to provide each drive unit 340 with a respective electrical signal derived from the same audio source such that the sound produced by the second radiating surfaces 314 is out of phase with respect to the sound produced by the first radiating surfaces 312.

(205) FIGS. 34(a) and 34(b) each show an example of drive circuitry 350, 350′ which may be included in the loudspeaker 301 of FIG. 33 and be configured to provide each drive unit 340 of the loudspeaker unit 301 of FIG. 33 with a respective electrical signal derived from the same audio source such that the sound produced by the second radiating surfaces 314 is out of phase with respect to the sound produced by the first radiating surfaces 312.

(206) The example drive circuitry 350 of FIG. 34(a) includes a digital signal processor (“DSP”) 352 configured to provide each drive unit 340 with a respective electrical signal via a respective amplifier 354, wherein the respective electrical signal is derived from an audio signal (in this case a digital audio signal) provided by the audio source at node 356. No manipulation of the electrical signals by the DSP 352 is required in order for the drive circuitry 350 to provide each drive unit 340 with a respective electrical signal derived from the same audio source such that the sound produced by the second radiating surfaces 314 is out of phase with respect to the sound produced by the first radiating surfaces 312. However, a DSP 352 is nonetheless preferred, since modification of the electrical signals provided to the drive units 340 e.g. to modify the phase, delay or amplitude of the electrical signal respectively provided to each drive unit 140 so as to optimise the sound provided to a user (e.g. in a manner described herein).

(207) The example drive circuitry 350′ of FIG. 9(b) includes an amplifier 354′ and wiring 355′ configured to maintain the polarity of the electrical signal(s) provided to the/each drive unit 340, e.g. with + and − wires supplying an audio signal provided by the audio source 356′ via the amplifier 354′ being connected to the/each drive unit 340 the same way around (unlike the situation in FIG. 9(b) where the wiring was used to reverse the polarity of electric signals provided to drive units configured to move a certain subset of diaphragms).

(208) The following drawings and corresponding discussion sets out some guiding principles for how the loudspeaker unit 301 of FIG. 33 could be implemented in a car headrest.

(209) FIGS. 35(a)-(c) illustrate a preferred listening position for use with a headrest that incorporates loudspeaker unit formed of two monopole loudspeakers arranged back to back, in this case with the diaphragm of one of the monopole loudspeakers having a first radiating surface that faces in a forwards direction F and with the diaphragm of the other monopole loudspeaker having a first radiating surface that faces in a backwards direction B. A first vent 337-1 is configured to allow sound to propagate out from the loudspeaker unit in an upwards direction U, and a second vent 337-2 configured to allow sound to propagate out from the loudspeaker unit in a downwards direction D.

(210) As can be seen from FIG. 35(c), in this example each vent 337 takes the form of a plurality of holes.

(211) In the example shown in FIGS. 35(a)-(c), the volume displacement of the second radiating surface of each of the two loudspeakers is directed towards the vents 337-1, 337-2. In this way antiphase sound is created at the vents 337-1, 337-2, without the need for another pair of monopole loudspeakers.

(212) FIG. 35(d) shows a variant of the headrest, wherein the enclosure of the loudspeaker unit includes a partition configured to direct sound produced by the second radiating surface of each diaphragm out of a respective one of the vents 337-1, 337-2.

(213) It is to be noted that the examples shown in FIGS. 35(a)-(d) achieve force cancellation similar to that achieved by the loudspeaker described in connection with examples of the second aspect of the invention discussed above, but with fewer loudspeakers.

(214) A delay could be implemented between the two loudspeakers to increase the virtual distance between the poles, e.g. as described above with reference to FIG. 31.

(215) Examples which implement the principles of the loudspeaker unit 301 of FIG. 33 will now be described, with alike reference numerals indicating corresponding features that do not need to be described further, except where further explanation is provided.

(216) FIGS. 36(a)-(d) show a first example loudspeaker unit 301a which implements the loudspeaker unit 301 of FIG. 33 in a car headrest.

(217) In this example, there are two diaphragms 310a arranged in the manner depicted in FIG. 35, i.e. arranged back to back, with one diaphragm 310a having a first radiating surface 312a that faces in a forwards direction F and with the other diaphragm 310a having a first radiating surface 312a that faces in a backwards direction B.

(218) An enclosure which is provided by walls of the frame 330a and which encloses the central space 339a is configured to receive sound produced by the second radiating surfaces 314a. A first vent 337a-1 included in the enclosure is configured to allow sound to propagate out from the loudspeaker unit in an upwards direction U, and a second vent 337a-2 included in the enclosure is configured to allow sound to propagate out from the loudspeaker unit in a downwards direction D.

(219) In this example, each diaphragm 310a is a cone diaphragm, wherein a concave surface of each cone provides a respective first radiating surface 312a. Each diaphragm is suspended from the frame 330a via respective suspension elements which include for each loudspeaker a roll suspension 332a-1 and a spider 332a-2.

(220) Each drive unit 340a configured to move a respective diaphragm 310a is a conventional electromagnetic drive unit.

(221) An acoustically transparent grill 349a fixedly attached the frame 334a, in order to provide support for an acoustically transparent covering material.

(222) The headrest is covered by an acoustically transparent material, which has been omitted from FIGS. 36(a)-(d) so that the diaphragm can be viewed on the front of the headrest (FIG. 30(b)) and so that the vent can be viewed on the top of the headrest (FIG. 30(d)).

(223) The loudspeaker unit 301a is configured to be fixedly mounted to a car seat frame via mounting pins 382a.

(224) Note that the enclosure is essentially open on top and bottom, thus the pressure inside the enclosure (which is out of phase with that of the front side of the two loudspeakers) will create out of phase sources via the top and bottom vents.

(225) FIGS. 37(a)-(c) show a second example loudspeaker unit 301b which implements the loudspeaker unit 301 of FIG. 33 in a car headrest.

(226) In this example, the radiating surfaces of the loudspeakers have been maximised, and the volume enclosed by the enclosure minimised,

(227) Here, the diaphragm is made of extruded polypropylene which may act as a safety element configured to prevent the magnet unit(s) from passing through the diaphragm in a crash event.

(228) FIGS. 38(a)-(b) show a third example loudspeaker unit 301c which implements the loudspeaker unit 301 of FIG. 33 in a car headrest.

(229) In this example, the loudspeaker unit 301c includes two pairs of diaphragms, with one of the diaphragms included in each pair having a first radiating surface that faces in the forward direction F, and with the other of the diaphragms included in each pair having a first radiating surface that faces in the backwards direction B. A first vent is configured to allow sound to propagate out from the loudspeaker unit in an upwards direction U, and a second vent is configured to allow sound to propagate out from the loudspeaker unit in a downwards direction D.

(230) This may be useful e.g. to provide stereo sound to the different ears of a user or alternatively to compensate for movement of a user's head (as will now be described).

(231) Preferably, a seat assembly that includes the car headrest also includes a head tracking unit (not shown) configured to track head movement of a user sat in the seat.

(232) For the purposes of this description, the two diaphragms whose first radiating surfaces face in the forwards direction F are referred to as “forward facing diaphragms”.

(233) Preferably, the DSP 352 in the drive circuitry 350 is configured to modify the electrical signals provided to the drive units configured to move the forward facing diaphragms based on head movement as tracked by the head tracking unit so as to compensate for movement of the head of a user sat in the seat.

(234) Compensation for head movement may involve adjusting any one or more of amplitude (u), delay (t) and phase (ϕ) according suitable algorithms.

(235) In a simple example, the DSP 352 in the drive circuitry 350 may be configured to increase the amplitude of sound produced by one of the forward facing diaphragms if it is determined based on head movement as tracked by the head tracking unit that an ear of the user has moved further away from the first radiating surface of that diaphragm (e.g. by distance Δd as shown in FIG. 38(b)). Similarly, the drive circuitry may be configured to decrease the amplitude of sound produced by one of forward facing diaphragms if it is determined based on head movement as tracked by the head tracking unit that an ear of the user has moved closer to the first radiating surface of that diaphragm (e.g. by distance Δd as shown in FIG. 38(b)). The amount by which the amplitude of sound is increased/decreased may depend on the distance by which the relevant ear has moved (e.g. distance Δd as shown in FIG. 38(b)).

(236) The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

(237) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(238) For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

(239) Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

(240) Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

(241) It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/−10%.

(242) Experimental Data

(243) Experimental Data 1

(244) FIGS. 39(a)-(b) illustrate an experimental set up used to obtain experimental data 1.

(245) FIGS. 40(a)-(b) illustrate experimental data 1 obtained using the experimental set up of FIGS. 39(a)-(b).

(246) Experiments were performed to test the performance of a loudspeaker unit according to the first aspect of the invention.

(247) These experiments were performed using a loudspeaker unit in which two diaphragms were used as dipole loudspeakers and moved by a drive units that were supplied with electrical signals that were either the same (case 1=dipole mode) or in antiphase (case 2: quadrupole mode).

(248) Each diaphragm used had a size of 20 cm×27 cm, making a total surface area of 540 cm.sup.2, and fed with an electrical signal having a power of 1 W.

(249) The arrangement of the diaphragms is shown in FIG. 39(a) for case 1 where the electrical signals were in phase, and in FIG. 39(b) for case 2 where the electrical signals were in antiphase.

(250) In both cases 1 and 2, SPL was measured at different distances (6 cm, 12.5 cm, 25 cm, 50 cm, 100 cm) over a range of frequencies along a path 45° to a z axis, and the results of these measurements are shown in FIG. 40(a) for case 1 and 40(b) for case 2.

(251) As can be seen from a comparison of FIGS. 34(a) and 34(b) at 50 Hz: For the dipole mode of operation (case 1) as shown in FIG. 40(a), the SPL at 12.5 cm is 101 dB and at 100 cm is 74 dB, meaning a drop in SPL of 25 dB between these two distances For the quadrupole mode of operation (case 2) as shown in FIG. 40(b), the SPL at 12.5 cm is 97 dB and at 100 cm is 60 dB, meaning a drop in SPL of 37 dB between these two distances, i.e. an improvement of 12 dB compared to the dipole mode

(252) This shows that a loudspeaker unit configured to operate with multiple diaphragms moving out of phase with each other is able to provide an improved cocooning effect compared with a dipole loudspeaker having the same area of radiating surfaces

REFERENCES

(253) A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. [1] https://en.wikipedia.org/wiki/Equal-loudness_contour [2] http://www.linkwitzlab.com [3] https://www.techopedia.com/definition/31557/head-tracking [4] http://www.autoguide.com/auto-news/2017/08/two-companies-are-working-on-bringing-in-car-sensing-tech-to-new-cars.html [5] https://sharpbrains.com/blog/2014/09/02/general-motors-to-adopt-eye-head-tracking-technology-to-reduce-distracted-driving/ [6] http://www.patentlyapple.com/patently-apple/2016/08/apple-wins-patent-for-advanced-3d-eyehead-tracking-system-supporting-apples-3d-camera.html [7] “Face Recognition and Head Tracking in Embedded Systems”, Lenka Ivantysynova and Tobias Scheffer, Optik&Photonik, January 2015, pages 42-45.