DIPOLE LOUDSPEAKER FOR PRODUCING SOUND AT BASS FREQUENCIES

Abstract

A dipole loudspeaker for producing sound at bass frequencies. The dipole loudspeaker includes: a diaphragm 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 drive unit configured to move the diaphragm at bass frequencies such that the first and second radiating surfaces produce sound at bass frequencies, wherein the sound produced by the first radiating surface is in antiphase with sound produced by the second radiating surface; a frame, wherein the diaphragm is suspended from the frame via one or more suspension elements, wherein the frame is configured to allow sound produced by the first radiating surface to propagate out from a first side of the dipole loudspeaker and to allow sound produced by the second radiating surface to propagate out from a second side of the dipole loudspeaker. The diaphragm includes a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, wherein the diaphragm is configured to permit airflow through at least part of said region of porous material from the first radiating surface of the diaphragm to the second radiating surface of the diaphragm.

Claims

1-15. (canceled)

16. A dipole loudspeaker for producing sound at bass frequencies, the dipole loudspeaker including: a diaphragm 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 drive unit configured to move the diaphragm at bass frequencies such that the first and second radiating surfaces produce sound at bass frequencies, wherein the sound produced by the first radiating surface is in antiphase with sound produced by the second radiating surface; a frame, wherein the diaphragm is suspended from the frame via one or more suspension elements, wherein the frame is configured to allow sound produced by the first radiating surface to propagate out from a first side of the dipole loudspeaker and to allow sound produced by the second radiating surface to propagate out from a second side of the dipole loudspeaker; wherein the diaphragm includes a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, wherein the diaphragm is configured to permit airflow through at least part of said region of porous material from the first radiating surface of the diaphragm to the second radiating surface of the diaphragm.

17. A dipole loudspeaker according to claim 16, wherein the dipole loudspeaker is configured for use with an ear of a user located at a listening position that is in front of and 30 cm or less from the first radiating surface of the diaphragm.

18. A dipole loudspeaker according to claim 16, wherein the region of porous material has a specific airflow resistance in the range 50-500 Pa.Math.s/m.

19. A dipole loudspeaker according to claim 16, wherein the diaphragm includes a layer of porous material mounted on a supporting structure.

20. A dipole loudspeaker according to claim 19, wherein the supporting structure is a rigid perforated sheet of non-porous material, wherein the sheet includes a plurality of holes/cut-outs.

21. A dipole loudspeaker according to claim 20, wherein the plurality of holes/cut-outs having an area that is 50% or more of the area of the sheet when the holes/perforations are covered.

22. A dipole loudspeaker according to claim 19, wherein a lead wire configured to supply electrical energy to a voice coil of the drive unit is mounted to the supporting structure.

23. A dipole loudspeaker according to claim 16, wherein the diaphragm is an unsupported layer of porous material.

24. A dipole loudspeaker according to claim 16, wherein the dipole loudspeaker includes a supplementary loudspeaker configured to produce sound which propagates through the at least part of said region of porous material.

25. A dipole loudspeaker according to claim 24, wherein the supplementary loudspeaker is a mid-high frequency loudspeaker configured to produce sound across at least the range 500 Hz-10 kHz.

26. A dipole loudspeaker according to claim 16, wherein the drive unit is rigidly attached to the frame of the dipole loudspeaker.

27. A seat assembly including a seat and a dipole loudspeaker for producing sound at bass frequencies, the dipole loudspeaker including: a diaphragm 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 drive unit configured to move the diaphragm at bass frequencies such that the first and second radiating surfaces produce sound at bass frequencies, wherein the sound produced by the first radiating surface is in antiphase with sound produced by the second radiating surface; a frame, wherein the diaphragm is suspended from the frame via one or more suspension elements, wherein the frame is configured to allow sound produced by the first radiating surface to propagate out from a first side of the dipole loudspeaker and to allow sound produced by the second radiating surface to propagate out from a second side of the dipole loudspeaker; wherein the diaphragm includes a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, wherein the diaphragm is configured to permit airflow through at least part of said region of porous material from the first radiating surface of the diaphragm to the second radiating surface of the diaphragm; wherein the seat is configured to position a user who is sat down in the seat such that an ear of the user is located at a listening position that is in front of and 30 cm or less from the first radiating surface of the diaphragm.

28. A seat assembly including claim 27, wherein the dipole loudspeaker is mounted within a headrest of the seat.

29. A seat assembly according to claim 28, wherein the headrest includes a headrest material which at least partially encloses the dipole loudspeaker, wherein the headrest material which encloses the dipole loudspeaker is a porous material having a specific airflow resistance of less than 25 Pa.Math.s/m.

30. A seat assembly according to claim 27, wherein the headrest includes two of the dipole loudspeakers, wherein the seat is configured to position a user who is sat down in the seat such that a first ear of the user is located at a listening position that is in front of and 30 cm or less from the first radiating surface of the diaphragm of a first of the two dipole loudspeakers, and such that a second ear of the user is located at a listening position that is in front of and 30 cm or less from the first radiating surface of the diaphragm of a second of the two dipole loudspeakers.

Description

SUMMARY OF THE FIGURES

[0078] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0079] FIGS. 1A-1C show a first diaphragm illustrating the principles of the present invention.

[0080] FIGS. 2A-2B show a second diaphragm illustrating the principles of the present invention.

[0081] FIG. 3 shows a third diaphragm illustrating the principles of the present invention.

[0082] FIG. 4A shows a first seat headrest in which two dipole loudspeakers according to the present invention are mounted.

[0083] FIG. 4B shows a second seat headrest in which a dipole loudspeaker according to the present invention is mounted.

[0084] FIGS. 5A-D show additional dipole loudspeakers according to the present invention.

[0085] FIGS. 6A-C relate to experiment 1, discussed in more detail below.

[0086] FIG. 7 relates to experiment 2, discussed in more detail below.

[0087] FIGS. 8A-B relate to experiment 3, discussed in more detail below.

[0088] FIGS. 9A-B relate to experiment 4, discussed in more detail below.

[0089] FIGS. 10A-B relate to airflow resistant measurements, discussed in more detail below.

DETAILED DESCRIPTION OF THE INVENTION

[0090] 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.

[0091] FIGS. 1A-1C show a first diaphragm 110 illustrating the principles of the present invention.

[0092] In this example, the diaphragm 110 includes a layer 112 of porous material mounted on a supporting structure 120. The porous material may be an open cell foam or other porous material such as a textile, for example.

[0093] Here, the layer 112 of porous material is only shown as covering part of the supporting structure 120.

[0094] Preferably, the thickness and porosity of the layer 112 of porous material is chosen such that the layer 112 of porous material has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0095] Thus, in this example, the entirety of the layer 112 of porous material has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0096] In this example, the supporting structure 120 is a perforated sheet of non-porous material, wherein the sheet has an arbitrary shape and includes an arbitrary number of holes/cut-outs of arbitrary shape, in this case two holes 122.

[0097] In this example, the holes 122 in the perforated sheet 120 permit airflow through part of the region of porous material. Specifically, the holes 122 in the perforated sheet permit airflow through the parts of the region of porous material that are located over the holes. Thus, the diaphragm is configured to permit airflow through said parts of the region of porous material from a first radiating surface 114(i) of the diaphragm to a second radiating surface 114(ii) of the diaphragm 110.

[0098] A skilled person would appreciate that the perforated sheet 120 could have any shape and any number or shape of perforations to achieve a required openness or structural performance. Similarly, the layer 112 of porous material could have a required porosity and/or thickness such that the layer 112 of porous material has a specific airflow resistance in a desired range.

[0099] FIG. 1C shows the first diaphragm 110, wherein a supplementary loudspeaker 150 is configured to produce sound which propagates through one of the holes 122 in the perforated sheet, and therefore through one of the parts of the layer 112 of porous material that the diaphragm is configured to permit airflow through. The supplementary loudspeaker is preferably a mid-high frequency loudspeaker.

[0100] This configuration is able to work since mid-high frequency sound is able to pass through the layer 112 of porous material via the holes 122 with relatively little attenuation (see experiments discussed below). A skilled person would appreciate there is a balance in setting the specific airflow resistance of the layer 112 so as to not overly attenuating mid-high frequency sound, whilst still generating bass frequencies with an adequate SPL.

[0101] FIGS. 2A-2B show a second diaphragm 110′ illustrating the principles of the present invention.

[0102] Alike features corresponding to features described in relation to previous drawings have been given alike reference numerals.

[0103] In this example, the diaphragm is an unsupported layer 112′ of porous material. That is, the porous material forming the layer 112′ and the thickness of the layer 112′ are chosen such that the layer 112′ can be used as a diaphragm without the need to be mounted on a supporting structure.

[0104] Moreover, the porous material forming the layer 112′, and the thickness of the layer 112′ are chosen such that the layer 112′ has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0105] Thus, in this example, the entirety of the layer 112′ of porous material can be viewed as a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0106] The material used for the layer 112′ may be foamed silica, foamed aluminium, or any other porous solid having the required properties.

[0107] FIG. 3 shows a third diaphragm 110″ illustrating the principles of the present invention.

[0108] Again, alike features corresponding to features described in relation to previous drawings have been given alike reference numerals.

[0109] In this example, the layer 112″ of porous material is mounted on a perforated sheet 120″ having an adequately large coverage of perforations such that its specific airflow resistance is effectively zero.

[0110] Thus, the holes 122″ in the perforated sheet 120″ permit airflow through the entire layer 112″ of porous material from a first radiating surface 114(i)″ of the diaphragm to a second radiating surface 114(ii)″ of the diaphragm 110″.

[0111] Again, the thickness and porosity of the layer 112″ of porous material is preferably chosen such that the layer 112″ of porous material has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0112] Thus, in this example, the entirety of the layer 112″ of porous material can be viewed as a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0113] FIG. 3 shows the diaphragm 110″ being moved at bass frequencies such that the first and second radiating surfaces 114(i)″, 114(ii)″ produce sound at bass frequencies, wherein the sound produced by the first radiating surface 114(i)″ is in antiphase with sound produced by the second radiating surface 114(ii)″ (this is inherently true for all diaphragms). In order to be used as a dipole loudspeaker, the diaphragm 110″ should be suspended from a frame (not shown) so that sound produced by the first radiating surface 114(i)″ is able to propagates out from a first side of the dipole loudspeaker and so that sound produced by the second radiating surface 114(ii)″ propagates out from a second side of the dipole loudspeaker.

[0114] As is also shown in FIG. 3, a dipole loudspeaker including the third diaphragm 110″ may be configured for use with an ear of a user located at a listening position that is in front of and a distance d (e.g. 30 cm or less) from the first radiating surface of the diaphragm.

[0115] FIG. 4A shows a seat headrest 290a (in this example, a car seat headrest) in which a first dipole loudspeaker 200a according to the present invention is mounted and a second dipole loudspeaker 200a′ according to the present invention is mounted.

[0116] Both loudspeakers 200a, 200a′ are bass loudspeakers for producing sound at bass frequencies.

[0117] In this example, the two dipole loudspeakers 200a, 200a′ have different structures so as to illustrate different possibilities, though in most cases it is envisaged that both dipole loudspeakers included in the seat headrest 290a would have the same structure as each other.

[0118] The seat headrest 290a includes headrest material 295a which encloses the first and second dipole loudspeakers 200a, 200a′.

[0119] In this example, the headrest material 295a includes a porous foam material having an open cell structure providing comfort (such as reticulated polyurethane (“PU”), polyethylene (“PE”) or polyester foam) and a specific airflow resistance of less than 25 Pa.Math.s/m, which is itself covered by a finishing material 296a (such as a textile or perforated leather) having a specific airflow resistance of less than 25 Pa.Math.s/m. Note: the low specific airflow resistances of the headrest material and finishing material are chosen so as to avoid bass frequencies being impeded from propagating out of the seat headrest 290a.

[0120] Each dipole loudspeaker 200a, 200a′ includes a drive unit 230a 230a′ configured to move a diaphragm 210a, 210a′ at bass frequencies such that first and second radiating surfaces 214a(i), 214a(i)′, 214a(ii), 214a(ii)′ produce sound at bass frequencies, wherein the sound produced by the first radiating surface 214a(i), 214a(i)′ is in antiphase with sound produced by the second radiating surface 214a(ii), 214a(ii)′. Each drive unit 230a, 230a′ shown here is an electromagnetic drive unit.

[0121] The seat (not shown) is configured to position a user who is sat down in the seat such that a first ear 298a of the user is located at a listening position that is in front of and 30 cm or less from the first radiating surface 214a(i) of the diaphragm 210a of the first dipole loudspeaker 200a, and such that a second ear 298a′ of the user is located at a listening position that is in front of and 30 cm or less from the first radiating surface 214a(i)′ of the diaphragm 210a′ of the second dipole loudspeaker 200a′.

[0122] Each dipole loudspeaker 200a, 200a′ also includes a frame 240a, 240a′, wherein the diaphragm 210a, 210a′ is suspended from the frame 240a, 240a′ via one or more suspension elements 241a, 241a′ wherein the frame 240a, 240a′ is configured to allow sound produced by the first radiating surface 214a(i), 214a(i)′ to propagate out from a first side of the dipole loudspeaker 200a, 200a′ and to allow sound produced by the second radiating surface 214a(ii), 214a(ii)′ to propagate out from a second side of the dipole loudspeaker 200a, 200a′. Thus there is no enclosure configured to capture sound from one of the two radiating surfaces (as in a monopole loudspeaker). In this example, each frame 240a, 240′ is a perforated frame to further help bass and mid-high frequency sound to pass therethrough substantially unimpeded.

[0123] The frame 240a of the first dipole loudspeaker 200a is fixedly attached to a frame 292a of the seat headrest 290a. The frame 292a of the seat headrest 290a is itself part of a rigid seat frame of the seat of which the seat headrest 290a is a part, with the frame 292a of the seat headrest 290a being rigidly connected to the remainder of the rigid seat frame via mounting pins 294a, 294a′.

[0124] The rigid seat frame can be considered the “application”. Reference herein to the “application” in relation to a given loudspeaker is intended to refer to an external apparatus to which a loudspeaker described herein is attached to (preferably rigidly attached to, though this need not always be the case, see e.g. FIG. 4B discussed below).

[0125] Each dipole loudspeaker 200a, 200a′ also includes a supplementary loudspeaker 250a, 250b, which is preferably a mid-high frequency loudspeaker. Thus the (composite) loudspeaker is able to produce sound over a full audio frequency range (i.e. a range that includes including bass, mid and high frequencies).

[0126] The diaphragm 210a of the first dipole loudspeaker 200a includes a layer 212a of porous material mounted on a supporting structure 220a, which in this case is a perforated sheet 220a, holes in which are configured to permit airflow through the entire layer 212a of porous material from the first radiating surface 214a(i) to the second radiating surface 214a(ii) of the diaphragm 210a.

[0127] In this example, the entirety of the layer 212a of porous material has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m, and thus can be viewed as a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0128] The drive unit 230a is rigidly mounted to the frame 240a, and has the supplementary mid-high frequency loudspeaker 250a mounted therein. In this case, the drive unit 230a and diaphragm 210a are essentially the same as those described with reference to FIG. 6D below, and thus does not need to be described further here.

[0129] The supplementary mid-high frequency loudspeaker 250a is configured to produce sound which propagates through a part of the layer 212a porous material that airflow is permitted to flow through.

[0130] We now move on to consider the second dipole loudspeaker 200a′.

[0131] In the second dipole loudspeaker 200a′, a proximal end 211a(i)′ of the diaphragm 210a′ is suspended from the frame 240a′ by at least one proximal suspension element 241a′, which here is a rigid clamp. The rigid clamp 241a′ is an extension of the material of the frame 292a. The rigid clamp 241a′ clamps the proximal end 211a(i)′ of the diaphragm 210a′ and is configured to substantially prevent translational and rotational movement of the proximal end 211a(i)′ of the diaphragm 210a′ relative to the frame 240a′, whilst permitting translational movement of a distal end 211a(ii)′ of the diaphragm 210a′ which is opposite to the 211a(i)′ of the diaphragm 210a′. The drive unit 230a′ is configured to move the distal end 211a(ii)′ of the diaphragm 210a′.

[0132] The diaphragm 210a′ is thus suspended as a cantilever, and the loudspeaker 200a′ may thus be referred to as having a “cantilever” diaphragm. Note that a local corrugation 213a′ in the diaphragm 210a′ is used for voice coil placement, improving packaging, and optimizing the trajectory path of the voice coil, thereby minimizing the air gap width.

[0133] If the clamp 241a′ were configured to substantially prevent translational movement of the proximal end 211a(i)′ of the diaphragm 210a′ relative to the frame 240a′ whilst allowing rotational movement thereof (not shown), then the diaphragm 210a′ could be referred to as a “hinged” diaphragm.

[0134] Loudspeakers incorporating cantilever and hinged diaphragms, and the benefits thereof (e.g. reduced rub and buzz harmonic distortion), are described in more detail in GB1907267.7.

[0135] The diaphragm 210a′ of the second dipole loudspeaker 200a′ includes a layer 212a′ of porous material mounted on a supporting structure 220a′.

[0136] In this example, the entirety of the layer 212a′ of porous material has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m, and thus can be viewed as a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0137] In this example, the supporting structure 220a′ is a perforated sheet. This perforated sheet 220a′ has holes only in part of the perforated sheet 220a′, and thus the perforated sheet 220a is only configured to permit airflow through only part of the layer 212a′ of porous material (this part being the part of the layer 212a′ that covers the part of the perforated sheet 220a that includes holes, at the distal end 211a(ii)′ of the diaphragm 210a′).

[0138] The supplementary mid-high frequency loudspeaker 250a′ is configured to produce sound which propagates through a part of the layer 212a′ porous material that airflow is permitted to flow through.

[0139] FIG. 4B shows a seat headrest 290b (again, in this example, a car seat headrest) in which a dipole loudspeaker 200b according to the present invention is mounted.

[0140] Alike features described in relation to previous drawings have been given alike reference numerals.

[0141] The second loudspeaker 200b incorporates some of the principles described in more detail in PCT/EP2018/084636.

[0142] Here, the diaphragm 210b of the loudspeaker 200b is suspended from the frame 240b of the loudspeaker 200b by one or more primary suspension elements 241b (in this case two roll suspensions), and the frame 240b of the loudspeaker 200b is suspended from the frame 292b of the seat headrest 290b by a one or more secondary suspension elements 293b (in this case two roll suspensions).

[0143] The drive unit 230b of the loudspeaker 210b is attached to the frame 240b of the loudspeaker 200b.

[0144] The drive unit 230b is an electromagnetic drive unit that includes a magnet unit 232b that is configured to produce a magnetic field, and a voice coil (not shown) attached to the diaphragm 210b via a voice coil coupler 234b, which includes a voice coil former 235b

[0145] The frame 240b of the dipole loudspeaker 200b includes rigid supporting arms 240b-1 configured to hold the magnet unit 232b in front of a second radiating surface 214b(ii) of the diaphragm 210b.

[0146] In this example, the voice coil coupler 234b is an element which attaches the voice coil to the second radiating surface 214b(ii) of the diaphragm 210b. In this example, the voice coil coupler 234b is glued to both the voice coil and the diaphragm 210b, and includes lots of holes to allow airflow. The voice coil coupler 234b may be configured to prevent the magnet unit 232b from passing through diaphragm 210b in the event of a crash. The voice coil coupler 234b may be made e.g. of plastic.

[0147] The one or more secondary suspension elements 293b are preferably tuned to have a resonant frequency below the frequency of operation of the loudspeaker dipole 200b, thereby helping to reduce vibrations from reaching the frame 292b of the seat headrest 290b, and thus the frame of the seat to which the frame 292b of the seat headrest 290b is rigidly attached.

[0148] In this example, the diaphragm 210b is an unsupported layer 212b of porous material. That is, the porous material forming the layer 212b and the thickness of the layer 212b are chosen such that the layer 212b can be used as a diaphragm without the need to be mounted on a supporting structure (hence the use of a voice coil coupler 234b to prevent the magnet unit 232b from passing through diaphragm 210b in the event of a crash).

[0149] Moreover, the porous material forming the layer 212b, and the thickness of the layer 212b are chosen such that the layer 212b has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0150] Thus, in this example, the entirety of the layer 212b of porous material can be viewed as a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0151] The porous material used for the layer 212b may be foamed silica, foamed aluminium, or any other perforated solid having the required properties.

[0152] FIGS. 5A-D show additional dipole loudspeakers 300a-d according to the present invention.

[0153] Each dipole loudspeaker 300a-d is a bass loudspeaker for producing sound at bass frequencies.

[0154] Alike features described in relation to previous drawings have been given alike reference numerals.

[0155] Each drive unit 330a-d includes both a magnet assembly and a coil assembly.

[0156] The magnet assembly includes a magnet unit 330a-d configured to provide a magnetic field in an air gap, wherein the air gap extends around a movement axis of the drive unit (wherein the drive unit is configured to move the diaphragm in a direction parallel to the movement axis).

[0157] The coil assembly includes: an attachment portion 336a-d which provide an attachment between the coil assembly and the diaphragm; a voice coil 337a-d; a voice coil former 338a-d which extends from the attachment portion into the air gap, wherein the voice coil is mounted to the voice coil former so that the voice coil sits in the air gap when the diaphragm 310a-d is at rest; a tubular member 339a-d, which is positioned radially outwardly of the voice coil former with respect to the movement axis, and which overlaps the voice coil former along at least a portion of the movement axis.

[0158] Each drive unit also includes two suspension elements 341a-d attached to the tubular member 339a-d and a part of the magnet assembly (in this case a frame 340a-d rigidly connected to the magnet unit 330a-d) positioned radially outwardly of the tubular member. The diaphragm 310a-d is thus suspended from the magnet assembly via the two suspension elements 341a-d and the coil assembly.

[0159] For each dipole loudspeaker 300a-d, the diaphragm 310a-d includes a layer 312a-d of porous material mounted on a supporting structure 320a-d, which in this case is a perforated sheet 320a-d, holes in which are configured to permit airflow through the entire layer 312a-d of porous material from the first radiating surface 314a-d(i) to the second radiating surface 314a-d(ii) of the diaphragm 310a-d.

[0160] In each example, the entirety of the layer 312a-d of porous material has a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m, and thus can be viewed as a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m.

[0161] For the dipole loudspeakers 300a-c shown in FIGS. 5A-C, the diaphragm is planar and the layer 312a-c of porous material has a uniform thickness.

[0162] For the dipole loudspeaker 300d shown in FIG. 5D, the diaphragm 310d is curved to improve geometric stiffness, and the layer 312c of porous material has a non-uniform thickness, and thus the layer 312c has variable specific airflow resistance across the layer 312c. Nonetheless, the continually varying thickness of the layer 312c is preferably chosen such that the specific airflow resistance of the layer 312c (or at least part of the layer 312c) is in the range 5-5000 Pa.Math.s/m, more preferably in the range 50-500 Pa.Math.s/m, if measured in accordance with ISO 9053, e.g. as discussed below (under “Airflow resistance measurements”).

[0163] The dipole loudspeaker 300a shown in FIG. 5A does not include a supplementary loudspeaker.

[0164] The dipole loudspeaker 300b shown in FIG. 5B has a supplementary (mid-high frequency) loudspeaker 350b mounted adjacent to the drive unit 330b.

[0165] The dipole loudspeakers 300c, 300d shown in FIGS. 5C-D each have a supplementary (mid-high frequency) loudspeaker 350c, 350d mounted in the drive unit 330c, 330d.

[0166] In all cases where a supplementary loudspeaker 350b, 350b, 350d is present, the supplementary loudspeaker 350b, 350b, 350d is configured to produce sound which propagates through a part of the layer 312b, 312c, 312d of porous material that airflow is permitted to flow through.

[0167] We note there that the drive units 330a-d of the dipole loudspeakers 300a-d are constructed in a similar manner to the inertial exciters described in PCT/EP2019/084950. However, unlike the inertial exciters described in PCT/EP2019/084950 (in which a magnet assembly is suspended from a diaphragm via a coil assembly by at least one suspension), the drive units 330a-d shown in FIGS. 5A-D are grounded, i.e. intended to be rigidly attached to a frame of the dipole loudspeaker 300a-d (the frame of the dipole loudspeaker 300a-d is not shown here but is shown for example in FIG. 4A, where a magnet assembly of the drive unit 230a is rigidly attached to the frame 240a of the first dipole loudspeaker 200a). Thus for the drive units 330a-d shown in FIGS. 5A-D, a diaphragm 310a-d is suspended from a magnet assembly of the drive unit 330a-d, rather than the magnet assembly of the drive unit 330a-d being suspended from a diaphragm 310a-d.

[0168] Drive units 330a-d having the constructions shown in FIGS. 5A-D are advantageous because they help to provide stable pistonic movement of the diaphragm 310a-d and reduce rocking of the diaphragm 310a-d when the magnet assembly is rigidly attached to an external body (e.g. frame), i.e. when the drive unit 330a-d is “grounded”.

[0169] Also, drive units 330a-d having the constructions shown in FIGS. 5A-D drive are particularly well suited for use in a dipole loudspeaker because they obstruct a smaller area of the second radiating surface 314a-d(ii) of the diaphragm 310a-d compared with other drive unit constructions.

Experiments

Experiment 1

[0170] FIG. 6A shows an experimental apparatus used for experiment 1.

[0171] The experimental apparatus included a diaphragm 410 that included includes a layer 412 of porous material mounted on a supporting structure 420.

[0172] The layer 412 of porous material was 10 mm Basotect open cell foam.

[0173] Basotect is a trademark from BASF and is an open cell melamine foam with a well-defined flow resistivity of approximately 10 kPa.Math.s/m.sup.2. Therefore, it is often used as a reference open cell foam.

[0174] The supporting structure 420 was a 2 mm thick aluminium perforated plate having circular holes of diameter 5 mm arranged with a distance of 8 mm centre to centre (see inset circle). The aluminium plate was 32 cm in length, 20 cm wide, and was excited at a nodal line 25 cm from its base via a voice coil 437 mounted to a voice coil former 438 attached to the with a grounded magnet unit 432.

[0175] Note: the perforated plate used here is so open in structure, its specific airflow resistance that is close to zero, and therefore it allows airflow through substantially the entire layer 412 of porous material.

[0176] The diaphragm 410 was driven using by supplying the voice coil 437 with an electrical signal via a lead wire (note that the lead wire can be conveniently attached to the supporting structure 420), and the resulting SPL measured by the microphone 403.

[0177] Measurements were performed in the following conditions: [0178] A. With the layer 412 of porous material absent, and with the perforations in the perforated plate 420 left open (“open plate”) [0179] B. With the layer 412 of porous material absent, and with the perforations in the perforated plate 420 taped over with tape (“closed plate”) [0180] C. With the layer 412 of porous material present, and with the perforations in the perforated plate 420 left open (“foam+open plate”)

[0181] FIG. 6B shows the results of these measurements, with the lines being labelled A, B, C to correspond to the above descriptions.

[0182] What this shows is that the diaphragm 410 as shown in FIG. 6A (“foam+open plate”=line C in FIG. 6B) is able to produce SPL at bass frequencies which closely approximate the SPL produced by a non-porous diaphragm (“closed plate”=line B in FIG. 6B), making it well suited for producing a personal sound cocoon at bass frequencies (e.g. when mounted in a car headrest).

[0183] This is also illustrated by FIG. 6C, which shows the measured difference (“delta”) between the SPL produced by the “closed plate” and the “foam+open plate” configurations up to 500 Hz (dotted line) as well as a line of best fit for these measurements (solid line). This figure shows that there is an attenuation of ˜2.5 dB at 100 Hz for the “foam+open plate” configuration compared with the “closed plate” configuration.

Experiment 2

[0184] FIG. 7 shows an experimental apparatus used for experiment 2.

[0185] The experimental apparatus used here is the same as for experiment 1, except that an additional supplementary mid-high frequency loudspeaker 450 was mounted to produce sound which propagates through a part of the layer 412 of porous material that airflow is permitted to flow through via the perforated plate 420.

[0186] In this experiment, the diaphragm 410 and supplementary loudspeaker 450 were used to play sound in the bass and mid-high frequencies (respectively), with a person locating their ear so that they could listen to sound produced by the mid-high frequency loudspeaker after this sound had propagated through the layer 412 of porous material (and the supporting structure 420) of the diaphragm 410.

[0187] The person listening to this sound reported that the sound was great, that the sound produced in the mid-high frequencies was perceived to be audibly non-affected by the diaphragm 410 and that this sound accompanied the bass output of the diaphragm 410 very well so that a full frequency range performance was achieved without the mid high frequency loudspeaker 450 being seen visually and without it occupying space that serves the production of low frequencies (which requires maximal possible surface area to achieve the required volume displacement to produce large enough SPL at low frequencies).

Experiment 3

[0188] FIG. 8A shows an experimental apparatus used for experiment 3.

[0189] The experimental apparatus used here is the same as for experiment 1, except that a different perforated plate 420′ was used, as shown by the inset rectangle. Here the perforated plate 420′ used was 3 mm thick hardboard with irregularly spaced circular holes having a 55 mm diameter.

[0190] The plate was again 32 cm in length and 20 cm wide.

[0191] Note: the perforated plate 420′ used here allows airflow through the parts of the layer 412 of porous material located over and close to the holes, though there may be some parts of the layer 412 (e.g. which are located far away from the holes) through which airflow is not permitted by the perforated plate 420′.

[0192] The use of perforated plate 420′ was intended to demonstrate that a perforated plate with densely packed small holes are not required to obtain good results, and that good results can still be obtained with very large holes that provide little support and which are unevenly distributed.

[0193] In this experiment, measurements were performed with the layer 412 of porous material present, and with the perforations in the perforated plate 420 left open (“foam+open plate”), with the layer 412 of Basotect open cell foam having different thicknesses, including: [0194] A. 10 mm (equating to a specific airflow resistance of around 100 Pa.Math.s/m) [0195] B. 5 mm (equating to a specific airflow resistance of around 50 Pa.Math.s/m) [0196] C. 2.5 mm (equating to a specific airflow resistance of around 25 Pa.Math.s/m)

[0197] Measurements were also performed: [0198] D. With the layer 412 of porous material absent, and with the perforations in the perforated plate 420 left open (“open plate”) [0199] E. With the layer 412 of porous material absent, and with the perforations in the perforated plate 420 taped over with tape (“closed plate”)

[0200] FIG. 8B shows the results of these measurements, with the y-axis indicating the difference in SPL between the “foam+open plate” configuration and the “closed plate” configuration at 30 Hz (solid line) and at 100 Hz (broken line), and with the x-axis indicating the specific airflow resistance of the layer 412 of porous material used in the “foam+open plate” configuration.

[0201] This graph shows that increasing specific airflow resistance of the layer 412 of foam results in performance at bass frequencies which gets closer to that of a “closed plate”, but that crucially, adequate SPL levels can be produced with relatively low values of specific airflow resistance. For example, a specific airflow resistance of 50 Pa.Math.s/m can achieve near “closed plate” performance at 30 Hz.

Experiment 4

[0202] FIG. 9A shows an experimental apparatus used for experiment 4.

[0203] The experimental apparatus used here is the same as for experiment 3, except that an additional supplementary mid-high frequency loudspeaker 450 was mounted behind a hole in the perforated plate 420′ so that sound produced by the supplementary loudspeaker 450 propagates through part of the layer 412 of porous material that airflow is permitted to flow through via the hole in the perforated plate 420′.

[0204] The microphone was here mounted at 10 cm from supplementary loudspeaker 450.

[0205] Measurements were performed in the following conditions: [0206] A. With the layer 412 of porous material absent, and with the perforations in the perforated plate 420 left open (“open plate”) [0207] B. With the layer 412 of porous material present (5 mm thick Basotect), and with the perforations in the perforated plate 420 left open (“foam+open plate”)

[0208] FIG. 9B shows the results of these measurements, with the lines being labelled A, B to correspond to the above descriptions.

[0209] What this shows is that a SPL performance in mid-high frequencies, whilst being attenuated by a small amount, is not significantly affected by the presence of the layer 412 of porous material, noting that whilst SPL is slightly decreased when the layer 412 of porous material is present (solid line B), the attenuation is roughly the same across all frequencies, and thus a user's listening experience would not be badly affected.

[0210] A skilled person would appreciate that the extent of attenuation caused by the layer 412 of porous material would dependent on the thickness of this layer and the material chosen (see below discussion relating to measuring specific airflow resistance).

[0211] It can be seen from experiments 1-4 that there is a balance between making the specific airflow resistance of the layer 412 thick enough to produce adequately high SPL levels at bass frequencies, but not so thick that performance of the loudspeaker is compromised (either by making the diaphragm too heavy, or by causing too much attenuation of mid-high frequencies of a supplementary loudspeaker, if present).

Airflow Resistance Measurements

Measurement Technique

[0212] SO 9053 sets out standard methods (Method A or Method B) for conducting airflow measurements to measure Airflow Resistance—R [Pa.Math.s/m.sup.3], Specific Airflow Resistance—Rs [Pa.Math.s/m], and Airflow Resistivity—r [Pa.Math.s/m.sup.2] for a material sample having a given surface area (S) and thickness (t).

[0213] FIG. 10A shows an experimental apparatus that can be used to perform measurements in accordance with ISO 9053 (Method B). Here, a material sample having surface area (S) and uniform thickness (t) is placed between two rigid supports (dashed lines) that are very open (at least 50%) and that therefore have negligible specific airflow resistance.

[0214] In accordance with ISO 9053, Airflow Resistance—R [Pa.Math.s/m.sup.3]—of a material sample gives an actual measured material sample flow resistance that is dependent on the surface area (S) of the sample.

[0215] Using the experimental apparatus shown in FIG. 10A and in accordance with ISO 9053 (Method B), a value of R can be obtained using the relation:

[00001]$\begin{array}{cc}R=\frac{\Delta p}{q_V}& \left[1\right]\end{array}$

Where Δp is pressure difference across the sample [Pa] and qv is volumetric airflow rate [m.sup.3/s].

[0216] Specific Airflow Resistance—Rs [Pa.Math.s/m]—of a material sample gives an indication of sample flow resistance that is independent of the surface area (S).

[0217] In accordance with ISO 9053, a value of Rs can be obtained by multiplying R by the surface area of the measured sample [m.sup.2]:

[00002]$\begin{array}{cc}{R}_S=R.Math.S& \left[2\right]\end{array}$

[0218] Airflow Resistivity—r [Pa.Math.s/m.sup.2]—of a material sample gives an indication of sample flow resistance that is independent of the surface area (S) and thickness (t).

[0219] In accordance with ISO 9053 is obtained by dividing Rs by the thickness t [m] of the sample:

[00003]$\begin{array}{cc}r=\frac{R_s}{t}& \left[3\right]\end{array}$

[0220] The present disclosure sometimes makes reference to a region of porous material having a specific airflow resistance in a defined range of values (e.g. a region of porous material having a specific airflow resistance in the range 5-5000 Pa.Math.s/m).

[0221] This region of porous material may be the entirety of, or a part of, a layer of porous material.

[0222] If a region of porous material (that is the entirety of, or a part of, a layer of porous material) has a uniform thickness t in a thickness direction (where the thickness direction may be taken as being locally perpendicular to the surface of the layer), then the specific airflow resistance of that region may be straightforwardly be calculated using the equation [3], rewritten as:

[00004]$\begin{array}{cc}{R}_s=r.Math.t& \left[4\right]\end{array}$

[0223] However, as can be seen from FIGS. 4A and 5D, a layer of porous material may have a non-uniform thickness, e.g. the thickness of the layer of porous material may continuously vary across the surface of the layer of porous material.

[0224] If a region of porous material (that is the entirety of, or a part of, a layer of porous material) has a non-uniform thickness t in a thickness direction (where the thickness direction may be taken as being locally perpendicular to the surface of the layer), then a maximum thickness t.sub.max of the layer and a minimum thickness t.sub.max of the layer in that region should be obtained, and a maximum and minimum value of the specific airflow resistance are obtained by inserting the values of t.sub.max, t.sub.min in equation [4]. If these maximum and minimum values of specific airflow resistance fall within the defined range of values (e.g. 5-5000 Pa.Math.s/m), then the region of porous material can be deemed to have a specific airflow resistance falling within the defined range of values.

[0225] Similarly, if a region of porous material (that is the entirety of, or a part of, a layer of porous material) has a non-uniform resistivity r, then an average value of the resistivity (e.g. averaged over the volume of the material in the region) should be used to determine whether the region of porous material has a specific airflow resistance falling within the defined range of values.

[0226] Note, if the region of porous material is defined as being only a part of a layer of porous material, then the part of the layer of porous material should include the full extent of the porous material in a thickness direction of the layer. In other words, the region of porous material should not be defined to include only part of a layer of porous material in a thickness direction of the layer.

Measurement Results

[0227] The following tables set out airflow measurement results obtained by the inventors in accordance with ISO 9053 (Method B) with a surface area (S) of 72 cm.sup.2 for Basotect foam (Table 1) and polyurethane (“PU”) foam (Table 2):

TABLE-US-00001 TABLE 1 Airflow measurements results for Basotect foam (10 kg/m3) Thickness [mm] 5.5 10 20 32 50 Flow Resistance [Pa .Math. 9046 12961 29607 44985 70615 s/m.sup.3] Specific Flow Resistance 65 94 214 326 511 [Pa .Math. s/m] Flow Resistivity [Pa .Math. 11905 9382 10715 10175 10223 s/m.sup.2]

TABLE-US-00002 TABLE 2 Airflow measurements results for PU foam (65 kg/m3) Thickness [mm] 6.5 15 Flow Resistance [Pa .Math. s/m.sup.3] 17336 38421 Specific Flow Resistance [Pa .Math. s/m] 125 278 Flow Resistivity [Pa .Math. s/m.sup.2] 19304 18540

[0228] FIG. 10B shows a plot of these measurements, and demonstrates that specific airflow resistance is linearly related to thickness for a given material, noting that an Rs of 100 Pa.Math.s/m can be achieved using only 5 mm thickness of the example PU foam used here, as compared to 10 mm thickness of Basotect (albeit PU foam is considerably heavier).

[0229] These results show that a thicknesses of Basotect in the range ˜5 mm to ˜50 mm, and thicknesses of PU in the range ˜2.5 mm to ˜25 mm might be useful to obtain a specific airflow resistance in the range 5-5000 Pa.Math.s/m.

[0230] 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.

[0231] 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.

[0232] 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.

[0233] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0234] 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.

[0235] 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%.

REFERENCES

[0236] A number of documents including patent applications 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, and any applications which claim priority to them, is incorporated herein. [0237] PCT/EP2018/084636 [0238] PCT/EP2019/056109 [0239] PCT/EP2019/056352 [0240] PCT/EP2019/084950 [0241] GB1907267.7 [0242] ISO 9053-1:2018 published October 2018