Loudpseakers

Abstract

A loudspeaker comprising two acoustic diaphragms mounted to face in axially-opposed directions, two voice coils each having an axis and an axial length and being configured to reciprocate along its axis to drive one of the diaphragms, the axes being substantially parallel and both axes passing through both diaphragms, and at least one magnet forming part of a chassis assembly configured to provide two axially-extending gaps, one for each of the voice coils to reciprocate within, wherein the at least one magnet and the chassis assembly are adapted so that magnetic flux flows across the gaps in opposite directions, and wherein when in use the diaphragms are at their predetermined maximum negative excursions the voice coils overlap in the axial direction by between 10% and 90% of their average axial length, and wherein when in use the diaphragms are in a relaxed position, between their maximum negative and positive excursions, the voice coils do not overlap in the axial direction.

Claims

1. A loudspeaker comprising two acoustic diaphragms mounted to face in axially-opposed directions, two voice coils each having an axis and an axial length and being configured to reciprocate along its axis to drive one of the diaphragms, the axes being substantially parallel and both axes passing through both diaphragms, and at least one magnet forming part of a chassis assembly configured to provide two axially-extending gaps, one for each of the voice coils to reciprocate within, wherein the at least one magnet and the chassis assembly are adapted so that magnetic flux flows across the gaps in opposite directions, wherein the chassis assembly further comprises a spacer formed of a non-magnetic, conductive material shaped so as to extend axially and positioned so as to separate the two axially-extending magnetic gaps and wherein when in use the diaphragms are at their predetermined maximum negative excursions the voice coils overlap in the axial direction by between 50% and 90% of their average axial length, and wherein when in use the diaphragms are in a relaxed position, between their maximum negative and positive excursions, the voice coils do not overlap in the axial direction.

2. The loudspeaker according to claim 1 in which the axes are coaxial.

3. The loudspeaker according to claim 1 in which the two voice coils have the same axial length.

4. The loudspeaker according to claim 1 in which the mass of the diaphragm facing in one direction and the voice coil associated therewith is substantially the same as the mass of the diaphragm facing in the other direction and the voice coil associated therewith.

5. The loudspeaker according to claim 1 comprising a single magnet.

6. The loudspeaker according to claim 5 in which the magnet is shaped as a closed loop and extends axially so as to surround the two axially-extending magnetic gaps.

7. The loudspeaker according to claim 5 in which the magnet extends axially and is surrounded by the two axially-extending magnetic gaps.

8. The loudspeaker according to claim 1 comprising at least two magnets.

9. The loudspeaker according to claim 1 in which the chassis assembly comprises a yoke.

10. The loudspeaker according to claim 1 in which when in use the diaphragms move between their relaxed positions and their predetermined maximum negative excursions the voice coils do not overlap in the axial direction for the first 50% of that movement.

11. The loudspeaker according to claim 10 in which the voice coils do not overlap in the axial direction for the first 30% of their movement between the relaxed positions and their predetermined maximum negative excursions.

12. The loudspeaker according to claim 1 in which the relaxed position of one or both voice coils is located midway between its/their maximum negative and positive excursions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described by way of example and with reference to the accompanying figures, in which;

(2) FIG. 1 is a schematic cross-sectional view of a conventional ring magnet loudspeaker drive unit;

(3) FIG. 2 is a diagram showing approximate motor strength as a function of voice coil displacement;

(4) FIG. 3 is a cross-sectional schematic view of an embodiment of a mirrored coaxial array in accordance with the invention;

(5) FIG. 4 is another view of the mirrored coaxial array of FIG. 3 in use and illustrating the maximum overlap of the voice coils, with the voice coils at maximum negative excursion, and

(6) FIGS. 5(a) to 5(c) are schematic illustrations of alternative embodiments of mirrored coaxial arrays.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) FIG. 1 shows a conventional over-hung ring-magnet motor system 1. Normally two of these are placed back to back when used in a reaction-force cancelling/vibration-cancelling arrangement (as described in US 2014/211963). An annular magnet 2 surrounds a steel yoke 4 which is in the form of a central cylinder 6 with front and rear end plates 8, 10. There is a magnetic gap formed by a circular hole 12 in the front end plate 8, and the hole 12 leads directly to an axially-extending gap 14 between the magnet 2 and the cylindrical part 6 of the yoke 4. A voice coil 16 carrying a varying electric current reciprocates in the magnetic gap 12. The voice coil 16 is mounted at its outer end (the upper end as shown in the drawing) to a diaphragm (not shown) and the reciprocation of the voice coil causes the diaphragm to vibrate, creating acoustic waves as is well-known in the art. In use, the voice coil moves between a negative excursion (when the voice coil is displaced downwardly in the drawing) into the hole 12 and a positive excursion (when the voice coil is displaced upwardly in the drawing) out of the hole 12.

(8) The coil length L1 and the thickness of the plate determine the maximum excursion of the motor system. The force generated by the motor system for each unit electrical current flowing in the coil (the BL) is constant when the coil is fully inside the gap. When the coil offset is ½ L1 the BL will drop to around 50% and this is typically the approximate maximum excursion (E1) of the motor system.

(9) The total motor-system height (H1) is
H1=BP1+FP1+C1

(10) C1 is the distance between the gap and the yoke into which for the voice coil can move during usage. According to FIG. 2 the motor strength drops to a value close to zero if the voice coil moves completely out of the gap. However, in practice the voice coil is connected to a dynamic mechanical system and mechanical inertia can cause the voice coil to travel beyond this extent. Therefore it is common practice to build in some extra clearance margin to ensure that collision never occurs

(11) $C1=L1\left(1+\frac{\mathrm{clearance}\mathrm{margin}\%}{100}\right)$

(12) A higher clearance margin provides a better guarantee that a collision won't occur, but this is at the expense of motor system compactness. Typical clearance margins are in the range of 10% to 50% depending on the application and required compactness of the loudspeaker driver.

(13) Very often the thickness (BP1) of the back plate 10 and the thickness (FP1) of the front plate 8 will be the same or very close to the same. This is because both plates carry the magnetic flux in a similar orientation and therefore will have similar saturation when they are the same thickness (balancing saturation against steel quantity is the key aspect of motor system cost and performance optimisation).

(14) Putting this all together, for two drivers placed back-to-back the total height of the two motor systems is approximately

(15) $\mathrm{total}\mathrm{height}=4\times \mathrm{plate}\mathrm{thicknes}+2\times \mathrm{coil}\mathrm{length}\left(1+\frac{\mathrm{clearance}\mathrm{margin}\%}{100}\right)$

(16) FIG. 3 shows an embodiment of a mirrored coaxial array 11 in accordance with the invention. In FIG. 3, two cylindrical voice coils 26, 28 with different diameters each drive a diaphragm D, and are coaxially located using a single ring magnet 22 to provide magnetic flux in two axially-extending gaps 46, 48. The drawing shows the voice coils 26, 28 in their “rest”, or relaxed position, where there is no axial overlap. A non-ferrous cylindrical spacer 30 is provided to locate the steel yoke 34 in the correct location, and the spacer 30 also separates the two gaps 46, 48. The spacer 30 should be conductive to reduce the inductance of the two voice coils 26, 28. In use, the voice coils 26, 28 reciprocate through magnetic gaps 42,44 in the front and back end plates 38, 40 and into and out of axial gaps 46, 48 between a maximum positive excursion (when the two coils are furthest apart axially, when the coils would be further apart than as shown in FIG. 3) and a maximum negative excursion (when the two coils are axially closest together, as shown in FIG. 4).

(17) Normally the aim is for the motor strength (BL) of both voice coils 26, 28 to be identical and also for the maximum excursion of both voice coils to be identical. Commonly the thickness FP1, FP2 of both end plates 38, 40 will be the same. The lengths L1, L2 of both coils 26, 28 will normally be the same. Under these conditions the clearances for the two coils will be the same. Under these conditions the total thickness (height in the drawing) of the dual motor system 11 is

(18) $\mathrm{total}\mathrm{height}=2\times \mathrm{plate}\mathrm{thicknes}+\mathrm{coil}\mathrm{length}\left(1+\frac{\mathrm{clearance}\mathrm{margin}\%}{100}\right)$

(19) i.e. half of the conventional motor system thickness/height.

(20) At maximum negative excursion both coils 26, 28 will be displaced by approximately ½L1+½FP as shown in FIG. 4 and the coils overlap by a significant margin OL. This situation is quite extreme, since the motor strength is almost zero with the coils in this location, but it can easily occur at high power input levels and particularly due to the inertia of the moving parts of the loudspeaker driver.

(21) Assuming that the coil lengths L1, L2 are the same, and the end plate thicknesses FP1, FP2 are the same, the overlap (OL) at this coil position is given by

(22) $\max \mathrm{overlap}=\mathrm{coil}\mathrm{length}\left(1-\frac{\mathrm{clearance}\mathrm{margin}\%}{100}\right)$

(23) From this it is obvious that the overlap can be expressed as a percentage of the voice coil length

(24) $\max \mathrm{overlap}\%=100\times \frac{\max \mathrm{overlap}}{\mathrm{coil}\mathrm{length}}=100-\mathrm{clearance}\mathrm{margin}\%$

(25) Given typical clearance margins, the maximum voice coil overlap is between 50% and 90%.

(26) Since one magnet ring 22 is used for both magnetic gaps 42, 44 it is usually necessary to use a large volume of magnet 22 than with a typical single motor system. In some cases this might mean that the clearance margin is greater than normal to allow the thickness of the magnet ring 22 to be as large as possible. This is clearly a balance between the motor-system strength and the motor-system thickness that the designer must fine tune. Even in this situation the maximum overlap of the coils would be significant and is likely to be at least 10% and probably more than 25%.

(27) The magnetic field orientation in the two magnetic gaps 42, 44 is opposite. Typically this motor system will be required to deliver the same force on both coils 26, 28 but in opposite directions in order to create a “reaction-force cancelling” arrangement and therefore it will be necessary to connect one of the coils in the reverse direction.

(28) It's advantageous if both coils 26, 28 have the same motor-strength. This is relatively easily achieved since both magnetic gaps 42, 44 are in a series magnetic connection and the same magnetic field passes through both. Since approximately the same magnetic flux passes radially through both magnetic gaps 42, 44, the magnetic flux density in each magnetic gap is approximately proportional to the voice coil diameter. Therefore the flux density experienced by the smaller diameter voice coil 28 will be higher. However, this effect is balanced by the lower coil circumference of the smaller diameter voice coil 28 and as a result it is fairly easy to achieve approximately the same motor-strength BL on both coils 26, 28 (particularly as there are many geometric and coil parameters that can be adjusted to minimise the differences).

(29) In some cases it may not be possible or desirable to achieve the same motor strength. In this case it might be advantageous to drive the two coils 26, 28 with different signals in order to still achieve approximate reaction-force cancellation.

(30) It is possible that this motor system arrangement may have advantages when not used in a reaction force cancelling mode, where there is no particular relationship between the signals in the two coils. In this case the compactness and the overlap of the voice coils may still be advantageous.

(31) FIG. 5(a) shows an alternative embodiment in which the two coils 56, 58 are separated by a spacer 50 as in the previous embodiment, but with a disc 52 of magnetic material located inside the smaller coil 58. FIG. 5(b) shows another embodiment with two magnets 62,64 separated by a spacer 60. FIG. 5(c) shows a less useful modification having a single, ring magnet 72 located between the two coils 76, 78; this version is less useful because the difference in the coil diameters must be greater than in the previous embodiments in order to make space for the ring magnet 72, because the aluminium spacer in the other embodiments is helpful to minimise voice coil inductance and reduce distortion but cannot be used in this embodiment, and because the two gaps 82, 84 are now located magnetically in parallel so it is likely to be more difficult to achieve the same magnetic flux in both.

(32) It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, the central cylindrical part of the yoke may be solid as shown in FIG. 3, or have an axial hole as shown in FIG. 1. The yoke is described as being made of steel, but any ferromagnetic material could be used, and the spacer is described as being made of aluminium, but any non-magnetic conductive metal or alloy could be used. The magnets can be of any suitable type or manufacture; the spacer could be a solid cylinder, it could be formed of segments fitted together, and/or it could have axially-extending apertures. The axially-extending gaps could contain a sound-absorbent material (such as an acoustic foam, a fabric, an open-cell foam, and a closed-cell foam or other porous material) to reduce resonance, as we described in our earlier application, GB2567673.

(33) Where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.