This application claims priority to GB2009203.7, filed 17 June 2020.

Field of the Invention

The present invention relates to a loudspeaker which includes a drive unit. Corresponding methods are also disclosed.

Background

Traditional loudspeakers, an example of which is shown in Fig. 1 (a), typically include an acoustic radiator, typically referred to as a diaphragm, suspended from a magnet assembly including a frame mounted in a baffle or loudspeaker enclosure. Sound is produced as a result of movement of the diaphragm, actuated by a voice coil attached to the diaphragm, which interacts with a magnet unit which is part of the magnet assembly including the frame. The baffle or loudspeaker enclosure acts to inhibit cancellation between sound produced by the front and rear faces of the diaphragm.

Inertial exciters, an example of which is shown in Fig. 1 (b), typically are devices in which a magnet assembly is configured to be suspended from an acoustic radiator such as a panel or soundboard, and which are configured to apply inertial force to the acoustic radiator so as to cause the acoustic radiator to vibrate to produce sound. Inertial exciters are typically used in automotive, aviation and consumer products.

Loudspeakers incorporating inertial exciters are well known, with examples being disclosed in, for example [1 ]-[12].

Inertial exciters are capable of transmitting a wide bandwidth of mechanical vibration energy into acoustic radiators, typically panels or walls that are configured to sustain that vibration energy across their surface to produce acoustic output. For a loudspeaker incorporating an inertial exciter, the frequency spectrum of interest (the frequency spectrum across which the loudspeaker is able to produce sound) may be the audible range (20Hz-20kHz).

In order to produce sound over a wide bandwidth, inertial exciters typically need to have a coil assembly (the part of the inertial exciter that includes the voice coil) that has a low mass and is very stiff so as to maximize the efficiency across the audio bandwidth. Whereas the magnet assembly (the part of the inertial exciter that includes the magnet system) can have a much higher mass (and generally will have a higher mass in practice).

The mechanical fixation of the inertial exciter to the acoustic panel requires special attention: when one wants to make use of moving coil (MC) excitation combined with moving magnet (MM) excitation (these types of excitation are discussed in more detail below), ideally the exciter is mounted to the acoustic radiator only via the coupler, i.e. with the magnet assembly being suspended from the acoustic radiator via the coil assembly, thereby leaving the magnet assembly freely suspended. Fig. 2(a) shows a loudspeaker 1 incorporating a wide bandwidth inertial exciter implementing principles derived from the prior art. Fig. 2(b) is a graph showing force level vs frequency for the loudspeaker shown in Fig. 2(a).

In this example, a magnet assembly 2 including a magnet unit 10 and a frame 12 is suspended from an acoustic radiator 90 via a coil assembly including a voice coil 30 and a voice coil former 32. The voice coil 30 sits in an air gap 16 of the magnet unit 10 when the exciter 1 is at rest

The voice coil will generate a force F according to:

F = BLI where B is magnetic field, L is wire length and / is electric current (standard units).

The inertia of the magnet assembly 2 (which is typically significantly heavier than the voice coil assembly 4) allows the voice coil assembly 4 to transmit vibrational energy to the acoustic radiator 90. Excitation of the acoustic radiator 90 caused by movement of the voice coil assembly is referred to herein as “moving coil” or“MC” excitation.

Where the magnet assembly 2 is suspended from the acoustic radiator 90 via the coil assembly 4 (as in the example shown in Fig. 2(a)), resonance of the magnet assembly 2 is able to give additional vibrational energy to the acoustic radiator 90 around the resonant frequency of the magnet assembly 2. The resonant frequency of the magnet assembly 2 is defined by the mass of the magnet assembly 2 and the compliance of the suspension 60 from which the magnet assembly 2 is suspended. Excitation of the acoustic radiator 90 caused by resonance of the magnet assembly 2 is referred to herein as “moving magnet” or “MM” excitation.

As shown in Fig. 2(b), MM excitation provides a force boost at low frequencies (labelled “MM” in Fig.

2(b)), which is an advantage of systems in which the exciter is mounted to the acoustic radiator only via the coupler, as in the example of Fig. 2(a).

The force level provided by MC excitation (labelled “MC” in Fig. 2(b)) is boosted by the voice coil having a low weight and being very stiff.

The present inventors have observed a problem with the loudspeaker illustrated in Fig. 2(a). This problem is illustrated by Fig. 2(c).

In detail, when mounting the acoustic panel 90 (to which the wide bandwidth inertial exciter 1 is attached) vertically, e.g. in an interior door panel of a car, the gravitational force on the magnet assembly 2 tends to rotate its position relative to the voice coil 4 assembly over time. This is due to the compliance of the single suspension 60 (in this case a spider) that is configured to position the voice coil 30 in the air gap 16 (and does this job very well), but is not configured to inhibit rotation of the magnet assembly 2 relative to the voice coil assembly 4 when the acoustic radiator 90 is vertically mounted, e.g. as may be the case in a car door.

The prior art teaches some possible solutions to this problem, some of which are summarized as follows: • Solution A as shown in Fig. 3(a)(i) and Fig. 3(a)(ii) (“Free magnet system”)

- Good MC & MM operation; Minimal additional mass for MC; Similar to [1];

- Problem: Motor mass on single suspension makes it unstable regarding buckling as depicted in Fig. 2(c)

• Solution B as shown in Fig. 3(b)(i) and Fig. 3(b)(ii) (“Grounded magnet system”)

- Stable magnet system; Similar to [13] and classic loudspeaker;

- Problem: Large bracket for large panels, No MM excitation benefit, Not an inertial exciter design

• Solution C as shown in Fig. 3(c)(i) and Fig. 3(c)(ii) (“Bracket to panel”)

- Stable magnet system; Similar to [6], [7], [11], [12]

- Problem: Influence of panel acoustics, No MM benefit

• Solution D as shown in Fig. 3(d)(i) and Fig. 3(d)(ii) (“Centrally suspended motor”)

- MC and MM excitation; Reasonably stable; Similar to [4]

- Problem: Additional mass for MC operation, Breakup of large coupler causes a step in force profile

• Solution E as shown in Fig. 3(e)(i) and Fig. 3(e)(ii) (“Double suspended motor”)

- MC and MM excitation; Stable motor suspension; Similar to [8], [9], [10]

- Problem: Additional mass for MC operation, Breakup of large coupler causes a step in force profile

• Solution F as shown in Fig. 3(f)(i) and Fig. 3(f)(ii) (“Shaker”)

- Only MM operation (for use as a shaker); Stable motor suspension

- Problem: Not a wide bandwidth

Solution F uses an inertial exciter as a shaker to transmit a small bandwidth of mechanical vibration energy into structures such as a seat in a car or in a cinema to augment the experience via tactile stimulus. Generally, the frequency spectrum in which this seems enjoyable is very limited, e.g. 30Hz - 80Hz. The design of shakers is less complicated as compared to acoustic exciters because they rely solely on the inertial vibration energy of the moving magnet system (MM) since their scope is to transfer only low frequency vibration. The fixation of such shaker to the panel is also less critical and may involve heavier constructions without compromising performance. Of course a wide bandwidth inertial exciter (with a freely suspended magnet system as in solutions A, D, E) can also be used solely as a shaker.

The present inventors have observed that it is difficult to make an inertial exciter that successfully inhibits rotation of the magnet assembly relative to the voice coil assembly whilst allowing MM excitation and without adding significant weight to the voice coil assembly. Thus, it is difficult to produce an inertial exciter having good sound reproduction over a wideband bandwidth, without encountering rotation issues when the acoustic radiator is mounted vertically, e.g. as might be the case in a car door.

A dipole loudspeaker for producing sound at bass frequencies is disclosed in [14] In some examples, the frame from which the diaphragm is suspended is a first frame, wherein the diaphragm is suspended from the first frame via one or more primary suspensions, and wherein the first frame is suspended from a second frame via one or more secondary suspensions. This arrangement may be useful to reduce vibrations passing from the loudspeaker into the environment.

The present inventors have observed that it is desirable to provide a small drive unit able to provide stable pistonic movement of an acoustic radiator without having problems caused by rocking of the acoustic radiator when the loudspeaker is in use

The present invention has been devised in light of the above considerations.

Summary of the Invention

A first aspect of the invention provides:

A loudspeaker comprising: a mounting frame; an acoustic radiator; a drive unit, including: a magnet assembly including a magnet unit configured to provide a magnetic field in an air gap, wherein the air gap extends around a movement axis of the inertial exciter; a coil assembly including: an attachment portion which provides an attachment between the coil assembly and the acoustic radiator; a voice coil; a voice coil former 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 drive unit is at rest; a tubular member, 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; at least one drive unit suspension attached to the tubular member and a part of the magnet assembly positioned radially outwardly of the tubular member so that the acoustic radiator is suspended from the magnet assembly via the coil assembly by the at least one drive unit suspension; wherein the magnet unit assembly is suspended from the mounting frame by at least one mounting frame suspension.

In the context of this application, the term “mounting frame” is intended only to distinguish a frame from which the magnet unit assembly is suspended (by at least one mounting frame suspension) from other frames disclosed herein (e.g. a “magnet assembly frame” as described below).

In the context of this application, the terms “drive unit” in “drive unit suspension” and “mounting frame” in “mounting frame suspension” is intended only to distinguish between a suspension used to suspend the acoustic radiator from the magnet assembly (a “drive unit suspension”) and a suspension used to suspend the magnet unit assembly from the mounting frame (a “mounting frame suspension”). The tubular member, by being positioned radially outwardly of the voice coil former (preferably also of the air gap) with respect to the movement axis, facilitates the attachment of the at least one drive unit suspension, preferably two drive unit suspensions, to the part of the magnet assembly positioned radially outwardly of the tubular member.

This is advantageous because it helps to provide stable pistonic movement of the acoustic radiator and reduces rocking of the acoustic radiator when the loudspeaker is in use.

By having the magnet unit assembly suspended from the mounting frame, vibrations passing from the loudspeaker into the environment can be reduced, e.g. in an analogous mannerto that described in [14]

The movement axis may be defined as an axis along which the voice coil assembly is configured to move relative to the magnet assembly when the drive unit is activated by supplying electrical current carrying an audio signal to the voice coil.

The drive unit may be considered to be at rest when electrical current is not supplied to the voice coil.

Note that in order for the acoustic radiator to be suspended from the magnet assembly via the coil assembly, the magnet assembly should only be attached to the acoustic radiator via the coil assembly, i.e. with no rigid attachment between the magnet assembly and the acoustic radiator.

Preferably, the drive unit comprises: a first drive unit suspension attached to the tubular member and the part of the magnet assembly positioned radially outwardly of the tubular member; and a second drive unit suspension, separated from the first drive unit suspension in a direction extending parallel to the movement axis, wherein the second drive unit suspension is either: attached to the tubular member and the part of the magnet assembly positioned radially outwardly of the tubular member or is attached to the voice coil former and a part of the magnet assembly positioned radially inwardly of the voice coil former.

The use of two drive unit suspensions, separated in the direction of the movement axis, helps to significantly reduce the rotation described above with respect to Fig. 2(c) and maintain good performance without substantially increasing the weight of the coil assembly, noting that the tubular member allows a large separation of the first and second drive unit suspensions, and also noting that the tubular member has an inherently stiff shape and so can be formed from lightweight material.

For a typical application, the distance between locations at which the two drive unit suspensions attach to the part of the magnet assembly positioned radially outwardly of the tubular member may be at least 3mm, more preferably at least 5mm, more preferably at least 6mm as measured in a direction extending parallel to the movement axis. A skilled person would appreciate that actual distances will vary in practice depending on various factors including weight of the magnet assembly (larger weight requires larger distance) and design limitations (e.g. space in aperture in which loudspeaker is to be installed). The magnet assembly may include a magnet assembly frame to which the magnet unit is attached, wherein the part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one drive unit suspension is attached) is a part of the magnet assembly frame.

The part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one drive unit suspension is attached) could, for example, be a rim of the magnet assembly frame.

The part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one drive unit suspension is attached) may include a respective ledge for the/each drive unit suspension attached to the part of the magnet assembly positioned radially outwardly of the tubular member, thereby facilitating attachment of the drive unit suspension(s) to the part of the magnet assembly positioned radially outwardly of the tubular member.

The magnet assembly frame (included in the magnet assembly) may include apertures configured to allow a jig to be inserted to centre the tubular member during assembly.

Some optional features of the drive unit described herein are described with reference to: a first plane perpendicular to the movement axis which extends through the attachment portion; a second plane perpendicular to the movement axis which extends through the air gap.

Features described with reference to the first and second planes are preferably described with respect to the drive unit when the drive unit is at rest. As noted above, the drive unit may be considered to be at rest when electrical current is not supplied to the voice coil.

The part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one drive unit suspension is attached) may include: a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially outwardly of the tubular member is located between the first plane and the second plane; and a distal portion, wherein the distal portion of the part of the magnet unit positioned radially outwardly of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially outwardly of the tubular member).

The magnet assembly may include a part of the magnet assembly positioned radially inwardly of the voice coil former, wherein the part of the magnet assembly position radially inwardly of the voice coil former includes: a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former is located between the first plane and the second plane; and a distal portion, wherein the distal portion of the part of the magnet unit positioned radially inwardly of the voice coil former is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially inwardly of the voice coil former).

The part of the magnet assembly positioned radially inwardly of the voice coil former may include part of the magnet unit. The proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former may for example include part of the magnet unit, e.g. an extra magnet 114a as shown in Fig. 4(a)(i). The distal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former may for example include part of the magnet unit, e.g. a main magnet 112a as shown in Fig. 4(a)(i).

The tubular member may include: a proximal portion, wherein the proximal portion of the tubular member is located between the first plane and the second plane; a distal portion, wherein the distal portion of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the tubular member).

The voice coil former may include: a proximal portion, wherein the proximal portion of the voice coil former is located between the first plane and the second plane; a distal portion, wherein the distal portion of the voice coil former is located on an opposite side of the second plane from the proximal portion (of the voice coil former).

Note that if the tubular member has the distal portion (as described above), this allows the tubular member to reach past the air gap on the outside of the magnet unit, and allows the first and second drive unit suspensions to be separated by a larger distance, compared with an arrangement in which the two drive unit suspensions are attached to the voice coil former.

Preferably, the first drive unit suspension is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially outwardly of the tubular member.

Preferably, the second drive unit suspension is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially outwardly of the tubular member.

However, the second drive unit suspension could potentially instead attach to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former, whilst still allowing a wide separation between the first and second drive unit suspensions, thereby still helping to reduce the rotation discussed above with reference to Fig. 2(c).

Preferably, the drive unit comprises both: a first drive unit suspension that is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially outwardly of the tubular member; and a second drive unit suspension that is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially outwardly of the tubular member.

This arrangement allows the first and second drive unit suspensions to have a particularly large space between them, which helps to reduce the rotation discussed above with reference to Fig. 2(c). In this arrangement, the drive unit may optionally include a third drive unit suspension that is attached to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former (e.g. as shown in Fig. 5(b)).

The tubular member preferably extends around the magnet unit.

The tubular member preferably overlaps the magnet unit along at least a portion of the movement axis.

The tubular member may be shaped to include the attachment portion, e.g. so as to facilitate direct gluing (or some other attachment) of the tubular member to the acoustic radiator.

The tubular member may be shaped to include the attachment portion and the voice coil former.

The tubular member may include or be attached to a surface extending outwardly in a radial direction (with respect to the movement axis) from the distal portion of the tubular member to provide a surface for attaching the tubular member to the first drive unit suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard.

The tubular member may include or be attached to a surface extending outwardly in a radial direction (with respect to the movement axis) from the proximal portion of the tubular member to provide a surface for attaching the tubular member to the second drive unit suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard.

The wall of the tubular member could form an angle with respect to the movement axis, e.g. so that the distal portion of the tubular member is further from the movement axis than the proximal portion of the tubular member, thereby forming a frusto-conical tubular member. In this case, the angle is preferably no more than 15°.

The tubular member could have one or more extensions in radially outward direction (with respect to the movement axis) to provide a respective attachment surface for the/each drive unit suspension attached to the tubular member, thereby facilitating attachment of the/each drive unit suspension to the tubular member.

The width of the drive unit in the radial direction (perpendicular to the movement axis) will generally depend on design requirements.

The drive unit may include one or more wires configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil.

The electrical path provided by the one or more wires may extend from a connector formed on the magnet assembly (e.g. on a frame of the magnet assembly) to the voice coil.

The one or more wires may include wire from the voice coil winding and/or a lead wire which connects to the voice coil winding. The one or more wires may include a wire that passes through or around the tubular member. A coupling element (if present - see below) may be configured to guide said wire through or around the tubular member.

The one or more wires may include a wire that passes through or around (preferably through) a frame included in the magnet assembly.

The one or more wires may include two wires that meet at an electrical junction formed on an outwardly facing surface of the tubular member, e.g. at a solderpad or glue dot on the outwardly facing surface of the tubular member.

The magnet unit is preferably configured to provide a magnetic field in an air gap. The voice coil former and/or the tubular member may be cylindrical. But other shapes of air gap, voice coil former and tubular member are possible, e.g. oval, square.

Preferably the voice coil former is arranged around the movement axis.

The voice coil former preferably extends from the attachment portion in a direction which extends along the movement axis into the air gap.

The tubular member and voice coil former are each preferably made from lightweight materials such as paper, cardboard, Kapton, aluminium, kevlar, PE, ABS etc.

The tubular member and voice coil former are preferably made of the same material as each other, but could be made of different materials.

The tubular member and voice coil former may be formed integrally with each other (preferably also the attachment portion).

Preferably the attachment portion is arranged around the movement axis.

The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including a gluing surface configured to be glued to the acoustic radiator.

The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including bayonet features (e.g. projections) configured to engage with corresponding bayonet features (e.g. slots) on the acoustic radiator to provide a bayonet attachment between the attachment portion and the acoustic radiator.

The attachment portion may be a coupling element which is separately attached to the voice coil former and/or tubular member, e.g. by glue.

The coupling element could be a ring-shaped element, e.g. a cardboard or plastic ring.

The coupling element is not an essential element of the invention, since the attachment portion could be formed integrally with the voice coil former and/or the tubular member. Or the voice coil and tubular member could be configured to attach independently (e.g. by glue) to the acoustic radiator, in which case the attachment portion could include the glue and part of the acoustic radiator.

The/each drive unit suspension could take various forms.

Preferably, the/each drive unit suspension includes one or more corrugations. A drive unit suspension including one corrugation is preferred in some examples.

The at least one drive unit suspension may include a spider. The/each drive unit suspension may be a spider.

The at least one drive unit suspension may include a roll drive unit suspension. The/each drive unit suspension may be a roll drive unit suspension.

The at least one drive unit suspension may include a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis. The/each drive unit suspension may be a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis.

A potential advantage of a sheet material drive unit suspension could be a reduced height (in the movement axis direction) compared with classic drive unit suspensions which typically require a corrugation to facilitate deflection in the movement axis direction.

If there are two drive unit suspensions, each drive unit suspension including one or more corrugations, then the one or more corrugations in one drive unit suspension may mirror the one or more corrugations in the other spider, e.g. with respect to a plane perpendicular to the movement axis, e.g. to help cancel asymmetries in stiffness.

The magnet unit may include a central main magnet and a U-yoke.

In use, electrical current carrying an audio signal may be supplied to the voice coil which energises the voice coil and causes a magnetic field to be produced by the current in the voice coil, which interacts with the magnetic field produced in the air gap by the magnet unit, and causes the voice coil assembly to move relative to the magnet assembly. This relative movement is accommodated by the at least one drive unit suspension.

A second aspect of the invention provides:

A loudspeaker comprising: a mounting frame; an acoustic radiator; a drive unit, including: a magnet assembly including a magnet unit configured to provide a magnetic field in an air gap, wherein the air gap extends around a movement axis of the exciter; a coil assembly including: an attachment portion which provides an attachment between the coil assembly and the acoustic radiator; a voice coil; a voice coil former 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 drive unit is at rest; a tubular member, which is positioned radially inwardly 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; at least one drive unit suspension attached to the tubular member and a part of the magnet assembly positioned radially inwardly of the tubular member so that the acoustic radiator is suspended from the magnet assembly via the coil assembly by the at least one drive unit suspension; wherein the magnet unit assembly is suspended from the mounting frame by at least one mounting frame suspension.

The loudspeaker provided by the second aspect of the invention is similar to that provided by the first aspect of the invention, and provides essentially the same benefits as the loudspeaker provided by the first aspect of the invention, but with the components arranged in a different order in the radial direction with respect to the movement axis.

The loudspeaker provided by the second aspect of the invention permits use of a ring-shaped magnet, allow more magnet material to be used compared with the inner magnet type examples, and therefore enable more powerful drive units (and thus loudspeakers), as may be desirable in some cases.

A loudspeaker according to the second aspect of the invention may thus incorporate any one or more features described in connection with loudspeaker according to the first aspect of the invention, but with the ordering and direction of certain elements in the drive unit being altered in the radial direction (with respect to the movement axis) in order to provide equivalent benefits. Similarly, definitions described above with respect to the first aspect of the invention may be used in connection with the first aspect of the invention.

Some example features of loudspeaker according to the second aspect of the invention will now be described.

The movement axis may be defined as an axis along which the voice coil assembly is configured to move relative to the magnet assembly when the drive unit is activated by supplying electrical current carrying an audio signal to the voice coil.

The drive unit may be considered to be at rest when electrical current is not supplied to the voice coil. Note that in order for the acoustic radiator to be suspended from the magnet assembly via the coil assembly, the magnet assembly should only be attached to the acoustic radiator via the coil assembly, i.e. with no rigid attachment between the magnet assembly and the acoustic radiator.

Preferably, the drive unit comprises: a first drive unit suspension attached to the tubular member and the part of the magnet assembly positioned radially inwardly of the tubular member; and a second drive unit suspension, separated from the first drive unit suspension in a direction extending parallel to the movement axis, wherein the second drive unit suspension is either: attached to the tubular member and the part of the magnet assembly positioned radially inwardly of the tubular member or is attached to the voice coil former and a part of the magnet assembly positioned radially outwardly of the voice coil former.

For a typical application, the distance between locations at which the two drive unit suspensions attach to the part of the magnet assembly positioned radially inwardly of the tubular member may be at least 3mm, more preferably at least 5mm, more preferably at least 6mm as measured in a direction extending parallel to the movement axis. A skilled person would appreciate that actual distances will vary in practice depending on various factors including weight of the magnet assembly (larger weight requires larger distance) and design limitations (e.g. space in aperture in which loudspeaker is to be installed).

The magnet assembly may include a magnet assembly frame to which the magnet unit is attached, wherein the part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one drive unit suspension is attached) is a part of the magnet assembly frame.

The part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one drive unit suspension is attached) could, for example, be a hub of the magnet assembly frame.

The part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one drive unit suspension is attached) may include a respective ledge for the/each drive unit suspension attached to the part of the magnet assembly positioned radially inwardly of the tubular member, thereby facilitating attachment of the drive unit suspension(s) to the part of the magnet assembly positioned radially inwardly of the tubular member.

The magnet assembly frame (included in the magnet assembly) may include apertures configured to allow a jig to be inserted to centre the tubular member during assembly.

Some optional features of the drive unit described herein are described with reference to: a first plane perpendicular to the movement axis which extends through the attachment portion; a second plane perpendicular to the movement axis which extends through the air gap.

Features described with reference to the first and second planes are preferably described with respect to the drive unit when the drive unit is at rest. As noted above, the drive unit may be considered to be at rest when electrical current is not supplied to the voice coil. The part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one drive unit suspension is attached) may include: a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially inwardly of the tubular member is located between the first plane and the second plane; and a distal portion, wherein the distal portion of the part of the magnet unit positioned radially inwardly of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially inwardly of the tubular member).

The magnet assembly may include a part of the magnet assembly positioned radially outwardly of the voice coil former, wherein the part of the magnet assembly position radially outwardly of the voice coil former includes: a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former is located between the first plane and the second plane; and a distal portion, wherein the distal portion of the part of the magnet unit positioned radially outwardly of the voice coil former is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially outwardly of the voice coil former).

The part of the magnet assembly positioned radially outwardly of the voice coil former may include part of the magnet unit. The proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former may for example include part of the magnet unit, e.g. a washer 213a as shown in Fig. 5(a). The distal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former may for example include part of the magnet unit, e.g. a main magnet 212a as shown in Fig. 5(a).

The tubular member may include: a proximal portion, wherein the proximal portion of the tubular member is located between the first plane and the second plane; a distal portion, wherein the distal portion of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the tubular member).

The voice coil former may include: a proximal portion, wherein the proximal portion of the voice coil former is located between the first plane and the second plane; a distal portion, wherein the distal portion of the voice coil former is located is on an opposite side of the second plane from the proximal portion (of the voice coil former).

Note that if the tubular member has the distal portion (as described above), this allows the tubular member to reach past the air gap on the inside of the magnet unit, and allows the first and second drive unit suspensions to be separated by a larger distance, compared with an arrangement in which the two drive unit suspensions are attached to the voice coil former.

Preferably, the first drive unit suspension is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially inwardly of the tubular member. Preferably, the second drive unit suspension is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially inwardly of the tubular member.

However, the second drive unit suspension could potentially instead attach to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former (e.g. as shown in Fig. 5(c)), whilst still allowing a wide separation between the first and second drive unit suspensions, thereby still helping to reduce the rotation discussed above with reference to Fig. 2(c).

Preferably, the drive unit comprises both: a first drive unit suspension that is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially inwardly of the tubular member; and a second drive unit suspension that is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially inwardly of the tubular member.

This arrangement allows the first and second drive unit suspensions to have a particularly large space between them, which helps to reduce the rotation discussed above with reference to Fig. 2(c).

In this arrangement, the drive unit may optionally include a third drive unit suspension that is attached to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former (e.g. as shown in Fig. 5(b)).

The magnet unit preferably extends around the tubular member.

The tubular member preferably overlaps the magnet unit along at least a portion of the movement axis.

The tubular member may be shaped to include the attachment portion, e.g. so as to facilitate direct gluing (or some other attachment) of the tubular member to the acoustic radiator.

The tubular member may be shaped to include the attachment portion and the voice coil former.

The tubular member may include or be attached to a surface extending inwardly in a radial direction (with respect to the movement axis) from the distal portion of the tubular member to provide a surface for attaching the tubular member to the first drive unit suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard.

The tubular member may include or be attached to a surface extending inwardly in a radial direction (with respect to the movement axis) from the proximal portion of the tubular member to provide a surface for attaching the tubular member to the second drive unit suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard.

The wall of the tubular member could form an angle with respect to the movement axis, e.g. so that the distal portion of the tubular member is closer to the movement axis that the proximal portion of the tubular member, thereby forming a frusto-conical tubular member. In this case, the angle is preferably no more than 15°.

The tubular member could have one or more extensions in radially inward direction (with respect to the movement axis) to provide a respective attachment surface for the/each drive unit suspension attached to the tubular member, thereby facilitating attachment of the/each drive unit suspension to the tubular member.

The width of the drive unit in the radial direction (perpendicular to the movement axis) will generally depend on design requirements.

The drive unit may include one or more wires configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil.

The electrical path provided by the one or more wires may extend from a connector formed on the magnet assembly (e.g. on a frame of the magnet assembly) to the voice coil.

The one or more wires may include wire from the voice coil winding and/or a lead wire which connects to the voice coil winding.

The one or more wires may include a wire that passes through or around the tubular member. A coupling element (if present - see below) may be configured to guide said wire through or around the tubular member.

The one or more wires may include a wire that passes through or around (preferably through) a frame included in the magnet assembly.

The one or more wires may include two wires that meet at an electrical junction formed on an inwardly facing surface of the tubular member, e.g. at a solderpad or glue dot on the inwardly facing surface of the tubular member.

The magnet unit is preferably configured to provide a magnetic field in an air gap. The voice coil former and/or the tubular member may be cylindrical. But other shapes of air gap, voice coil former and tubular member are possible, e.g. oval, square.

Preferably the voice coil former is arranged around the movement axis.

The voice coil former preferably extends from the attachment portion in a direction which extends along the movement axis into the air gap.

The tubular member and voice coil former are each preferably made from lightweight materials such as paper, cardboard, Kapton, aluminium, kevlar, PE, ABS etc.

The tubular member and voice coil former are preferably made of the same material as each other, but could be made of different materials. The tubular member and voice coil former may be formed integrally with each other (preferably also the attachment portion).

Preferably the attachment portion is arranged around the movement axis.

The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including a gluing surface configured to be glued to the acoustic radiator.

The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including bayonet features configured to engage with corresponding bayonet features on the acoustic radiator to provide a bayonet attachment between the attachment portion and the acoustic radiator.

The attachment portion may be a coupling element which is separately attached to the voice coil former and/or tubular member, e.g. by glue.

The coupling element could be a ring-shaped element, e.g. a cardboard or plastic ring.

The coupling element is not an essential element of the invention, since the attachment portion could be formed integrally with the voice coil former and/or the tubular member. Or the voice coil and tubular member could be configured to attach independently (e.g. by glue) to the acoustic radiator, in which case the attachment portion could include the glue and part of the acoustic radiator.

The/each drive unit suspension could take various forms.

Preferably, the/each drive unit suspension includes one or more corrugations. A drive unit suspension including one corrugation, e.g. a roll drive unit suspension, is preferred in some examples.

The at least one drive unit suspension may include a spider. The/each drive unit suspension may be a spider.

The at least one drive unit suspension may include a roll drive unit suspension. The/each drive unit suspension may be a roll drive unit suspension.

The at least one drive unit suspension may include a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis. The/each drive unit suspension may be a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis.

A potential advantage of a sheet material drive unit suspension could be a reduced height (in the movement axis direction) compared with classic drive unit suspensions which typically require a corrugation to facilitate deflection in the movement axis direction. If there are two drive unit suspensions, each drive unit suspension including one or more corrugations, then the one or more corrugations in one drive unit suspension may mirror the one or more corrugations in the other spider, e.g. with respect to a plane perpendicular to the movement axis, e.g. to help cancel asymmetries in stiffness.

The magnet unit may include a ring-shaped main magnet and a T-yoke.

In use, electrical current carrying an audio signal is supplied to the voice coil which energises the voice coil and causes a magnetic field to be produced by the current in the voice coil, which interacts with the magnetic field produced in the air gap by the magnet unit, and causes the voice coil assembly to move relative to the magnet assembly. This relative movement is accommodated by the at least one drive unit suspension.

We shall now describe a number of features that may be applied to either/both of a loudspeaker according to the first aspect of the invention, or a loudspeaker according to the second aspect of the invention:

The acoustic radiator could have various shapes (e.g. flat, curved, small, large, geometric, free-form).

The loudspeaker may be configured as a dipole loudspeaker. A loudspeaker according to the first or second aspect of the invention is particularly well suited for use as a dipole loudspeaker because its construction is such that it can obstruct a small area of the radiating surface of the acoustic radiator to which it is attached (often referred to as the “second radiating surface” herein).

The mounting frame may be part of, or may be configured to fixedly attach to, a rigid supporting structure, such as a car seat frame.

The at least one mounting frame suspension may be tuned to have a resonance frequency that is below a frequency spectrum over which the loudspeaker is configured to operate, e.g. so as to limit the force on a supporting structure. The at least one mounting frame suspension may be tuned to have a resonance frequency that is 20Hz or lower, more preferably a resonance frequency in the range 10 Hz to 20 Hz.

Note that if there is more than one mounting frame suspension (e.g. two roll suspensions, as in the example of Fig. 6(a)) then it is the resulting stiffness of all mounting frame suspensions that (together with the mass of the drive unit and acoustic radiator) which define the resonance frequency.

The one or more mounting frame suspensions may be tuned to have a resonance frequency that is lower than a resonance frequency that the one or more drive unit suspensions are tuned to have.

The mounting frame may include a rigid body which extends around the movement axis. The rigid body is preferably located radially outwards from the magnet unit, relative to the movement axis.

In some examples, the loudspeaker may include a mounting frame suspension and a drive unit suspension that are both part of a single piece of material (which may be an elastic material, in some examples). In some examples, the loudspeaker may include: a first mounting frame suspension and a first drive unit suspension that are both part of a first piece of material (which may be an elastic material, in some examples); and a second mounting frame suspension and a second drive unit suspension that are both part of a second piece of material (which may be an elastic material, in some examples).

Having two suspensions which are part of a same piece of material helps facilitate manufacture of the loudspeaker.

In some examples, the at least one mounting frame suspension may include one or more roll suspensions, preferably at least two roll suspensions.

The material used for a roll suspension does not need to be elastic (e.g. it can be a textile). This is because a roll suspension allows for axial movement because it has excess material in its roll that can “roll off’ during excursion. However, because of this excess material, a single roll suspension will not generally provide an adequate level of axial stability against rocking, whereas with two roll suspensions it can.

In some examples, the at least one mounting frame suspension may be formed of an elastic material, preferably a single piece of elastic material. The elastic material may be a single piece of elastic foam.

When a single piece of elastic material is used, we rely on its elasticity to provide the compliance of the suspension. So, if it wants to rock it has to stretch the elastic material. The elastic material should be chosen according to design requirements. Experimentation shows that a single piece of elastic foam works well for a wide range of purposes.

Preferably, the acoustic radiator has a width in at least one direction perpendicular to the movement axis that is larger than a width of the drive unit in the same direction. This means, for example, that the acoustic radiator should have a width in at least one direction perpendicular to the movement axis that is larger than a width of the drive unit (including the magnet assembly and the coil assembly) in that direction. The at least one direction may include two orthogonal directions.

In other words, preferably the acoustic radiator extends beyond the sides of the drive unit in at least one direction perpendicular to the movement axis.

By having the diaphragm extend beyond the contours of the drive unit in at least one direction perpendicular to the movement axis, one can maximize the diaphragm’s surface area within a given headrest application. This is especially meaningful if the loudspeaker is configured to move the diaphragm at bass frequencies and is configured to be used as a dipole loudspeaker for cocooning purposes, e.g. as described in [14], where a radiating surface of 100cm ² or more may be beneficial.

More generally, having the diaphragm extend beyond the contours of the drive unit allows the loudspeaker to better approximate a perfect dipole. In some examples, the acoustic radiator may have, in at least one direction perpendicular to the movement axis, a width that is at least 1 .5 times (possibly even 2 times) the width of the drive unit in the same direction. In a headrest implementation (e.g. as discussed in [14]), this may allow the acoustic radiator to have a geometry which follows the contour of the headrest, which may help bring the diaphragm closer to the ear of a user.

Preferably, the attachment portion is attached to the acoustic radiator at multiple locations on the acoustic radiator, wherein a centre of mass of the acoustic radiator is located between two of the locations at which the attachment portion is attached to the acoustic radiator.

In some examples, the acoustic radiator may have a laminate structure formed of at least two layers. The at least two layers may include a first layer of a first material having a first density (mass per unit volume), and a second layer of a second material having a second density, wherein the first density is lower than the second density.

In this way, the (denser, and therefore heavier) second layer can provide additional stiffness to the first layer.

In some examples, the acoustic radiator may have a laminate structure formed of at least three layers. The at least three layers may include a first layer of a first material having a first density (mass per unit volume), wherein the first layer is sandwiched between a second layer of a second material having a second density, and a third layer of a third material having a third density, wherein the first density is lower than both the second density and third density.

In this way, the (denser, and therefore heavier) second and third layers can provide additional stiffness to the first layer.

The third material may be the same as the second material, e.g. as in the example shown in Figs. 6(e)(i) and 6(e)(ii) below.

The second layer may only partially cover a face of the first layer to which the second layer is attached, e.g. so that only the minimum amount of (denser, and therefore heavier) second material is used in order for the acoustic radiator to have a desired stiffness. This may help to optimize the weight of the acoustic radiator within a desired pistonic frequency range (e.g. up to 200Hz before a first break-up mode occurs).

For example, the second layer may cover 75% or less of the surface area of the face of the first layer to which the second layer is attached, preferably 50% or less of the surface area of the face of the first layer to which the second layer is attached.

The third layer (if present) may only partially cover a face of the first layer to which the third layer is attached, e.g. so that only the minimum amount of (denser, and therefore heavier) third material is used in order for the acoustic radiator to have a desired stiffness. This may help to optimize the weight of the acoustic radiator within a desired pistonic frequency range (e.g. up to 200Hz before a first break-up mode occurs). For example, the third layer may cover 75% or less of the surface area of the face of the first layer to which the third layer is attached, preferably 50% or less of the surface area of the face of the first layer to which the third layer is attached.

Preferably, the first material includes (preferably is) polystyrene and the second material includes (preferably is) balsa. Preferably, the third material (if present) includes (preferably is) balsa.

The acoustic radiator may have: a first radiating surface which faces in a forward direction, away from the drive unit; and a second radiating surface which faces in a backward direction, toward the drive unit.

The coil assembly of the drive unit may be attached to the second radiating surface of the acoustic radiator (via the attachment portion).

The acoustic radiator may be curved so that the first radiating surface is concave and so that its second radiating surface is convex. Again, this may allow the acoustic radiator to have a geometry which follows the contour of the headrest, which may help bring the diaphragm closer to the ear of a user.

The acoustic radiator may have, at its periphery (e.g. outer edge), an extension which extends along the movement axis in the backward direction. As described in more detail below, this can help maximize the acoustic output and minimize blowing noises in a gap between the acoustic radiator and a surrounding waveguide.

The/each drive unit suspension may each extend in a linear direction in a plane perpendicular to the movement axis, e.g. for increased linearity of movement, for silent operation and/or for avoiding trapped air that can make blowing noises.

The drive unit may be 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.

The bass frequencies at which the drive unit is configured to move the diaphragm preferably includes frequencies across the range 60-80Hz, more preferably a frequencies across the range 50-100Hz, more preferably a frequencies across the range 40-100Hz, and may include frequencies across the range 40- 160Hz. At these frequencies, the present inventor has found that the loudspeaker is able to produce a particularly useful personal sound cocoon, for reasons discussed in detail in [14]

Moving the diaphragm at frequencies below 40Hz may be useful for some applications, but not for others (such as in a car, where below 40Hz background noise tends to be too loud).

The loudspeaker may be configured as a subwoofer.

In some examples, the first and second radiating surfaces may each have a surface area of at least 100cm ² , e.g. for reasons as described in [14] Preferably, a frame of the magnet assembly (e.g. “magnet assembly frame” as discussed above) and the mounting frame overlap at one or more locations as viewed in a plane perpendicular to the movement axis. Preferably, a frame of the magnet assembly (e.g. “magnet assembly frame” as discussed above) and the mounting frame overlap at two or more locations as viewed in a plane perpendicular to the movement axis, e.g. in a series of locations which are (continuously or discontinuously) located along a path (e.g. a circular path) which extends around the movement axis. Such overlaps can provide crash protection, e.g. as described below with reference to Fig. 6(f).

The loudspeaker may be configured to be mounted within a headrest of a seat, e.g. by including one or more mounting formations configured to facilitate mounting of the loudspeaker within the headrest.

In a third aspect, the present invention may provide a seat assembly including a seat and a loudspeaker according to the first aspect or second aspect of the present invention.

Preferably, the seat is configured to position a user who is sat down in the seat such that at least one ear of the user is located at a listening position (preferably each ear of a user is located at a respective listening position) that is 40cm or less (more preferably 30cm or less, more preferably 25cm or less, more preferably 20cm or less, more preferably 15cm or less) from the first radiating surface of the loudspeaker.

The loudspeaker may be mounted within a headrest of the seat (“seat headrest”). Since a typical headrest is configured to be a small distance (e.g. 30cm or less) from the ear(s) of a user who is sat down in a seat, this is a particularly convenient way of configuring the seat 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 a small distance (e.g. 30cm or less) from the first radiating surface of the loudspeaker.

A seat headrest typically has a front surface configured to face towards the head of a user sat in the seat, and a back surface configured to face away from the head of a user sat in the seat. The loudspeaker is preferably mounted within the headrest of the seat e.g. with the first radiating surface of the loudspeaker facing the front surface of the headrest, e.g. with a principal axis of the first radiating surface extending out through the front surface of the headrest.

The loudspeaker may be configured as a dipole loudspeaker, e.g. the loudspeaker may be mounted in the seat headrest so that the seat headrest is configured to allow sound produced by the first radiating surface of the acoustic radiator to propagate out through the front surface of the headrest and to allow sound produced by a second radiating surface of the acoustic radiator to propagate out from the back surface of the headrest.

The seat may have a rigid seat frame.

The mounting frame of the loudspeaker may be part of or fixedly attached to the rigid seat frame.

The seat (e.g. part of a seat headrest in which the loudspeaker is mounted) may include a waveguide which at least partially (preferably entirely) surrounds the acoustic radiator and is configured to guide sound produced by the first and/or second radiating surface of the acoustic radiator. Preferably, a gap between the waveguide and a periphery of the acoustic radiator is less than 5mm, more preferably less than 2mm (e.g. in the range 1 mm to 2mm), at one or more (preferably all) locations at the periphery (e.g. edge) of the acoustic radiator.

The seat may be a vehicle seat, for use in a vehicle such as a car (“car seat”) or an aeroplane (“plane seat”).

The seat could be a seat for use outside of a vehicle. For example, the seat could be a seat for a computer game player, a seat for use in studio monitoring or home entertainment.

In a fourth aspect, the present invention may provide a vehicle (e.g. a car or an aeroplane) having a plurality of seat assemblies according to the third aspect of the invention.

A fifth aspect of the invention provides:

A method of manufacturing a loudspeaker according to the first or second aspect of the invention.

The method may include pre-assembling the coil assembly, before suspending the magnet assembly from the coil assembly by the at least one drive unit suspension.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Summary of the Figures

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

Fig. 1(a) shows an example traditional loudspeaker.

Fig. 1(b) shows an example inertial exciter.

Fig. 2(a) shows a loudspeaker incorporating a wide bandwidth inertial exciter implementing principles derived from the prior art.

Fig. 2(b) is a graph showing force level vs frequency for the loudspeaker shown in Fig. 2(a).

Fig. 2(c) illustrates a problem with the inertial exciter shown in Fig. 2(a).

Fig. 3(a)(i) and (ii) illustrate “Solution A” as taught by the prior art.

Fig. 3(b)(i) and (ii) illustrate “Solution B” as taught by the prior art.

Fig. 3(c)(i) and (ii) illustrate “Solution C” as taught by the prior art.

Fig. 3(d)(i) and (ii) illustrate “Solution D” as taught by the prior art.

Fig. 3(e)(i) and (ii) illustrate “Solution E” as taught by the prior art.

Fig. 3(f)(i) and (ii) illustrate “Solution F” as taught by the prior art. Fig. 4(a)(i)-(v) show a first drive unit 101a that exemplifies a drive unit of the inner magnet type, and a loudspeaker 180a incorporating the first drive unit 101a.

Fig. 4(b)(i)-(iv) show a second drive unit 101 b that exemplifies an inertial exciter of the inner magnet type, and a loudspeaker 180b incorporating the first drive unit 101b.

Fig. 4(c) shows a third drive unit 101c that exemplifies a drive unit of the inner magnet type.

Fig. 4(d) shows a fourth drive unit 101 d that exemplifies a drive unit of the inner magnet type.

Fig. 4(e) shows a fifth drive unit 101e that exemplifies a drive unit of the inner magnet type.

Fig. 4(f) shows a sixth drive unit 101 f that exemplifies a drive unit of the inner magnet type.

Fig. 4(g) shows a seventh drive unit 101 g that exemplifies a drive unit of the inner magnet type.

Fig. 4(h) shows an eighth drive unit 101 h that exemplifies a drive unit of the inner magnet type.

Fig. 4(i)(i)-(viii) shows a ninth drive unit 101 i that exemplifies a drive unit of the inner magnet type.

Fig. 4(j)(i)-(ii) show a tenth drive unit 101 j that exemplifies a drive unit of the inner magnet type.

Fig. 4(k) shows an eleventh drive unit 101k that exemplifies a drive unit of the inner magnet type.

Fig. 4(l) shows a twelfth drive unit 1011 that exemplifies a drive unit of the inner magnet type.

Fig. 5(a) shows a first drive unit 201a that exemplifies a drive unit of the outer magnet type.

Fig. 5(b) shows a second drive unit 201b that exemplifies a drive unit of the outer magnet type.

Fig. 5(c) shows a third drive unit 201 c that exemplifies a drive unit of the outer magnet type.

Fig. 5(d) shows a fourth drive unit 201 d that exemplifies a drive unit of the outer magnet type.

Fig. 6(a) shows a first example loudspeaker.

Fig. 6(b) shows a second example loudspeaker.

Fig. 6(c) shows a third example loudspeaker.

Figs. 6(d)(i)-(iii) shows a fourth example loudspeaker.

Figs. 6(e)(i)-(ii) shows a fifth example loudspeaker.

Fig. 6(f) shows a sixth example loudspeaker.

Fig. 6(g) shows a seventh example loudspeaker.

Fig. 6(h) shows an eighth example loudspeaker.

Fig. 6(i) shows a seat headrest incorporating ninth and tenth example loudspeakers. Detailed Description of the Invention

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.

For the purpose of this description, example drive units are divided into two types, referred to as “inner magnet” type according to the first aspect of the invention and “outer magnet” type according to the second aspect of the invention.

Drive unit - Inner magnet type examples

A first drive unit 101 a that exemplifies an drive unit of the inner magnet type is shown in Fig. 4(a)(i).

Here, we note for completeness that the drive unit 101 a is referred to as a drive unit because it is intended for use in a loudspeaker where an acoustic radiator is suspended from a magnet assembly of the drive unit.

Were the drive unit 101a intended for use in a loudspeaker where a magnet assembly of the drive unit is suspended from an acoustic radiator, then the drive unit 101a might instead be referred to as an “inertial exciter”. The terms “drive unit” and “inertial exciter” can thus be used interchangeably in relation to the drive unit 101a, until the drive unit 101 a is incorporated into a loudspeaker, at which point the drive unit 101a should be referred to as an “inertial exciter” only if the loudspeaker has a magnet assembly of the drive unit suspended from an acoustic radiator.

The drive unit 101a of Fig. 4(a) comprises a magnet assembly 102a and a coil assembly 104a.

The magnet assembly 102a includes a magnet unit 110a and a frame 120a to which the magnet unit 110a is attached.

In this example, the magnet unit 110a includes a main magnet 112a, a washer 113a an extra magnet 114a and a U-yoke 115a. The magnet unit 110a is configured to provide a magnetic field in an air gap 116a. The air gap 116a extends around a movement axis 106a of the drive unit 101a.

The frame 120a includes a base portion 122a which extends radially outwardly with respect to the movement axis 106a (in this example from a base of the U-yoke 115a), and a rim 124a which extends axially with respect to the movement axis 106a, that is at least partly along the movement axis 106a. The rim 124a of the frame 120a is positioned at the periphery of the base portion 122a, and is positioned radially outwardly of the magnet unit 110a.

The rim 124a of the frame 120a is positioned radially outwardly of a tubular member 140a, and thus serves as the “part of the magnet assembly positioned radially outwardly of the tubular member” referenced in the “Summary of the invention” section of this document, above.

In this example, the main magnet 112a, washer 113a, extra magnet 114a, U-yoke 115a, and air gap 116a are circular, though other forms are possible. In this example, the washer 114a and U-yoke 116a may be made of steel, though other materials are possible.

In this example, the coil assembly 104a includes a voice coil 130a, a voice coil former 132a, a tubular member 140a and an attachment portion 150a.

In this example, the attachment portion 150a is a coupling element which is separately attached to the voice coil former and tubular member, e.g. by glue. The coupling element 150a is configured to provide an attachment between the coil assembly 104a and an acoustic radiator (not shown) by including a gluing surface 151a configured to be glued to the acoustic radiator. The coupling element 150a could for example be a plastic or cardboard ring-shaped element.

The voice coil former 132a extends axially with respect to the movement axis 106a from the coupling element 150a into the air gap 116a. The voice coil 130a is mounted to the voice coil former 132a so that the voice coil 130a sits in the air gap 116a when the drive unit 101a is at rest.

The tubular member 140a is positioned radially outwardly of the voice coil former 132a with respect to the movement axis 106a. The tubular member 140a also overlaps the voice coil former 132a along a portion of the movement axis (this portion corresponding to the full length of the voice coil former 132a).

In this example, the voice coil former 132a and tubular member 140a are cylindrical, though other shapes are possible.

Two planes are depicted in Fig. 4(a)(i).

A first plane 108a is perpendicular to the movement axis 106a and extends through the attachment portion which as noted above is the coupling element 150a.

A second plane 109a is perpendicular to the movement axis 106a and extends through the air gap 116a. The rim 124a of the frame 120a includes: a proximal portion, wherein the proximal portion of the rim 124a is located between the first plane 108a and the second plane 109a; and a distal portion, wherein the distal portion of the rim 124a is located is on an opposite side of the second plane 109a from the proximal portion of the rim 124a.

The tubular member 140a similarly includes: a proximal portion, wherein the proximal portion of the tubular member 140a is located between the first plane 108a and the second plane 109a; and a distal portion, wherein the distal portion of the tubular member 140a is located is on an opposite side of the second plane 109a from the proximal portion of the of the tubular member 140a.

The drive unit 101a includes: a first drive unit suspension 160a that is attached to the distal portion of the tubular member 140a and the distal portion of the rim 124a; and a second drive unit suspension 165a that is attached to the proximal portion of the tubular member 140a and the proximal portion ofthe rim 124a. Each drive unit suspension 160a, 165a in this example is a spider including multiple corrugations. Such drive unit suspensions are well known in the art.

Thus, when the coil assembly 104a is attached to the acoustic radiator via the attachment portion/coupling element 150a, the acoustic radiator is suspended from the magnet assembly 102a via the coil assembly 104a by the first and second drive unit suspensions 160a, 165a.

As can be seen from Fig. 4(a)(i), the rim 124a of the frame 120a includes a first ledge 125a to which the first drive unit suspension 160a is attached, and a second ledge 126a to which the second drive unit suspension 165a is attached.

In this example, the first and second drive unit suspensions 160a, 165a are each shown as a respective spider having multiple corrugations.

The drive unit 101a includes wires 134a, 135a configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil 130a.

The electrical path provided by the wires 134a, 135a extend from a connector 138a formed on an outwardly facing surface of the rim 124a of the frame 120a to the voice coil 130a.

In this example, the wires include part of the voice coil winding 134a as well as a lead wire 135a. In this example, the voice coil winding 134a extends around the tubular member 140a as guided by the coupling element 150a.

The voice coil winding 134a and lead wire 135a meet at an electrical junction formed at a solderpad or glue dot 136a on an outwardly facing surface of the tubular member 140a.

Fig. 4(a)(ii) shows a loudspeaker 180a including the drive unit 101a of Fig. 4(a)(i) and an acoustic radiator 190a suspended from the magnet assembly 102a, wherein the coil assembly 104a of the drive unit 101 a is attached to the acoustic radiator 190a via the attachment portion/coupling element 150a so that the acoustic radiator 190a is suspended from the magnet assembly 102a via the coil assembly by the first and second drive unit suspensions 160a, 165a.

Preferably, the magnet assembly 102a is itself suspended from a mounting frame via at least one mounting frame suspension (not shown here, but shown in the examples discussed below under the heading “Loudspeakers in which the magnet assembly is suspended from a mounting frame”).

In use, electrical current carrying an audio signal is supplied to the voice coil 130a via the connector 138a and wires 134a, 135a. This energises the voice coil 130a and causes a magnetic field to be produced by the current in the voice coil 130a, which interacts with the magnetic field produced in the air gap 116a by the magnet unit 110a, and causes the voice coil assembly 104a to move relative to the magnet assembly 102a. This relative movement is accommodated by the first and second drive unit suspensions 160a, 165a.

Because the acoustic radiator 190a is suspended from the magnet assembly 102a via the coil assembly 104a by the first and second drive unit suspensions 160a, 165a, the loudspeaker is able to be moved by MC and MM excitation. Because the voice coil former 132a and tubular member 140a are tubular, they provide good stiffness even when made of a lightweight material such as paper, cardboard, Kapton, aluminium, kevlar etc. Thus, the voice coil assembly 104a can have low weight and good stiffness, as is needed for good wide bandwidth performance from MC excitation.

Moreover, because the tubular member 140a has a distal portion which overlaps the voice coil former 132a so as to extend beyond the air gap 116a, i.e. to the opposite side of the second plane 109a from the proximal portion of the tubular member 140a, it is possible to have a large distance between the first and second drive unit suspensions 160a, 165a, which helps inhibit rotation of the voice coil assembly 104a relative to the magnet assembly 102a when the loudspeaker is vertically mounted.

Note that this is achieved whilst providing one interface (the glue surface of the coupling element 150a) with the acoustic radiator 190a, and also whilst permitting MC excitation. The low mass of the voice coil assembly (see above) help to achieve acoustic sensitivity and balance in the upper frequency band, as depicted in Fig. 4(a)(iii).

Fig. 4(a)(iv) shows a method step involved in assembling the drive unit 101 a which makes use of a conventional centering jig 195a to align the voice coil former 132a in the air gap 116a before the components of the voice coil assembly 104a are glued together. The coupling element 150a may be flush with an inwardly facing surface of the voice coil former 132a to facilitate use of the centering jig 195a.

Fig. 4(a)(v) shows an alternative or additional method step involved in assembling the drive unit 101a in which apertures are incorporated into the frame 120a to allow a centering jig 196a to be inserted into the apertures during assembly, e.g. to help with aligning the voice coil former 132a in the air gap 116a.

Preferably the voice coil assembly (including the coupling element 150am the voice coil 130a, voice coil former 132a and the tubular member 140a) could be pre-assembled on a separate jig (not shown) before being assembled in the magnet assembly 102a.

Various alternative inner magnet examples will now be described. Alike features have been given alike reference numerals where appropriate and are not described in further detail, except where necessary.

A second drive unit 101 b that exemplifies an drive unit of the inner magnet type is shown in Fig. 4(b)(i).

The coupling element 150b of the drive unit 101 b is shown in Fig. 4(b)(ii) and includes bayonet features in the form of radial extensions 151 b configured to engage with corresponding bayonet features 191 b on the acoustic radiator 190b shown in Fig. 4(b)(iii) to provide a bayonet attachment between the coupling element 150b and the acoustic radiator 190b. The bayonet features 191 b on the acoustic radiator preferably form slots for accommodating the radial extensions 151 b. The resulting loudspeaker 180b is shown in Fig. 4(b)(iv).

The above-described bayonet feature could facilitate assembly and replacement of the drive unit 101 b to the acoustic radiator 190b. The above-described bayonet features could be combined with adhesives or filler (e.g. grease) to avoid rattling during operation. The adhesive or filler could have temperature dependent properties so that by applying heat the drive unit 101 b can be replaced.

A third drive unit 101c that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(c).

In this example, the tubular member 140c includes a collar 141 c that provides a flat face to facilitate gluing of the first drive unit suspension 160c, which in this example could be a fabric damper, a metal or plastic spiral spring, a rubber element, etc.

A fourth drive unit 101 d that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(d).

In this example, the a ring 141 d, e.g. made of cardboard or plastic, is attached to the distal portion of the tubular member 140d to provide a flat surface 141 d to facilitate gluing of the first drive unit suspension 160d.

A fifth drive unit 101e that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(e).

In this example, the tubular member 140e is integrally formed with the attachment portion 150e by appropriately shaping the tubular member 140e to include the attachment portion 150e. This allows the tubular member 140e to be glued directly to the voice coil former 132e, and avoids the use of a coupling element as described in previous examples. In this example, the attachment portion 150e is a flat face of the tubular member 140e that is configured to be glued to the acoustic radiator (not shown).

The tubular member 140e could be made of paper, cardboard, Kapton, aluminium, kevlar, PE, ABS etc.

A sixth drive unit 101 f that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(f).

The drive unit 101 f is the same as the fifth drive unit 101 e shown in Fig. 4(e), except that holes are formed in the attachment portion 150f to enhance the glue attachment to the acoustic radiator (not shown).

A seventh drive unit 101 g that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(g).

In this example, the coupling element 150g is attached only to the voice coil former 132g, with the tubular member 140g being attached to the voice coil former 132g.

An eighth drive unit 101 h that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(h).

In this example, the tubular member 140h forms an angle with respect to the movement axis, thereby forming a frusto-conical tubular member 140h. In this case, the angle is preferably no more than 15°.

A tubular member 140h shaped in this way could facilitate the making of the tubular member 140h from paper or from plastic in a deep draw process.

In this example, the tubular member 140h is again integrally formed with the attachment portion 150h by appropriately shaping the tubular member 140h to include the attachment portion 150h.

A ninth drive unit 101 i that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(i)(i). This example is essentially the same as the first drive unit 101 a shown in Fig. 4(a)(i), except that in this case the first and second drive unit suspensions 160i, 165i include only a single corrugation, and the single corrugations mirror each other (in a plane 108i perpendicular to the movement axis 106i) to help cancel asymmetries in stiffness between the two drive unit suspensions 160i, 165i. The first and second drive unit suspensions 160i, 165i may in this case be roll drive unit suspensions, e.g. made of rubber, textile or foam.

Fig. 4(i)(ii) show the attachment between the frame 120i and the drive unit suspensions 160i, 165i. In this particular example, the rim of the frame 120i is provided in two parts, 124i(i) and 124i(ii).

Example dimensions are drawn on Fig. 4(i)(i) and Fig. 4(i)(ii), noting that the distance between locations at which the two drive unit suspensions 160i, 165i attach to the rim of the magnet assembly is 6.3mm in this example, which is large given the overall size of the drive unit 101 i.

Figs. 4(i)(iii)-(viii) are 3D views showing the drive unit 101 i from various angles.

A tenth drive unit 101 j that exemplifies an drive unit of the inner magnet type is shown in Fig. 4(j)(i).

The drive unit 101 j shown in Fig. 4(j)(i) is the same as the drive unit 101 a shown in Fig. 4(a)(i) except that the drive unit includes an alternative form of first and second drive unit suspensions 160j, 165j.

The alternative form of drive unit suspension used for the first and second drive unit suspensions 160j, 165j is shown in more detail in Fig. 4(j)(ii).

As can be seen most clearly from Fig. 4(j)(ii), the alternative form of first and second drive unit suspensions 160j, 165j is a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis 106j, whilst inhibiting movement in a direction perpendicular to the movement axis 106j.

A suitable material for the alternative form of first and second drive unit suspensions 160j, 165j could be a fiber- re in forced plastic, e.g. a polymer matrix reinforced with glass fibres or carbon fibres, or a metal, e.g. steel spring material.

An eleventh drive unit 101 k that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(k).

The drive unit 101 k shown in Fig. 4(k) is the same as the drive unit 101 a shown in Fig. 4(a)(i) except that in this example the second drive unit suspension 165k is attached to a proximal portion of the voice coil former 132k and a proximal portion of a part of the magnet assembly positioned radially inwardly of the voice coil former (in this case the extra magnet 114k).

Note, that in this case the drive unit 101 k has: a first drive unit suspension 160k that is attached to a distal portion of the tubular member 140k and the distal portion of the rim 124k; and a second drive unit suspension 165k that is attached to a proximal portion of the voice coil former 132k and a proximal portion of a part of the magnet assembly positioned radially inwardly of the voice coil former 132k (in this case the extra magnet 114k). Thus, this arrangement still allows for a wide separation between the first and second drive unit suspensions 160k, 165k, thereby helping to inhibit rotation of the magnet assembly 102k relative to the voice coil assembly 104k.

Besides providing drive unit suspension, the second drive unit suspension 165k can also serve as a dust cover to prevent dust in the airgap 116k prior to mounting the drive unit 101 k to an acoustic radiator.

In this example, the first drive unit suspension 165k is a roll drive unit suspension including only one corrugation.

A twelfth drive unit 1011 that exemplifies a drive unit of the inner magnet type is shown in Fig. 4(1).

The drive unit 1011 shown in Fig. 4(l) is the same as the drive unit 101 a shown in Fig. 4(a)(i) except that a third drive unit suspension 1681 is attached to a proximal portion of the voice coil former 1321 and a proximal portion of a part of the magnet assembly positioned radially inwardly of the voice coil former (in this case the extra magnet 1141).

Besides providing drive unit suspension, the third drive unit suspension 1681 can also serve as a dust cover to prevent dust in the airgap 1161 prior to mounting the acoustic radiator to the drive unit 1011.

Drive unit - Outer magnet type examples

A first drive unit 201 a that exemplifies a drive unit of the outer magnet type is shown in Fig. 5(a).

The drive unit 201a shown in Fig. 5(a) includes many features which are common to the drive unit 101a shown in Fig. 4(a)(i). Alike features have been given alike reference numerals where appropriate and are not described in further detail, except where necessary.

The magnet assembly 202a includes a magnet unit 210a and a frame 220a to which the magnet unit 210a is attached.

In this example, the magnet unit 210a includes a (ring-shaped) main magnet 212a, a (ring-shaped) washer 213a and a T-yoke 215a (which looks like an upside down “T” as drawn). The magnet unit 210a is configured to provide a magnetic field in an air gap 216a. The air gap 216a extends around a movement axis 206a of the drive unit 201 a.

The outer magnet type examples can be useful as they allow more magnet material to be used compared with the inner magnet type examples, and therefore enable more powerful exciters, as may be desirable in some cases.

In this example, the frame 220a includes a base portion 222a which extends radially inwardly with respect to the movement axis 206a (in this example from a base of the T-yoke 215a).

In this example, the frame 220a also include a hub 224a which extends axially with respect to the movement axis 206a, that is at least partly along the movement axis 206a. The hub 224a of the frame 220a is positioned at the centre of the base portion 222a, and is positioned radially inwardly of the tubular member 240a. In this example, the tubular member 240a is positioned radially inwardly of the voice coil former 232a with respect to the movement axis 206a, and overlaps the voice coil former 232a along at least a portion of the movement axis 206a.

The drive unit 201a includes: a first drive unit suspension 260a that is attached to a distal portion of the tubular member 240a and the distal portion of the hub 224a; and a second drive unit suspension 265a that is attached to the proximal portion of the tubular member 240a and the proximal portion of the hub 224a.

The proximal portions of the tubular member 240a and hub 224a are located between the first plane 208a and the second plane 209a as defined above. The proximal portions of the tubular member 240a and hub 224a are located on an opposite side of the second plane 209a from the proximal portions.

As can be seen from Fig. 5(a), the hub 224a of the frame 220a includes a first ledge 225a to which the first drive unit suspension 260a is attached, and a second ledge 226a to which the second drive unit suspension 265a is attached.

In this example, the drive unit 201a includes a lead wire 234a configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil 130a.

In this example, the electrical path provided by the lead wire 234a extend from a connector 238a formed on an outwardly facing surface of the base portion 222 of the frame 220a (outward in the sense of facing away from the hub 224a) to the voice coil 230a.

In this example, the lead wire 234a extends through the frame 220a.

In this example, the coupling element 250a is similar to that shown in Fig. 4(a)(i).

In use, electrical current carrying an audio signal is supplied to the voice coil 230a via the connector 238a and lead wire 234a. This energises the voice coil 230a and causes a magnetic field to be produced by the current in the voice coil 230a, which interacts with the magnetic field produced in the air gap 216a by the magnet unit 210a, and causes the voice coil assembly 204a to move relative to the magnet assembly 202a. This relative movement is accommodated by the first and second drive unit suspensions 260a, 265a.

Various alternative inner magnet examples will now be described. Alike features have been given alike reference numerals where appropriate and are not described in further detail, except where necessary.

A second drive unit 201 b that exemplifies a drive unit of the outer magnet type is shown in Fig. 5(b).

This example is that same as that shown in Fig. 5(a), except that a third drive unit suspension 268b is attached to the voice coil former 232b and to a part of the magnet assembly 202b (in this case the washer 213b) positioned radially outwardly of the voice coil former 232a.

Besides providing drive unit suspension, the third drive unit suspension 268b can also serve as a dust cover to prevent dust in the airgap 216b when the drive unit 201 b is in use. A third drive unit 201c that exemplifies a drive unit of the outer magnet type is shown in Fig. 5(c).

This example is that same as that shown in Fig. 5(a), except that in this example the second drive unit suspension 265c is attached to the voice coil former 232b and to a part of the magnet assembly 202b (in this case the washer 213b) positioned radially outwardly of the voice coil former 232a.

Note, that in this case the drive unit 201c has: a first drive unit suspension 260c that is attached to a distal portion of the tubular member 240c and the distal portion of the hub 224c; and an second drive unit suspension 265c that is attached to a proximal portion of the tubular member 240c and a proximal portion of a part of the magnet assembly positioned radially outwardly of the tubular member 240c (in this case the washer 213b).

Thus, this arrangement still allows for a wide separation between the first and second drive unit suspensions 160k, 165k, thereby helping to inhibit rotation of the magnet assembly 202c relative to the voice coil assembly 204c.

Besides providing suspension, the second drive unit suspension 265c can also serve as a dust cover to prevent dust in the airgap 216c when the drive unit 201 b is in use.

A fourth drive unit 201 d that exemplifies a drive unit of the outer magnet type is shown in Fig. 5(d).

This example is that same as that shown in Fig. 5(b), except that:

• the tubular member 240d is integrally formed with the attachment portion 250d by appropriately shaping the tubular member 240d to include the attachment portion 250d.

• holes are formed in the attachment portion 250d to enhance the glue attachment to the acoustic radiator (not shown)

Loudspeakers in which the magnet assembly is suspended from a mounting frame

In all the examples that follow, an acoustic radiator is suspended from the magnet assembly by at least one drive unit suspension, and the magnet assembly is suspended from a mounting frame by at least one mounting frame suspension. Although such an arrangement is only explicitly shown in the examples that follow, a skilled person would appreciate that such an arrangement could equally be adopted with all of the drive units previously discussed.

In the examples loudspeakers that follow, the drive units includes many features which are common to the drive unit 101a shown in Fig. 4(a)(i). Alike features have been given alike reference numerals where appropriate and such features are not described in further detail, except where necessary. In some examples, the acoustic radiator is omitted for clarity, but it would be appreciated in all cases that the loudspeaker would have an acoustic radiator attached to the coil assembly, wherein the attachment portion provides the attachment between the coil assembly and the acoustic radiator.

In all the examples that follow, the loudspeakers incorporate drive unit of the outer magnet type, but a skilled person would readily appreciate that drive units of the inner magnet type could equally be used. In all the examples that follow, the loudspeaker is preferably configured to move the diaphragm at bass frequencies and is configured to be used as a dipole loudspeaker, e.g. as described in [14]

Fig. 6(a) shows a first example loudspeaker 300a, in which there is an acoustic radiator 390a attached to the coil assembly 304a, wherein the attachment portion 350a provides the attachment between the coil assembly 304a and the acoustic radiator 390a.

As shown in Fig. 6(a), the loudspeaker 300a additionally includes a mounting frame 380a, wherein the magnet unit assembly 302a is suspended from the mounting frame 380a by at least one mounting frame suspension 370a, 375a. In this example, the magnet unit assembly is suspended from the mounting frame by two mounting frame suspensions 370a, 375a, which are in this example both roll suspensions, wherein the single corrugations mirror each other (in a plane perpendicular to the movement axis) to help cancel asymmetries in stiffness between the two mounting frame suspensions 370a, 375a.

In this example, the mounting frame 380a is configured to fixedly (i.e. rigidly) attach to a rigid supporting structure, such as a car seat frame, by including one or more mounting formations. In this example, the mounting formations are holes 392a, through which bolts can pass in order to bolt the mounting frame 380a to the rigid supporting structure.

To facilitate the use of the bolt, the acoustic radiator 390a may include one or more access holes 392a to configured to provide access to the one or more mounting formations. The one or more access holes 392a may be closed with tape 393a after access is no longer needed, so as to reduce/avoid loss of acoustic performance due to the presence of the one or more access holes. Other acoustic radiator constructions are of course possible.

As shown in Fig. 6(a), the acoustic radiator 390a has a width in a direction d perpendicular to the movement axis 306a that is larger than a width of the drive unit 301a in the same direction. In other words, the acoustic radiator 390a extends beyond the sides of the drive unit 301a in at least one direction (d) perpendicular to the movement axis.

The acoustic radiator 390a has: a first radiating surface 394a which faces in a forward direction F, away from the drive unit 301 a; and a second radiating surface 394a’ which faces in a backward direction B, toward the drive unit; wherein the coil assembly 304a of the drive unit 301a is attached to the second radiating surface 394a’ of the acoustic radiator (via the attachment portion 350a).

Fig. 6(b) shows a second example loudspeaker 300b, wherein the acoustic radiator has been omitted, for clarity.

In this example, the loudspeaker 300b includes: a first mounting frame suspension 360b and a first drive unit suspension 370b that are both part of a first piece of material (which may be an elastic material, in some examples); and a second mounting frame suspension 365b and a second drive unit suspension 375b that are both part of a second piece of material (which may be an elastic material, in some examples). The magnet unit 310b in this case includes a two-pole piece with a recession 317b at the voice coil location. This recession 317b can be filled, e.g. with copper.

The first and/or second drive unit suspension 360b, 365b may include one or more holes/interruptions to help silence its operation. In this particular example, the first drive unit suspension 360b is perforated to achieve this effect.

Fig. 6(c) shows a third example loudspeaker 300c.

In this example, the acoustic radiator is curved 390c so that the first radiating surface 394c is concave and so that the second radiating surface 394c’ is convex.

The first and/or second drive unit suspension may include one or more interruptions to help facilitate the attachment of a lead wire 334c. In this particular example, the second drive unit suspension 365c include an interruption through which the lead wire 334c passes.

Again, the acoustic radiator 390c has a width in a direction d perpendicular to the movement axis that is larger than a width of the drive unit 301c in the same direction.

Figs. 6(d)(i)-(iii) show a fourth example loudspeaker 300d.

In this example, the acoustic radiator 390d is curved so that the first radiating surface 394d is concave and so that its second radiating surface 394d’ is convex.

Again, the acoustic radiator 390d has a width in a direction d perpendicular to the movement axis that is larger than a width of the drive unit 301d in the same direction.

The attachment portion 350d is attached to the acoustic radiator 390d at multiple locations on the acoustic radiator 390d, wherein a centre of mass of the acoustic radiator 390d is located between two of the locations at which the attachment portion 350d is attached to the acoustic radiator, preferably such that the acoustic radiator is driven (substantially) at its centre of mass. In this example, the centre of mass is located on the movement axis 306d, and is therefore between the two locations at which the attachment portion 350d is attached to the acoustic radiator 390d in the plane of cross-section depicted in Fig. 6(d)(iii) (these two locations correspond to the two points labelled by reference numeral 350d in Fig. 6(d)(iii)).

In this example, the drive unit suspensions 360d, 365d each extend in a linear direction in a plane perpendicular to the movement axis, e.g. for increased linearity of movement, for silent operation and/or for avoiding trapped air that can make blowing noises. Interruptions in the drive unit suspensions 360d, 365d are provided by gaps between the linear suspensions.

Figs. 6(e)(i)-(ii) show a fifth example loudspeaker 300e, wherein Fig. 6(e)(i) is a view of the front (forwardfacing) surface of the loudspeaker 300e, and Fig. 6(e)(ii) is a view of the back (backward-facing) surface of the loudspeaker 300e.

In this example, the loudspeaker 300e shares many of the features of the example loudspeaker shown in Fig. 6(b), but in this case the acoustic radiator 390e has a laminate structure formed of three layers: a layer of polystyrene 395e sandwiched between two layers of balsa 395e’, 395”, wherein a first layer of balsa 395e’ is attached to a first (forward-facing) face of the layer of polystyrene 395e, and a second (backward-facing) layer of balsa 395e’ is attached to a second face of the layer of polystyrene 395e. .

The first layer of balsa 395e’ covers less than 75% or less of the surface area of the face of the layer of polystyrene 395e to which the first layer of balsa 395e’ is attached, The first layer of balsa 395e’ and the part of the face of the layer of polystyrene 395e exposed behind the first layer of balsa 395e’ provides a first radiating surface 394e of the acoustic radiator 390e.

The second layer of balsa 395e” covers less than 75% or less of the surface area of the face of the layer of polystyrene 395e to which the second layer of balsa 395e” is attached, The second layer of balsa 395e” and the part of the face of the layer of polystyrene 395e exposed behind the second layer of balsa 395e” provides a second radiating surface 394e’ of the acoustic radiator 390e.

The first and second layers of balsa 395e’, 395e” help to stiffen the acoustic radiator 390e.

The polystyrene in the layer of polystyrene 395e is preferably expanded, but could be extruded instead. Example parameters for the layer of polystyrene 395e may be:

• Density: as low as possible e.g. 15kg/m ³ up to 60kg/m ³

• E modulus (which is related to density) may be in the range 1 ,4MPa - 4,0MPa

• Thickness may be up to 10mm, preferably 3mm to 5mm.

The balsa in the layers of balsa 395e’, 395e” may be a sheet of balsa wood, or a balsa veneer. If a sheet of balsa is used, then a grain direction of the balsa preferably extends in a direction in which the balsa is longest. Example parameters for each layer of balsa 395e’, 395e” may be:

• Density: 100kg/m ³ to 150kg/ ³

• Bending modulus along the grain may be 3.5GPa - 5GPa (note that bending modulus rises with frequency)

• Thickness may be 0.5mm - 1 ,5mm

Note that each layer of balsa 395e’, 395e” covers only part of a surface formed from the layer of polystyrene 395e, e.g. so that only the minimum amount of balsa is used to achieve a desired stiffness.

Fig. 6(f) shows a sixth example loudspeaker 300f.

In this example, the acoustic radiator 390f has a laminate structure formed of at least two layers wherein the at least two layers include a first layer of a first material having a first density, and a second layer of a second material having a second density, wherein the first density is lower than the second density. In some examples, there may be an additional third layer of the second material, wherein the first layer is sandwiched between the second layer of the second material and the third layer of the second material having the second density. The at first and second materials may be, for example:

• balsa-foam • paper-foam

• paper-honeycomb

• carbon fibre-foam

• glass fibre-paper

A laminate structure including two layers of the same material are also possible, e.g. balsa-balsa.

In this example, the attachment portion 350f of the drive unit 301 f includes alignment features configured to engage with corresponding alignment features 391 f (here drawn as shoes) on the acoustic radiator 390f to facilitate alignment and easy attachment of the attachment portion 350f and acoustic radiator 390f. This attachment may be mechanical (e.g. screw, bayonet or heat weld) or chemical (adhesive).

This example shows two alternative forms of crash protection (circled with dotted lines), i.e. two locations at which a frame of the magnet assembly 302f and the mounting frame 380f overlap as viewed in a plane (e.g. plane 308f) perpendicular to the movement axis 306f. These crash protection features help to prevent the magnet assembly 302f from protruding out from a headrest during a crash of a vehicle in which the loudspeaker 300f is mounted within the headrest.

Of course, it would be possible to have just one of the crash protection features as shown in Fig. 6(f), rather than both. It would equally be possible for either/both of these crash protection features to be present continuously (or discontinuously) along a path extending around the movement axis.

Fig. 6(g) shows a seventh example loudspeaker 300g.

In this example, the acoustic radiator 390g has, at its periphery (outer edge), an extension which extends along the movement axis in the backward direction B.

For the purposes of demonstrating different solutions, Fig. 6(g) shows two possible forms of the extension.

In a first form, the extension 397g is an integral part of the acoustic radiator which is folded to extend along the movement axis in the backward direction.

In a second form, the extension is provided by a foam strip 397g’ attached (e.g. by adhesive) to the remainder of the acoustic radiator 390g at the periphery of the acoustic radiator 390g.

A seat (e.g. part of a seat headrest in which the loudspeaker is mounted) may include a waveguide 378g which at least partially (preferably entirely) surrounds the acoustic radiator 390g and is configured to guide sound produced by the first and/or second radiating surface of the acoustic radiator 390g.

Preferably, a gap between the waveguide and a periphery of the acoustic radiator 390g less than 5mm, more preferably less than 2mm (e.g. in the range 1 mm to 2mm), at one or more (preferably all) locations at the periphery of the acoustic radiator.

Fig. 6(h) shows an eighth example loudspeaker 300h. In this example, there is only one mounting frame suspension 370h, wherein this mounting frame suspension 370h is formed of elastic foam, preferably a single piece of elastic foam, e.g. which may have properties as described in [15], though other forms of elastic foam are of course possible.

Fig. 6(i) shows a seat headrest 10OOi incorporating ninth and tenth loudspeakers 300i, 300G, and additionally incorporating two mid-high frequency loudspeakers 101 Oi, 1010i’.

The two mid-high frequency loudspeakers 101 Oi, 1010i’ may be of a cardioid type, e.g. as described in [16], though other forms of mid-high frequency loudspeakers are of course possible.

Also shown in Fig. 6(i) are:

• an electronic unit 1015i

• headrest chassis 1030i to which the mounting frame 380i of the loudspeaker 300i is attached (the headrest chassis 1030i is part of a rigid seat frame)

• acoustic transparent finishing material 1020i, e.g. perforated leather, textile, etc (noting that parts can be left non transparent for aesthetic reasons)

• Open cell elastic foam 1025i, preferably acoustic transparent in front of the acoustic radiators 300i, 300G (this can be combined with areas of non-transparent foam to increase support comfort)

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.

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.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventor does not wish to be bound by any of these theoretical explanations.

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

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.

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

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] US6618487B1

• [2] US4506117A

• [3] US8247930B2

• [4] US7372968B2

• [5] US4550428A

• [6] US6965679B1

• [7] US2005/180587A1

• [8] US4675907A

• [9] US4354067A

• [10] US4750208A

• [11] DE102004009902A1

• [12] US9621994B1

• [13] US5734132

• [14] WO2019/121266

• [15] GB2008724.3

• [16] GB2004076.2