GB2504691A

GB2504691A - Exciter coupling arrangement improves frequency response for a bending wave loudspeaker

Info

Publication number: GB2504691A
Application number: GB201213921A
Authority: GB
Inventors: Libin Wang; Andreas Buos; Chao Peng
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2012-08-06
Filing date: 2012-08-06
Publication date: 2014-02-12
Anticipated expiration: 2032-08-06
Also published as: GB2504691B; GB201213921D0

Abstract

In a bending wave loudspeaker the vibration exciter 110 is coupled to a primary bending wave radiator 120. A secondary bending wave radiator 130 provides for the sound output from the loudspeaker and is mechanically coupled in face to face contacting relationship with the primary bending wave radiator 120. The exciter 110 is able to excite bending wave vibrations in the primary substrate 120, and the primary substrate is able to excite bending wave vibrations in the secondary substrate 130. By modifying the mechanical properties (stiffness, impedance) of the substrates 120, 130 the overall frequency response of the loudspeaker is modified, particularly at low frequencies (fig 2). This arrangement may be used to employ vehicle panels including trim panels as bending wave loudspeakers.

Description

AUDIO APPARATUS AND METHOD

FIELD OF THE INVENTION

The present invention relates to audio apparatus and to a method of generating sound. In particular, but not exclusively, embodiments of the invention are directed to bending wave audio loudspeakers or transducers and to a method of generating sound by means thereof.

Aspects of the invention relate to a loudspeaker, an audio apparatus, a motor vehicle and a method.

BAG KG ROUND

It is known to provide a bending wave audio loudspeaker in the form of a substantially flat panel. An exciter is coupled to the panel and arranged to excite acoustic bending waves in the panel, which radiates sound waves into a surrounding medium such as air.

Bending wave loudspeakers have the feature that they may occupy a substantially smaller volume than a conventional cone-driven loudspeaker of similar power.

The present inventors have recognised that bending wave loudspeaker systems may be deployed advantageously in motor vehicles where a reduction in audio package space and enhanced aesthetic appeal may be enjoyed. It is desirable to provide an improved bending wave loudspeaker system.

STATEMENT OF THE INVENTION

Embodiments of the invention may be understood with reference to the appended claims.

Aspects of the present invention provide a loudspeaker, an audio apparatus, a vehicle and a method.

In one aspect of the invention for which protection is sought there is provided a bending wave loudspeaker comprising: a vibration exciter; a primary substrate providing a primary bending wave radiator; and a secondary substrate providing a secondary bending wave radiator, wherein the primary and secondary substrates are provided in face to face contacting relationship with one another, the exciter being arranged to excite bending wave vibrations in the primary substrate, the primary substrate being arranged in turn to excite bending wave vibrations in the secondary substrate.

It is to be understood that the primary and secondary substrates are each sufficiently stiff to support bending waves.

Embodiments of the invention have the advantage that a substantial improvement in frequency response of a bending wave loudspeaker system may be enjoyed. It is to be understood that the secondary substrate alone may be such that bending waves of a certain frequency or range of frequencies are not excited sufficiently therein in the absence of the primary substrate. The provision of a primary substrate in the manner described above in which those bending waves are sufficiently excited results in a substantial improvement in frequency response of the loudspeaker system. This is because bending waves that are not sufficiently excited in the secondary substrate alone may be excited in the primary substrate and transferred to the secondary substrate by the mechanical coupling between the first and second substrates.

Thus, the primary substrate may be considered to force' the establishment of bending waves in the secondary substrate. Bending waves of insufficiently high amplitude in the secondary substrate in the absence of the primary substrate may therefore be increased in amplitude.

In other words, it is to be understood that without a primary substrate, the total acoustic radiation energy (so called sound power") radiated into a 2pi sound field will be lower than if both a primary substrate and a secondary substrate are employed.

The primary and secondary substrates may be mechanically coupled in face to face contacting relationship.

In some embodiments the primary and secondary substrates may be integrally formed, for example as a substantially single or unitary body. In some embodiments the primary substrate may be formed to have a different stiffness to the secondary substrate or a different areal density, as described below, by forming the substrates as a substantially single or unitary body but varying composition or structure or other physical or chemical characteristic of the body. This may be accomplished in some embodiments for example by varying a density of a material, for example by adding a foaming agent to a portion of a material injected into a mould, such that the primary substrate is formed to have a lower stiffness and lower areal density than the secondary substrate by virtue of the presence of voids. Other arrangements are also useful. For example a variation in a thermal treatment such as a rate of cooling of different portions of a body such as a cast or injection moulded body may result in density and/or stiffness variation within the body.

In some embodiments the primary substrate may comprise an insert. For example the primary substrate may comprise a substrate embedded in a component providing the secondary substrate. For example the primary and secondary substrates may be formed by injection moulding of a polymer to form the secondary substrate and at the same time at least partially (or fully) encapsulating the primary substrate with polymer. Such embodiments may have the advantage that a subsequent mechanical joining step in order mechanically to couple the primary and secondary substrates may not be required.

Advantageously the primary substrate may be selected to be a substrate which in isolation from the secondary substrate has a first sound pressure-frequency response characteristic, the secondary substrate in isolation from the primary substrate having a second sound pressure-frequency response characteristic different from the first.

It is to be understood that a plot of sound pressure radiated by a loudspeaker a given distance from the speaker as a function of frequency may be referred to as a "frequency response curve". The term originates from the measurement of sound pressure level at frequencies between 2OHz-2OKHz.

It is to be understood that in some embodiments the primary substrate has a smaller areal density than the secondary substrate in order to provide a higher sound pressure level. At the same time the primary substrate may have a lower stiffness than the secondary substrate in order to enable low frequency sound pressure waves to be radiated. In some embodiments, a bending wave loudspeaker is provided that is arranged to radiate sound in substantially only the frequency range 4OHz-5KHz. In some embodiments this frequency range can be covered with substrates of medium stiffness.

It is to be understood that if it is desirable to provide a bending wave loudspeaker of high fidelity over the range 2OHz-2OKHz, a (single) relatively high stiffness substrate is typically provided, capable of radiating higher frequencies. Consequently, a separate sub-woofer system (comprising a substrate of lower stiffness) is typically required in order to obtain adequate sound pressure levels at lower (bass range) frequencies.

Advantageously an amount of sound power generated by the loudspeaker in or over a first frequency range may be greater than that generated in the same frequency range by the secondary substrate in isolation, for a given excitation thereof.

Optionally the first frequency range may be one selected from amongst from substantially 20Hz to substantially 200Hz, from substantially 20Hz to substantially 1kHz and from substantially 20Hz to substantially 5kHz.

It is to be understood that the primary substrate in isolation has a higher radiation efficiency compared to the secondary substrate, because the areal density of the secondary substrate is higher.

Surprisingly, when the primary and secondary substrates are driven by the exciter the assembly may generate audio frequency waves in the first frequency range having an amplitude much higher than in the absence of the primary substrate, i.e. with the exciter coupled to the secondary substrate directly. That is, the secondary substrate in combination with the primary substrate has an amplitude/frequency response that is an improvement in the first frequency range at least over excitation of the secondary substrate alone.

It is to be understood that, in general, bending wave loudspeaker systems such as one comprising the primary substrate alone, the secondary substrate alone, or a combination of primary and secondary substrates in face to face contact, are mass controlled. It is found that an amplitude of vibrations of a given substrate when a given force is applied by an exciter is reduced as a mass of one or more substrates driven increases.

However, in some embodiments, due to a larger number of bending wave modes which may be excited when a primary and secondary substrate are combined in the manner described, an increase in sound power radiated may be enjoyed. In some embodiments an increase in sound power radiated in a diffuse sound field inside a car may be enjoyed.

It is to be understood that, in the case where the secondary substrate is formed to have a relatively stiff, heavy structure as found for example in car interior trim panels or external body panels, the substrate may attenuate audio frequencies in the lower frequency range, which may correspond to the first frequency range. By providing a primary substrate that does not attenuate these frequencies to the same extent in face to face mechanical contact therewith, a substantial improvement in an amount of acoustic power radiated into a 2pi sound field may be enjoyed. As noted above, in some embodiments this is due to an increase in the number of vibrating modes.

Advantageously the primary substrate may have a stiffness that is lower than that of the secondary substrate.

The use of a primary substrate having a lower stiffness may improve (increase) an amount of lower frequency sound power radiated. The lower frequency may correspond to frequencies in the first range described above.

It is to be understood that an ideal substrate may be a substrate having an isotropic stiffness (or compliance) characteristic. That is, a stiffness or compliance of the substrate throughout is substantially identical about either of two orthogonal axes in a plane of the substrate. In practice, a substrate may have a certain degree of anisotropy. In some cases anisotropy may be caused for example by a manufacturing process in which a substrate is rolled out in one direction, for example paper, cardboard or metal materials.

Advantageously the primary substrate may have a stiffness that is lower (i.e. a compliance that is greater) than that of the secondary substrate.

Alternatively in some embodiments the primary substrate may be more stiff than the secondary substrate.

Alternatively or in addition the primary substrate may have an area density that is lower than that of the secondary substrate.

Advantageously the primary substrate may be lighter (having a lower area density) than the secondary substrate.

The primary and secondary substrates may be substantially coextensive.

Advantageously the secondary substrate may extend laterally beyond the primary substrate in at least one direction therefrom.

Optionally the secondary substrate extends laterally beyond the primary substrate around substantially the whole of the primary substrate.

It is to be understood that in flat panel bending wave speaker design it is important to excite as many bending modes of vibration as possible. Advantageously therefore, coupling between primary and secondary substrates may cover substantially the whole area of the primary substrate.

In some embodiments, the secondary substrate may be part of the interior of a vehicle and therefore be much bigger in size than the primary substrate. Since the secondary substrate is normally much bigger in size, the primary substrate can be arranged to make a full face-face contact.

Optionally the primary or secondary substrates are constructed from one selected from amongst paper, cardboard, aluminium, steel, a plastics material, optionally a plastics material selected from amongst polythene, HDPE, PET, PEEK, ABS, polycarbonate, an architectural material, optionally plasterboard.

Further optionally one or both of the primary and secondary substrates are constructed from one selected from amongst a honeycomb structured material, a foam material, a composite material and a corrugated material.

Optionally the secondary substrate comprises a panel of a vehicle.

Optionally the secondary substrate comprises a vehicle exterior trim panel, a vehicle interior trim panel, or a vehicle body panel.

Advantageously the primary substrate and second substrate may have substantially the same mechanical impedance. In addition or instead the primary substrate and second substrate may have substantially the same acoustic impedance.

Optionally the mechanical and/or acoustic impedance of the primary panel may have a value that is within one selected from amongst 10% and 20% of the corresponding value of the secondary panel.

In a further aspect of the invention for which protection is sought there is provided an audio apparatus comprising a loudspeaker according to the preceding aspect.

The apparatus may comprise an audio amplifier arranged to drive the loudspeaker to generate Sound.

In an aspect of the invention for which protection is sought there is provided a motor vehicle comprising audio apparatus according to the preceding aspect.

Optionally the secondary substrate comprises an interior trim panel of the vehicle.

Alternatively the secondary substrate may comprise an exterior panel.

The exterior panel may comprise one selected from amongst a wing panel, a bonnet, a window, a roof, a boot lid and an exhaust system component.

In one aspect of the invention for which protection is sought there is provided a method of producing sound by means of a bending wave loudspeaker, the method comprising driving by means of a vibration exciter a primary substrate of the loudspeaker thereby to excite bending wave vibrations in the primary substrate, the primary substrate providing a primary bending wave radiator; and driving by means of the primary substrate a secondary substrate provided in face to face contacting relationship with the primary substrate thereby to excite bending wave vibrations in the secondary substrate, the secondary substrate providing a secondary bending wave radiator.

It is to be understood that the secondary substrate may have material properties which may limit the frequency spectrum of sound radiated therefrom. However, by coupling mechanical vibration energy of a primary substrate into the secondary substrate, an increase in quality of sound radiation may be enjoyed. That is, sound radiated may more faithfully reproduce sound provided in the form of an audio signal to the exciter.

It is to be understood that by driving the primary substrate in face to face contact with the secondary substrate an amount of sound power radiation generated by the loudspeaker may be increased compared with driving the secondary substrate only. In some embodiments, an increase in the number of vibrating modes of the secondary substrate may occur due to driving of the primary substrate in face to face contact with the secondary substrate.

Thus it is to be understood that in some embodiments the increase of acoustic power radiation into a 2pi Sound field may be caused by an increase in the number of vibrating modes.

The method may comprise the step of driving the primary substrate to increase the amount of sound power radiated by the loudspeaker over a prescribed frequency range.

The prescribed frequency range may be the range from substantially 20Hz to substantially 5kHz.

In some embodiments the step of driving the primary substrate to increase the amount of sound power radiated by the loudspeaker over a prescribed frequency range may include the step of increasing the amount of acoustic power radiated into a 2pi sound field by increasing the number of vibrating modes of the loudspeaker.

Advantageously the method may comprise the step of selecting the secondary substrate to be an automotive panel.

Further advantageously the method may comprise the step of selecting the secondary substrate to be one selected from amongst an interior automotive trim panel and an exterior automotive panel.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination. Features described with reference to one embodiment are applicable to all embodiments, unless such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which: FIGURE 1 is a schematic illustration of a bending wave loudspeaker according to an embodiment of the present invention; and FIGURE 2 is a plot of sound pressure level at a distance of lm ahead of a bending wave loudspeaker according to an embodiment of the invention as a function of frequency.

DETAILED DESCRIPTION

FIG. 1 shows a bending wave loudspeaker 101 according to an embodiment of the present invention. The loudspeaker 101 has an exciter 110 having electrical terminals hOT arranged to be connected to an audio amplifier (not shown). The exciter is mechanically coupled to a primary substrate 120 which is in turn mechanically coupled to a secondary substrate 130. In the embodiment shown the exciter is connected to the primary substrate in face to face contact therewith by means of an adhesive. The primary and secondary substrates 120, 130 are also bonded in face to face contact with one another by means of an adhesive.

It is to be understood that other means for mechanically connecting the exciter and the primary and secondary substrates 120, 130 to one another are also useful including mechanical fixing elements. Importantly, the coupling between the primary and secondary substrates 120, 130 is such that bending (or flexural) modes established in the primary substrate 120 may be communicated directly to the secondary substrate 130.

When an audio frequency electrical signal is applied to the terminals hOT of the exciter 110 the exciter 110 is configured to apply vibrational excitations to the primary substrate 120.

These are communicated to the secondary substrate 130 by virtue of the mechanical coupling noted above.

In the embodiment shown the primary substrate 120 is formed from a corrugated plastics board material of rectangular shape. The substrate 120 has a sandwich structure, wherein an inner sheet 122 of a plastics material of corrugated or undulating form is sandwiched between first and second outer sheets 121, 123, also formed of plastics material. In some embodiments the inner sheet 122 is bonded to the outer sheets 121, 123 by means of heat welds although an adhesive may be used in some embodiments.

Other types of primary substrate material are also useful, such as aluminium/plastics material/aluminium sandwich panels. The plastics material may be polypropylene although other materials are also useful. Honeycomb sandwich panels with different skins such as metal, plastics, wood or other material are also useful in some embodiments. Monolith substrates are also useful in addition to or instead of sandwich or other composite panels.

The secondary substrate 130 is in the form of an interior trim panel of a motor vehicle. In one embodiment the substrate 130 is an interior door trim panel. In some other embodiments the substrate 130 is part of a dashboard of a motor vehicle. Other panels may also be used including internal or external panels.

The trim panel 130 of FIG. 1 is fabricated from an acrylonitrile butadiene styrene (ABS) plastics material. It is to be understood that in some embodiments the secondary trim panel may be formed from a different plastics material, such as polythene, high-density polyethylene (HDPE), polyethylene terephthalate (PET), polyether ether ketone (PEEK), polycarbonate, or any other suitable plastics material. Non-plastics materials are also useful including metals, wood, glass or any other suitable material.

Motor vehicle trim panels are typically relatively stiff and heavy. As such their ability to radiate bass audio frequencies (e.g. in the range from around 50Hz to around 5kHz, or 50Hz to around 3kHz, or 50Hz to around 200Hz) is inferior to that of lighter, less stiff (more compliant) panels. Bass audio frequencies tend to be attenuated to a much larger extend that higher frequency sounds. If an exciter 110 is coupled directly to such a panel, the faithfulness of the reproduction of bass audio frequencies may be found to be inadequate.

In some applications it is important that faithful reproduction of sounds in the bass range is obtained, for example when it is required to generate the sound of an internal combustion engine or an exhaust. This may be desirable in vehicles having artificial sound generators for generating interior or exterior sounds to enhance user enjoyment of a vehicle.

The present applicant has found that, surprisingly, if a lighter, less stiff primary substrate such as that described above fabricated from a corrugated plastics panel material is bonded directly to a secondary substrate in the form of a motor vehicle trim panel and excited by means of an exciter 110, a substantial improvement in a listener's enjoyment of bass frequencies may be obtained.

The coupling of the primary panel to the secondary panel allows substantially improved bass reproduction as well as excellent reproduction in the mid and treble frequency ranges without a requirement to provide a separately excited sub-woofer speaker. ii

Conventional piston-driven speaker systems typically include three types of speaker in a single installation -a sub-woofer bass speaker, a mid-range speaker and a high frequency speaker or tweeter -in order to cover the majority of the audio frequency range required for voice and music reproduction. Separate exciters (or drivers) are required for each of these speakers. In contrast, embodiments of the present invention enable a high quality audio speaker driven by a single exciter, reproducing faithfully a bass frequency range as well as a mid frequency range, to be provided integral to an existing trim panel of a motor vehicle.

It is to be understood that when driven by an audio amplifier, the exciter 110 is arranged to excite bending wave vibrations in the primary substrate 120. The bending wave vibrations established in the primary substrate 120 in turn excite bending wave vibrations in the secondary substrate 130.

FIG. 2 is a schematic plot of sound pressure level at a distance of one metre from three different loudspeakers as a function of frequency for a given amplitude of excitation over a range of frequencies from 20Hz to 20kHz. Trace Ti is a plot obtained when the primary substrate 120 only is bonded to an exciter 110. Trace T2 is a plot obtained when the secondary substrate (motor vehicle trim panel) 130 only is bonded to an exciter 110. Trace T3 is a plot obtained from loudspeaker 101 as described above, with the exciter 110 bonded to the primary substrate 120 and the primary substrate 120 bonded to the secondary substrate 130. It can be seen that a substantial improvement in frequency response is obtained when the primary and secondary substrates 120, 130 are bonded together (trace T3) compared with that from the secondary substrate 130 alone (trace T2).

Thus it is to be understood that embodiments of the present invention have the advantage that an audio performance of a motor vehicle trim panel acting as a panel speaker may be substantially enhanced by coupling a primary substrate of lower stiffness thereto between the trim panel and exciter. In some embodiments a primary panel of lower stiffness and lower weight than the secondary panel may be particularly advantageous.

It is to be understood that the primary and secondary substrates need not be flat, and non-flat planar substrates such as curved substrates are also useful.

In some embodiments the secondary panel 130 may be an exterior vehicle panel or component such as a bonnet (or hood), a front wing, a rear wing, a tailgate, a lid of a boot (or trunk), a roof, a floor pan, a door, a lighting cluster lens or any other suitable component.

The primary panel may be attached to the component providing the secondary panel on a concealed interior side thereof where appropriate, and arranged to be excited by an exciter as described above. The vehicle may have a controller arranged to drive the exciter. The controller may be arranged to excite the exciter with a waveform corresponding to a sound of an engine, an exhaust or other sound. The waveform generated by the controller may be responsive to a speed of an engine or motor of the vehicle. In some embodiments the waveform may be responsive to a virtual speed of an engine in some cases. For example, in the case of a hybrid electric vehicle operating with the engine off, the controller may be arranged to synthesise an engine sound that corresponds to the sound that would be generated by the engine if the engine were switched on. In the case of an electric vehicle, the controller may be arranged to synthesise an engine sound that corresponds to the sound that would be generated by the vehicle if the vehicle had an engine.

In some embodiments the controller may be arranged to synthesise an engine sound such that an actual sound of the vehicle in combination with an engine already installed in the vehicle provides enhanced user enjoyment. For example in some embodiments a vehicle having a six cylinder engine may be made to sound like a vehicle having an eight cylinder engine. Furthermore, a sound of exhaust gases passing through an exhaust pipe of a vehicle may be synthesised to enhance user enjoyment of a sound of the exhaust.

As described above, the primary and secondary substrates 120, 130 may be formed to have mechanical impedances that are as close as possible to one another within the appropriate constraints such as cost, size, weight and one or more other parameters. In some embodiments the primary and secondary substrates 120, 130 may be formed to have acoustic impedances that are as close as possible to one another within the appropriate constraints. In some embodiments the mechanical and/or acoustic impedances may be substantially different from one another.

Throughout the description and claims of this specification, the words comprise" and "contain" and variations of the words, for example comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

CLAIMS: 1. A bending wave loudspeaker comprising: a vibration exciter; a primary substrate providing a primary bending wave radiator; and a secondary substrate providing a secondary bending wave radiator, wherein the primary and secondary substrates are provided in face to face contacting relationship with one another, the exciter being arranged to excite bending wave vibrations in the primary substrate, the primary substrate being arranged in turn to excite bending wave vibrations in the secondary substrate.
2. A loudspeaker as claimed in claim 1 wherein the primary substrate is selected to be a substrate which in isolation from the secondary substrate has a first sound pressure-frequency response characteristic, the secondary substrate in isolation from the primary substrate having a second sound pressure-frequency response characteristic different from the first.
3. A loudspeaker as claimed in claim 1 or claim 2 arranged wherein an amount of sound power generated by the loudspeaker over a first frequency range is greater than that generated over the first frequency range by the secondary substrate in isolation, for a given excitation thereof.
4. A loudspeaker as claimed in claim 3 wherein the first frequency range is one selected from amongst from substantially 20Hz to substantially 200Hz, from substantially 20Hz to substantially 1kHz and from substantially 20Hz to substantially 5kHz.
5. A loudspeaker as claimed in any preceding claim wherein the primary substrate has a stiffness that is lower than that of the secondary substrate.
6. A loudspeaker as claimed in any preceding claim wherein the primary substrate has an area density that is lower than that of the secondary substrate.
7. A loudspeaker as claimed in any preceding claim wherein the primary substrate has a volume density that is lower than that of the secondary substrate.
8. A loudspeaker as claimed in any preceding claim wherein the primary and secondary substrates are substantially coextensive.
9. A loudspeaker as claimed in any one of claims 1 to 7 wherein the secondary substrate extends laterally beyond the primary substrate in at least one direction therefrom.
10. A loudspeaker as claimed in claim 9 wherein the secondary substrate extends laterally beyond the primary substrate around substantially the whole of the primary substrate.
11. A loudspeaker as claimed in any preceding claim wherein the primary or secondary substrates are constructed from one selected from amongst paper, cardboard, aluminium, steel, a plastics material, optionally a plastics material selected from amongst polythene, HDPE, PET, PEEK, ABS, polycarbonate, an architectural material, optionally plasterboard.
12. A loudspeaker as claimed in any preceding claim wherein one or both of the primary and secondary substrates are constructed from one selected from amongst a honeycomb structured material, a foam material, a composite material and a corrugated material.
13. A loudspeaker as claimed in any preceding claim wherein the secondary substrate comprises a panel of a vehicle.
14. A loudspeaker as claimed in any one or claims 1 to 12 wherein the secondary substrate comprise a vehicle exterior trim panel, a vehicle interior trim panel, or a vehicle body panel.
15. A loudspeaker as claimed in any preceding claim wherein the primary substrate and second substrate have substantially the same mechanical and/or acoustic impedance.
16. A loudspeaker as claimed in claim 15 wherein the mechanical and/or acoustic impedance of the primary panel has a value that is within one selected from amongst 10% and 20% of the corresponding value of the secondary panel.
17. Audio apparatus comprising a loudspeaker as claimed in any preceding claim.
18. Apparatus as claimed in claim 17 comprising an audio amplifier arranged to drive the loudspeaker to generate sound.
19. A motor vehicle comprising audio apparatus as claimed in claim 17 or 18.
20. A motor vehicle as claimed in claim 19 wherein the secondary substrate comprises an interior trim panel.
21. A motor vehicle as claimed in claim 19 wherein the secondary substrate comprises an exterior panel.
22. A motor vehicle as claimed in claim 21 wherein the exterior panel comprises one selected from amongst a wing panel, a bonnet, a window, a roof, a boot lid and an exhaust system component.
23. A method of producing sound by means of a bending wave loudspeaker, the method comprising driving by means of a vibration exciter a primary substrate of the loudspeaker thereby to excite bending wave vibrations in the primary substrate, the primary substrate providing a primary bending wave radiator; and driving by means of the primary substrate a secondary substrate mechanically coupled in face to face contacting relationship with the primary substrate thereby to excite bending wave vibrations in the secondary substrate, the secondary substrate providing a secondary bending wave radiator.
24. A method as claimed in claim 23 comprising the step of driving the primary substrate in face to face contact with the secondary substrate to increase the number of vibrating modes of the secondary substrate.
25. A method as claimed in claim 24 comprising the step of driving the primary substrate to increase the amount of sound power radiated by the loudspeaker over a first frequency range.
26. A method as claimed in claim 25 wherein the first frequency range is one selected from amongst from substantially 20Hz to substantially 200Hz, from substantially 20Hz to substantially 1 kHz and from substantially 20Hz to substantially 5kHz.
27. A method as claimed in any one of claims 23 to 26 comprising the step of selecting the secondary substrate to be an automotive panel.
28. A method as claimed in claim 27 comprising the step of selecting the secondary substrate to be one selected from amongst an interior automotive trim panel and an exterior automotive panel.
29. A loudspeaker, audio apparatus, motor vehicle or method substantially as hereinbefore described with reference to the accompanying drawings.