ACOUSTIC DEVICE

Abstract: An acoustic device has a panel (11) capable of supporting resonant bending wave modes and transducers (19) mounted on the panel. Sometimes, the acoustic response of panel and transducers may have a whole-body resonance peak at a particular frequency. To control this, a mass (21) may be added to the panel, for example on a compliant coupling (23).

Description: TITLE: ACOUSTIC DEVICE DESCRIPTION FIELD OF THE INVENTION This invention relates to an acoustic device, in particular to an acoustic device using distributed resonant bending wave modes in a plate. BACKGROUND ART Distributed mode loudspeakers are described in O97/09842. A bending wave transducer excites resonant modes distributed in a plate, and creates sound thereby. However, such loudspeakers can suffer adverse effects from whole body modes, which are lower frequency vibrations of the plate moving as a whole. An example of such a mode can occur when a panel is placed in a shallow enclosure. If the outer edge of the panel is held in place, the centre of the panel can vibrate backwards and forwards; the frequency of this vibration will be affected by on the cushion of air trapped by the enclosure and can occur at frequencies around lOOOHz, which are relatively critical for sound production. Another example of a whole body mode may be the vibration mode associated with the panel vibrating on its resilient suspension. The discrete resonance caused by whole body modes may cause peaks in the frequency response. This is disadvantageous, so distributed mode devices would be normally designed so that they operate outside the frequency range of such modes. However, this is not always possible and so there is a need to reduce the effect of such modes on the performance of a distributed mode acoustic device. SUMMARY OF INVENTION According to the invention there is provided an acoustic device comprising a panel capable of supporting bending waves, and having a whole-body resonance at a whole-body resonance frequency, a transducer for exciting distributed bending waves modes in the panel to cause the panel to emit sound, and a mass fixed to the panel at a location spaced from the transducer to control the whole-body resonance. Preferably the mass is fixed to the panel by a coupling which decouples the mass from the panel at higher frequencies. The coupling may be a compliant coupling, such as a spring or compliant metal sheet to substantially decouple the mass from resonant bending waves m the panel at frequencies much higher than the whole-body resonance frequency. The compliance may also result from the elasticity of the skin and core of the panel. Alternatively or additionally, compliant adhesive may form the compliant coupling fixing the mass to the panel. At frequencies much higher than the resonance frequency of the mass on the compliant coupling the mass may be substantially decoupled from the panel. The compliance of the compliant coupling may be selected so that the resonance frequency of the mass on the compliant coupling is suitable for desired acoustic results; the resonance frequency of mass and compliant coupling may be above the whole body mode frequency. The fundamental frequency of bending wave modes is given m the isotropic case by f 0 = π/ V(B/μ) , where A is the area of the panel, B the bending stiffness, and μ the areal density m Kg/m 2 . In prior art distributed mode loudspeakers, this fundamental frequency is arranged to be suitably low, so that the panel has sufficiently distributed modes m the operating frequency range. However, it is possible m some applications that the panel may need to be stiffer than may be optimal, i.e. the panel may need to be "over-stiff", with a value of bending stiffness B higher than would normally be chosen to reduce the fundamental frequency. In such cases reduction of the effects of the whole body mode may be particularly useful. The mass fixed to the panel may be m proportion to the effective moving mass of the panel, i.e. between 10% or 20% of the effective moving mass and 200% or 300% of that mass. Lightweight multimedia speakers may have, for example, an effective moving panel mass of say 8 gram; for such an application a mass between 1 and 10 gram may need to be fixed to the panel. However, the mass must be varied depending on a number of parameters, such as the mass of the system, the relevant spring constants and indeed the whole mechanical circuit of the assembly. The mass is preferably fixed to the panel m a near- central location. This choice is not to do with modal drive but takes into account the consideration that the mass should be located relatively near to the centre of mass of the panel so that the added mass can control whole body modes. The preferred location may be one which couples more weakly to the lower resonant modes other than the whole body mode. The location may be found by systematic experiment or computed analysis of the mechanical circuit of the panel. An enclosure may be provided behind the panel; the enclosure may be open or closed, m particular the enclosure may conveniently be shallow. The enclosure may act as a baffle. The invention also relates to a method of making an acoustic device, following the steps of providing a panel capable of supporting bending waves, and a transducer on the panel, exciting distributed bending waves modes in the panel using the transducer to cause the panel to emit sound, determining the frequency response of the panel, identifying a whole-body resonance at a whole-body resonance frequency, and applying a mass to the panel at a particular location to control the whole-body resonance. The frequency response of the panel may be determined by using systematic analysis or mathematical calculation. Once one acoustic device has been made, it may simply be replicated by using a like panel, with a like transducer and a like mass mounted thereupon in a like manner, without needing to determine the whole-body resonance frequency for every unit made. The step of fixing the mass to the panel may be carried out using a compliant coupling. BRIEF DESCRIPTION OF THE DRAWINGS A specific embodiment of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which Figure 1 shows a side view of a loudspeaker according to the invention, Figure 2 shows a view of a loudspeaker according to the invention, Figure 3 shows the frequency response of the loudspeaker without the additional mass, and Figure 4 shows the frequency response of the loudspeaker with the additional mass. DETAILED DESCRIPTION A panel 11 is mounted on a frame 13. The panel 11 has a thickness of 0.35mm, a length of 206.5mm, a width of 147mm, a mechanical impedance of 6.22Nm, a fundamental frequency of 203Hz and a coincidence frequency of 7.8kHz. The frame 13 is a rear structure that defines and encloses a shallow enclosure 15 of volume 0.6 litre. An optimised panel of the same material might have thickness 0.35mm, mechanical impedance of 6.22 Nm, length 333mm, width 284mm, fundamental frequency 78Hz and coincidence frequency of 7.8KHz. The fundamental frequency of the panel according to the invention is much higher, 203Hz against 78Hz. The panel 11 is resiliently supported on the frame 13 by 4mm thick double sided foam tape 17 adhered around the perimeter of the panel 11, to provide a relatively compliant support. Other edge supports, such as a clamped support or a simply supported support may be provided if useful . As shown m Figure 2, two exciters 19 are mounted in positions taught to be advantageous by WO97/09842. The exciters are 25mm exciters wired m series to give a d.c. resistance of 9Ω; a parallel connection may also be possible . Figure 3 shows the measurement of the sound pressure level of the panel measured at lm as a function of frequency. A substantial peak around 500Hz-lKHz is believed to show the effect of the whole body mode caused by the panel 11 vibrating on a cushion of air m the enclosure 15, the whole body mode being controlled by the compliance of air m the enclosure. A mass 21, i.e. a small weight, was then added to the panel 88mm along and 63 mm from the side. The mass was fixed with a piece of thin compliant plastic 23 with adhesive on each side, similar to conventional sticky tape. The mass was 3.1g. As can be seen m Figure 4, the peak is substantially controlled without causing significant change at higher frequencies . The intention is that the mass 21 selectively controls the output, sufficiently reduces the sensitivity and lowers the resonant frequency substantially at the whole body mode frequency. The mass 21 becomes increasingly decoupled from the panel 11 at higher frequencies as the panel becomes more densely modal . The position of the mass clearly is important for fine-tuning the control. The position, mass and compliance of the mounting of the mass on the panel may be adjusted to control the response of the system, for example to provide a more even acoustic response as a function of frequency.

Claims: CLAIMS 1. An acoustic device comprising a panel capable of supporting bending waves, and having a whole-body resonance at a whole-body resonance frequency, a transducer for exciting distributed bending waves modes m the panel to cause the panel to emit sound, and a mass fixed to the panel such that the mass controls the whole-body resonance. 2. An acoustic device according to claim 1 wherein the mass is fixed to the panel by a coupling which substantially decouples the mass from the panel at higher frequencies . 3. An acoustic device according to claim 2 wherein the coupling is a compliant coupling which substantially decouples the mass from the panel at frequencies much higher than the whole-body resonance frequency. 4. An acoustic device according to any preceding claim wherein the frequency response of the panel, absent said mass, has a peak at the whole body frequency, and wherein the peak m frequency response is substantially smoothed m the acoustic device having the said mass fixed to the panel . 5. An acoustic device according to any preceding claim wherein the mass is between 10% and 300% of the effective moving mass of the panel . 6. An acoustic device according to any preceding claim wherein the mass is fixed to the panel m a location close to the centre of mass of the panel . 7. An acoustic device according to any preceding claim further comprising a rear structure defining an enclosure behind the panel . 8. An acoustic device according to any preceding claim further comprising a baffle around the edge of the panel . 9. A method of making an acoustic device comprising providing a panel capable of supporting bending waves, and a transducer on the panel, exciting distributed bending waves modes m the panel using the transducer to cause the panel to emit sound determining the frequency response of the panel, identifying a whole-body resonance at a whole-body resonance frequency, and applying a mass to the panel at a particular location to control the whole-body resonance. 10. A method according to claim 9 further comprising the step of making a further acoustic device by substantially replicating said panel with the mass and transducer applied. 11. A method according to claim 9 or 10 wherein the step of fixing the mass to the panel is carried out using a compliant coupling. 12. A method according to any of claims 9 to 12 wherein the frequency response of the panel, absent said mass, is determined to have a peak at the whole body frequency, and wherein the peak m frequency response is substantially smoothed m the acoustic device having the said mass applied to the panel.

Inventors: ELLIS CHRISTIEN, ELLIS, CHRISTIEN
Applicants: ELLIS CHRISTIEN, NEW TRANSDUCERS LTD, ELLIS, CHRISTIEN, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: H04R
IPC codes: H04R1, H04R7