ACOUSTIC DEVICE

Abstract: An acoustic device has a plurality of resonant bending wave modes along the length of a member (1). The fundamental frequency of resonant bending wave modes in directions perpendicular to the length is much higher, so that the lower frequency resonant bending wave modes are substantially one directional. A plurality of transducers (5) may be spaced across the width of the member (1) at a preferred position (3) along the length of the panel (1).

Description: TITLE: ACOUSTIC DEVICE The invention relates to an acoustic device, and in particular to an acoustic device of the type that uses resonant bending wave modes . A prior resonant bending wave device is described in WO97/09842. This document describes a panel having resonant bending wave modes in the area of the panel. A transducer may be provided at a preferential location on the panel for exciting the resonant modes, to produce a loudspeaker or microphone. Such a device is known as a distributed mode loudspeaker. US 3 347 335 describes a loudspeaker in which bending waves are sent along a beam. In this device the bending waves are excited at one end of the beam and a non- reflecting termination is provided at the other. Since the termination is non-reflecting, the bending waves will travel down the beam, be absorbed and will not reflect back to form resonant modes . According to the invention there is provided an acoustic device comprising a member having a modal axis along which axis there are a plurality of resonant bending wave modes, and non-modal axes perpendicular to the modal axis, wherein the fundamental frequency of the resonant modes along each non-modal axis is at least five times the fundamental frequency of the resonant modes along the modal axis . Preferably, the fundamental frequency of the resonant modes along each non-modal axis is at least ten times the fundamental frequency along the modal axis . The higher the fundamental frequency along the non-modal axis compared to along the modal axis, the more the acoustic device can be said to be "one-dimensional". The member may be a panel with the modal axis along the length of the panel and a non-modal axis along the width of the panel . The panel need not be flat . When a resonant bending wave mode is excited in a panel it will cause the panel to displace by a small amount out of the plane of the panel . The amount of this displacement will vary along a direction in the plane of the panel, and it is the direction along which the displacement varies and not the direction of the displacement itself that is meant when a bending wave mode is said to be along a particular direction. The fundamental frequency along a particular axis is the frequency of the lowest bending wave mode along that axis. The density of modes along an axis is related to the fundamental frequency along that axis: in a broad frequency range there will be more resonant modes along an axis with a low fundamental frequency than along an axis with a higher fundamental frequency. For comparison, the prior art document WO97/09842 teaches interleaving the frequencies of the modes along the long and short axes, which requires similar fundamental frequencies. That document teaches isotropic panels with aspect ratios of 1.134 or 1.41, which correspond to ratios of fundamental frequencies of 1.285 and 2 respectively. The fundamental frequency f 0 along an axis of a panel may be related to the panel bending stiffness B (about a perpendicular axis) and the panel length along the axis by the proportional relationship (which assumes constant mass per unit area) (f 0 ) 2 α B/L 4 . It will be seen that in order to achieve a high ratio of the fundamental frequency along the width axis over that along the length axis the width may be less than half, preferably less than a third of the length. The sound emitted from a panel is anisotropic at frequencies where resonant bending wave modes along the modal axis, but not the non-modal axis, are excited. In such frequency ranges sound is preferentially emitted into a plane perpendicular to the panel through the modal axis, and reduced in a plane perpendicular to the modal axis through the non-modal axis. This can give rise to enhancement of the sound into the plane through the modal axis at these frequencies. Accordingly the panel may be particularly suitable for use with piezoelectric transducers, which have a frequency response which tails off at low frequencies. The increased low frequency sound output can compensate for this tailing off of excitation to provide a more even sound overall . The preferential sound radiation into a single plane can also be useful in some specific applications, for example to direct sound into a horizontal plane in a room and avoid sending too much sound to a ceiling or floor of the room. The preferential emission of sound into a plane is greatest for a flat panel, rather than a rod, and increases with increased width. However, this assumes that the one-dimensionality can be maintained and that modes along the non-modal axis of the panel are not excited. This latter condition requires a narrow width. In order to achieve the contradictory requirements of one- dimensional behaviour but with a panel of significant width a highly anisotropic panel may be used. The panel may be stiffer to bend about the modal axis than about the non-modal axis. The bending stiffness of the panel about the modal axis panel may be at least 1.5 times that about the non-modal axis, further preferably at least twice as stiff. Since the resonant bending wave modes along an axis cause bending about a perpendicular axis, if the panel is stiffer to bend about the modal axis this will reduce the number of modes along the non-modal axis . A panel having anisotropic bending stiffness may be made of a material having a corrugated or cellular structure, with the cells or corrugations running in the plane of the panel along the non-modal axis. In embodiments, a transducer may be provided to excite the resonant bending wave modes. The transducer may preferably be placed at a location at which spaced away from the nodes of the lower modes along the modal direction. To achieve this, the transducer may be placed at a preferred location along the length of the member, for example at substantially 4/9, 3/7 or 5/13 of the length along the modal axis. These locations are similar to those taught in WO97/09842, except that in that document the preferred locations have these coordinate values in both directions. The transducer need not be placed on the modal axis, but may be placed laterally thereof . A plurality of transducers may be provided. To provide multiple transducers at one preferred location a plurality of transducers may be placed side by side across the width of the panel. This can provide increased output. Alternatively, a single transducer may extend across the width of the panel at a preferred location. Such a transducer can be effective even if it only causes bending along one axis. A bending transducer extending across the width of the panel may be able to provide greater power than a single point-like transducer for use on a two-dimensional panel which cannot have a significant spatial extent. It may also be possible to excite the panel at a less-preferred location, for example a location nearer one end than the preferred location. It is possible to vary the bending stiffness along the modal axis so that other positions than those mentioned above become preferred. Alternatively, it may be possible to damp or clamp the panel in some way to improve the efficiency of the panel even when excited at a less preferred location. For a better understanding of the invention a specific embodiment will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an acoustic device according to the present invention, Figure 2 shows the output of the panel shown in Figure 1 as a function of frequency at three directions in a plane perpendicular to the panel and along the modal direction, and Figure 3 shows the output of the panel shown in Figure 1 as a function of frequency at three directions in a plane perpendicular to the panel and along the non-modal direction. A rectangular panel 1 is substantially flat extending in the x (length) and y (width) directions as shown. The panel is anisotropic in bending stiffness and is much narrower than it is long. It is also much stiffer about the x axis than the y axis. Accordingly, the fundamental frequency is much less along the x axis, the modal axis, than along the non-modal y- axis. Therefore, there are many more resonant bending wave modes along the x axis than along the y axis. A plurality of transducers 5 are arranged spaced apart from one another in the y direction along a line 3 extending across the width of the panel. The line 3 is spaced from one end of the panel along the length of the panel at a distance of four ninths of the length of the panel in the x direction. The plurality of transducers can input more power into the panel than would be possible with a single transducer. There are fewer constraints in applying multiple transducers in the present apparatus than there are in applying multiple transducers to a distributed mode panel according to WO97/09842, since in such two-dimensional panels the transducer position is constrained to a preferred location whereas in one- dimensional panels the location is constrained merely to be a preferred distance along the length of the panel . The transducers 5 are connected to a conventional amplifier by leads 7; they are conventional bending wave transducers . They can be piezoelectric transducers . The sound pressure level in dB produced by such a panel has been measured as a function of frequency. Figure 2 shows the sound pressure level "on axis", i.e. perpendicular to the plane of the panel, and at two further directions offset by 45° and 60° from that axis towards the x direction. Figure 3 shows the sound pressure level "on axis", i.e. perpendicular to the plane of the panel, and at two further directions offset by 45° and 80° from that axis towards the y direction. Thus Figure 3 shows sound pressure levels emitted sideways and Figure 2 shows sound pressure levels emitted along the length of the panel . The sound pressure levels are measured at a distance of lm from the panel. The panel measured is made from a corrugated polymer sold under the trade mark "Correx" . It is about 2.83 times stiffer about the modal axis than about the non- modal axis. The sound energy is not very directional in the plane of the modal axis (see Figure 2) . The high frequencies are radiated to a very wide angle, and the mid frequencies are only slightly reduced off axis. This curve is similar to the curve obtained from a classic distributed mode panel as taught, for example, by WO97/09842. In contrast, in the plane of the non-modal axis the sound pressure level is strongly reduced away from the axis at high frequencies, and maintained at mid frequencies (see Figure 3). The measurements show that little sound is emitted sideways. In order to increase the effect, the width of the panel can be increased. When the panel is wide, the wavefronts become cylindrical and the low frequency output rises at 3dB per octave as the frequency is lowered. This can compensate for a falling output from a piezoelectric driver at these frequency ranges. However, as can be seen from the relation presented above there are limits to the width of the panel in order that the fundamental frequencies remain different enough for effective one- dimensional behaviour.

Claims: CLAIMS 1. An acoustic device for operation in a predetermined frequency range comprising a member having a modal axis, and a non-modal axis orthogonal to the modal axis, wherein the member can support a plurality of resonant bending wave modes in the predetermined frequency range along the modal axis, and the fundamental frequency of resonant bending wave modes along the non-modal axis is at least five times the fundamental frequency of the resonant bending wave modes along the modal axis. 2. An acoustic device according to claim 1 wherein the fundamental frequency of the resonant modes along the non- modal axis is at least ten times the fundamental frequency along the modal axis . 3. An acoustic device according to claim 1 or 2 wherein the member is in the form of a panel having a length and a width wherein the modal axis is along the length of the panel and the non-modal axis across the width of the panel . 4. An acoustic device according to claim 3 wherein the width of the panel is less than half the length of the panel . 5. An acoustic device according to claims 3 or 4 wherein the bending stiffness of the panel about the modal axis is at least 1.5 times the bending stiffness of the panel about the non-modal axis . 6. An acoustic device according to claim 5 wherein the panel has a corrugated or cellular structure, with the corrugations or cells running along the non-modal axis. 7. An acoustic device according to any preceding claim further comprising a transducer coupled to the member for exciting the resonant bending wave modes. 8. An acoustic device according to claim 6 wherein the transducer is placed at a location spaced away from the nodes of a predetermined plurality of lower frequency resonant bending wave modes . 9. An acoustic device according to claim 8 wherein the transducer is placed at or laterally of a position substantially 4/9, 3/7 or 5/13 along the modal axis of the member from either end of the member. 10. An acoustic device according to any of claims 7 to 9 wherein the transducer is a piezoelectric transducer. 11. An acoustic device according to any of claims 7 to 10 comprising a plurality of transducers. 12. An acoustic device according to any of claims 3 to 6 further comprising a plurality of transducers arranged across the width of the panel .

Inventors: BANK GRAHAM, HARRIS NEIL, BANK, GRAHAM, HARRIS, NEIL
Applicants: BANK GRAHAM, HARRIS NEIL, NEW TRANSDUCERS LTD, BANK, GRAHAM, HARRIS, NEIL, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: H04R
IPC codes: H04R1, H04R7, H04R17