ACOUSTIC DEVICE ACCORDING TO BENDING WAVE PRINCIPLE

Abstract: An acoustic device has an outer shell (1) supporting bending waves and enclosing a volume (2). The bending waves couple to the enclosed volume to provide coupled resonant modes. A transducer (3, 5) is coupled to the shell to excite the coupled resonant modes. The coupling of the enclosed volume can improve the distribution of resonant modes.

Description: ACOUSTIC DEVICE ACCORDING TO BENDING WAVE PRINCIPLE DESCRIPTION FIELD OF THE INVENTION This invention relates to acoustic devices of the type that use members that support bending wave action over the surface of the member, the bending waves in turn coupling to the ambient. Such devices may be used, for example, as loudspeakers or microphones . BACKGROUND TO THE INVENTION The International patent application WO97/09842 and related applications describe speakers and other acoustic devices having an acoustic member and a transducer coupled to the acoustic member. In these devices, the various parameters of the member may be adjusted so that resonant bending wave modes in the member are distributed evenly in frequency. The resonant bending wave modes may also be distributed over the surface of the member. Preferential locations for mounting the transducer on the member are also disclosed. A typical preferential mounting location is at a near-centre location, but not at the centre. However, other preferential locations may also be available depending on the shape of member. However, it is not always easy to provide sufficient modal density, especially at lower frequencies. Accordingly, it would be advantageous if improved modal density or other enhancement could be provided, especially to the lower or mid frequency response . SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided an acoustic device, comprising a substantially continuous outer shell supporting bending waves, the shell being bent to at least partially enclose an air volume so that the bending waves couple to the volume to provide coupled resonant modes, and a transducer coupled to the shell for coupling an electrical signal in the transducer with the coupled modes and hence in turn to ambient sound.. In the device according to the invention, in addition to the resonant bending wave modes available in conventional distributed mode devices, additional modes are present. Calculations will be presented later showing the increased number of modes with air coupled to the outer shell. Accordingly, the device according to the invention may increase the number of modes present in a predetermined frequency range . Preferably, the coupled resonant modes are evenly distributed in frequency over a predetermined frequency range. Usefully, this frequency range is around 1 to 2 or 3 octaves above the fundamental resonant frequency. It is in this range that the resonant bending wave modes are most sparse and in which distributing the bending and in-plane modes gives the greatest benefit. The outer shell may be fully closed to totally surround the volume. Alternatively, ports or vents may be provided in the outer shell . The ports or vents may be designed to provide specific resonance effects, and in particular enhance or control the output in the lower acoustic frequency range . Although it is not necessary that the outer shell fully surrounds the enclosed volume the outer shell must be substantially continuous so that it may demonstrate effective acoustic action. In other words, the outer shell must not have too many perforations or windows. Highly perforate members may not be suitable as acoustic radiators, since radiation from the front of the member will destructively interfere with radiation from the rear that is emitted in antiphase to radiation from the front. Moreover, the shell should be sufficiently continuous for coupling of the shell to the enclosed volume to be significant. Tests on panels have shown very low coupling of perforate members with ambient air. Accordingly, the outer shell may define holes in its surface of total area no greater than 20% of the surface area, preferably no greater than 10% and further preferably no greater than 5%. Air inside the volume may also exhibit cavity resonances . The acoustic device according to the invention supports resonant bending wave modes over the surface of a three- dimensional shell and coupling to a volume at least partially enclosed by the shell. In contrast, conventional distributed mode loudspeakers have resonant bending wave modes distributed over a single panel. In the aforementioned O97/09842 it is suggested to mount a distributed mode panel to the front of a frame. In such prior art devices any resonant bending wave modes are substantially restricted to the panel area, not the frame. Accordingly, such devices do not provide the improved modal density and hence acoustic performance offered by devices according to the invention. Another prior publication, WO 98/31188, describes a flat panel mounted in a tray. The tray is highly perforate, with more window area than solid area, and accordingly not substantially continuous. The tray will therefore not couple effectively to the ambient nor to the air within the tray. The outer shell may be of constant thickness. Alternatively, the thickness of the outer shell may be varied, either slowly or continuously or more rapidly. Ribs or other extensions may be provided on the outer shell. The shell may be a single integral shell. Alternatively, the outer shell may comprise a combination of members mechanically coupled to create the desired acoustical radiator structure. The boundary condition at the joins between the members may usefully be specified. The outer shell may be in the form of a polyhedron or part thereof having a plurality of individual faces. Each face may have a natural resonant frequency and the natural resonance frequencies may be selected to have different values. This can increase the modal density of the outer shell as a whole. Further, the different natural resonant frequencies may be selected so that the resonant modes on different faces are at interleaved frequencies. This approach may be particularly useful to increase the modal density for the lower 10 to 20 resonant modes. The individual faces or separate panel members need not have uniform mechanical properties and they may vary in stiffness, isotropy of stiffness, damping, or thickness. The acoustic device may be a loudspeaker, the transducer being an exciter. The acoustic device may comprise front and rear faces in the form of panels together with at least one further panel providing a path for resonant modes from front to rear and so coupling the front and rear panels. The front and rear panels may be substantially planar. The front and rear panels may be driven by separate discrete exciters, or a single exciter may be coupled to both the front and the rear panel. In embodiments, the voice coil of an exciter may be coupled to one of the panels and the magnet assembly of the exciter to the other of the panels. Since the magnet assembly is heavy, the coupling of it to the panel will result in high frequency roll-off. This may enhance the bass response of the acoustic device. A plurality of exciters may be provided. The exciters may be driven in- phase, out of phase or in any suitable phase relationship to one another. In a conventional flat panel loudspeaker any in-plane compression waves produce little or no acoustic output. This is because in-plane compression and expansion of a flat panel does not couple to ambient air. In contrast, in a device according to the invention the outer shell is bent, so in-plane compression and expansion causes shrinking and expansion of the shell, locally or globally, which acts as a mechanism to couple the compression waves to the enclosed volume and to the ambient. Accordingly, in-plane compression waves may usefully contribute to the coupled modes. In fact, the resonant modes may in some embodiments couple bending waves, in-plane modes and the enclosed volume. The modal density may accordingly be improved. According to a second aspect of the invention, there is a provided an acoustic device, comprising a substantially continuous shell enclosing a volume, supporting a plurality of resonant modes coupling the shell and the enclosed volume, the resonant modes spanning the shell, and a transducer coupled to the shell for coupling an electrical signal in the transducer with the resonant modes and hence in turn to ambient sound. In the device according to the second aspect of the invention, the resonant modes span the surface, from front to back, side to side and top to bottom. This allows good coupling of the modes over the whole surface with the enclosed volume. It is not necessary that the modes cover the whole surface; the shell may for example have ports or areas that do not resonate. According to a further aspect of the invention there is provided a method of driving an acoustic device comprising a substantially continuous outer shell supporting bending waves, the shell being bent to at least partially enclose an air volume so that the bending waves couple to the volume to provide coupled resonant modes, and two transducers coupled to the shell, including driving the two transducers in phase with a common electrical signal, so that the transducers drive the coupled modes of the shell and volume in a monopole configuration, and radiating sound energy from the coupled modes into the ambient air. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings in which: - Figure 1 shows sections through a loudspeaker according to a first embodiment of the invention having an ellipsoidal shell; Figure 2 shows a section through a loudspeaker according to a second embodiment; Figure 3 shows a section through a loudspeaker according to a third embodiment having a port ; Figure 4 shows a section through a modification of the port; Figure 5 shows a view of a loudspeaker according a fourth embodiment to the invention in the form of an open box; Figure 6 shows a view of a fifth embodiment of the invention having a closed box; Figure 7 illustrates various excitation techniques that can be used with the loudspeaker shown in Figure 6; Figure 8 illustrates the response to excitation in an open box without air; Figure 9 illustrates the response to excitation in the open box shown in Figure 8, including the effects of air coupling; Figure 10A shows the velocity response at the exciter as a function of frequency for the box modelled in Figure 8, without air; Figure 10B shows the velocity response at the exciter as a function of frequency for the box modelled in Figure 9, including the effects of air; Figure 11 shows the pressure inside the box of Figure 9B Figure 12 shows the modes in a loudspeaker having a closed box with two exciters driven in antiphase; Figure 13 shows the modes in the loudspeaker modelled in Figure 12 with two exciters driven in phase; Figure 14 shows the velocity response of the device modelled for Figures 12 and 13; Figure 15 shows the velocity response of a six sided closed box in which the bending stiffness of the front face does not match that of the rear face; Figure 16 shows views of a loudspeaker according to the invention in the form of a truncated pyramid; Figure 17 shows a view of a loudspeaker according to the invention in the form of a tetrahedron; Figure 18 shows a view of a loudspeaker according to the invention in the form of a dodecahedron; Figure 19 shows a view of a loudspeaker according to the invention in the form of a cylinder; Figure 20 shows a view of a loudspeaker according to the invention in the form of a cone section; Figure 21 shows the root mean square (rms) central difference of mode frequency as a function of aspect ratio of the front face of the device of Figure 5 , Figure 22 shows velocity profiles for three exciter positions of the device of Figure 5 having a front face aspect ratio of 2:1; and Figure 23 shows a figure of merit for exciter location as a function of exciter position on the device used in the model for Figure 22. DETAILED DESCRIPTION Referring to Figure 1, a closed ellipsoid shell (1) encloses a volume (2) and has transducers (3) , (5) mounted on the interior of the shell at opposed positions on the minor axis of the ellipsoid. The shell (1) supports resonant modes formed from resonant bending wave components coupled to the enclosed volume . The transducers couple an electrical signal to the coupled resonant modes of the shell and the volume. In the present embodiment, the transducers are exciters than can, in use, be driven to excite coupled modes to produce an acoustic output. The transducer may be of conventional type in which a voice coil moves relative to a grounded magnet assembly when electrical current is passed through the voice coil. The transducer may be inertial, in which case the magnet assembly is free and the force of the voice coil acts against the inertia of the magnet assembly. Alternatively, a grounded transducer may be used in which case the magnet assembly is supported. In the present embodiment, commercially available exciters normally used to drive distributed mode panels are used in an inertial configuration. By driving the transducers in a known polarity it is possible to produce a desired polar behaviour in emitted sound. For a monopole source, the transducers can be driven in phase, whereas to produce a dipole the transducers can be driven in antiphase. Alternatively, the exciters can be driven at any suitable phase relation. The invention allows a loudspeaker to be used unbaffled. In a conventional pistonic or distributed mode loudspeaker using a single diaphragm or panel the sound radiated from the rear is in antiphase with the sound radiated from the front. Accordingly, to avoid interference effects the sound radiated from the rear has to be prevented from reaching the front, by enclosing the diaphragm in a box or providing a baffle around the loudspeaker. By driving the loudspeaker according to the present invention as a monopole with two transducers operating in phase it is possible to avoid the need for such baffles. The coupled modes may be made up of two types of shell vibrations coupled to the enclosed volume. One of these types is bending waves that bend the shell out of the local plane of the shell. The other type is an expansion or contraction in the plane of the shell . A totally flat plate would not provide such coupled expansion-contraction modes with the resonant bending wave modes. Although a flat plate can have oscillation modes of expansion and contraction, these simply move the plate in its own plane and do not affect the motion of ambient air molecules. Accordingly, such modes in a flat plate have little or no acoustic effect. In contrast, if the plate is sufficiently bent back in on itself, or even forms a completely closed body, the in-plane compression wave modes cause a global expansion and contraction of the body, which can couple to air and hence have an acoustic effect. The actual modes of vibration of the shell need not be pure bending wave modes, nor pure compression wave modes. Rather, the modes may interfere and couple with one another to provide coupled modes. These modes may still however retain a predominant bending wave character. These waves in the shell then couple to the included volume to produce coupled resonant modes . It is not essential that the transducers are mounted on the minor axis. It may be convenient to mount them off axis, as illustrated in Figure 2, or indeed at any suitable location. It is preferred to mount the transducers at a position that is selected for optimum or desired response. The use of a regular geometry, such as an ellipsoid, makes this easier. Alternatively, approaches such as finite element analysis can be used to investigate suitable transducer locations. In general, approaches similar to those used for distributed mode loudspeakers may be suitable; in particular asymmetric transducer locations may prove suitable. An example of this will be discussed later with reference to Figures 21 to 23. A single transducer may be sufficient for some applications; others may require several transducers spaced over the shell . The placement of transducers may influence the directivity of coupling of the outer shell to the ambient. The provision of an enclosed volume allows the use of ports to control the resonance inside the volume. Figure 3 illustrates a port (7) in the form of a simple hole in one end of the ellipsoid. Alternatively, a ducted port (9) may be provided as illustrated in Figure 4. The port may result in effects analogous to effects that such ports produce in conventional pistonic-type loudspeakers, or pipes. The ports may have an asymmetric cross -sect ion. As indicated above, it is not necessary that the volume is wholly enclosed by the shell. Rather, all that is required is that the shell is sufficiently bent back on itself that resonant modes in the shell couple to the air in the enclosed volume so as to produce an acoustic effect. Figure 5 illustrates an open box comprising a large front face (11) surrounded by a frame (13) comprising four side faces (15) at right angles to the front face (11) . A single transducer (3) is provided, connected to an amplifier via audio connections (9) . The side faces (15) are all acoustically coupled to the front face (11) . Resonant bending wave modes in the front face (11) do not simply remain in the front face but couple round to the side faces (15) . A box can also be implemented in the form of a sealed enclosure (Figure 6) , with front and rear faces (11) , (17) and four side faces (13) joining the front (11) to the rear (17) face to form a sealed enclosure containing a volume. Two transducers (3) , (5) are provided, one on each face (11), (17). Also shown in Figure 6 is an electrical circuit (19) that can switch between inverting and non-inverting drive of the two transducers. A double pole double throw switch (21) switches between a parallel drive of the transducers and an anti-parallel drive. The transducers or front and rear faces can be uncoupled, as shown in Figure 7A. Alternatively, the magnet assemblies of two conventional moving coil transducers can be coupled together as shown in Figure 7B. As a further alternative, a single transducer can have its voice coils connected to the front face (11) and the magnet assembly connected to the rear face, (17) . The magnet assembly is much heavier than the voice coil, and so this assembly will preferentially couple low frequencies to the rear face. Accordingly, this arrangement can be used to increase the bass response of a loudspeaker. Front and rear faces may be reversed. Finite element calculation of the response of an acoustic device has been performed for a five-sided device similar to that shown in Figure 5. For convenience, this configuration will be referred to as an open box. Figure 8 shows the behaviour of the box in response to excitation, in the absence of air, at 178Hz (Figure 8A) , at 348Hz (Figure 8B) and at 1000Hz (Figure 8C) . Figure 9 shows the behaviour in the presence of air at the same frequencies, i.e. at 178Hz (Figure 9A) , at 348Hz (Figure 9B) and at 1000Hz (Figure 9C) . As can be seen, the responses are not restricted to any one of the planar surfaces, but instead couple over the whole of the five surfaces of the box. Moreover, the presence of air beneficially adds to the complexity of the shapes . The velocity response at the transducer as a function of frequency is shown in Figure 10: Figure 10A shows the results without air and Figure 10B shows the results with air. A large value indicates a high velocity achieved by excitation at that frequency. Particularly large velocities occur at resonance. As can be seen, the response with no air shows a smaller number of larger peaks. This is characteristic of a smaller number of resonant modes. The response when there is air enclosed shows a larger number of peaks, each of which is smaller. This is characteristic of a larger number of weaker modes. As can be seen, the coupling of the modes in the panel to the enclosed volume increases the number of resonant modes and so improves the acoustic device. What is surprising is that this effect is so marked even in an open box. The air pressure inside the box at 348 Hz is shown in Figure 11. The asymmetric air pressure pattern can clearly be seen. It is this air pressure distribution that causes the complex mode shapes of the air coupled resonant modes. Similar calculations have been carried out for an acoustic device similar to that shown in Figure 6, i.e. a closed six-sided box having a front, a rear and four side faces . Some of these results are shown in Figure 12 and 13. All the calculations include air. Again, the coupled modes couple over all of the surfaces of the box, front and back, left and right sides together with the top and bottom. Figure 12A illustrates the mode at 178 Hz caused by driving the closed box with the velocity in phase. Since the front and rear faces face the opposite directions, this is achieved with the transducer on the front panel moves the panel outwards when the transducer on the rear panel moves the panel inwards. This may be achieved by electrically connecting the transducers out of phase, for example using the switch shown in Figure 6. Figure 12B shows the oscillation at lOOOHz. Figures 13A and 13B show the same frequency modes with the box acting as a monopole with the front and rear panel moving in antiphase, i.e. with the electrical connections to the transducers in phase. As can be seen, a complex response is again obtained. Figures 12C and 13C shows the air pressure inside the box driven at lOOOHz as a dipole and a monopole respectively, corresponding to the response of the box shown in Figures 12B and 13B. Figure 12C clearly shows an asymmetric response, even though the drive and the box are symmetric. Figure 13C shows the very different pressure response just caused by driving the same box in a different way. Some of the transducer velocity against frequency graphs obtained with the closed box are presented in Figure 14. Figure 14A and 14B show that the response on the front and rear faces (driven as a monopole) of a symmetrical closed box match, as might have been expected. Figure 14C shows the significantly less even, and hence worse, results obtained for a dipole drive of the same box. Of course, all the above results are just calculations but they do show improvement possible using a shell bent back on itself to enclose a volume. Figure 15 shows the velocity response graph for a box in which the front face has a different stiffness from the rear, driven by two transducers one on the front (shown in Figure 15A) and one on the rear face (Figure 15B) . As can be seen, the response of the front face is beneficially different from that of the rear face. Accordingly, the use of asymmetry can beneficially increase the modal density in frequency. It should be noted that there is a difference between devices such as that illustrated in Figure 5 with a plurality of faces, and devices such as that of Figure 1 in the form of a continuous curve. The joins (23) between the faces act as hinges and so resonant bending wave modes do not simply travel from one facet to the next. Rather, a more complex coupling of the modes occurs. Other multi-faced structures are also possible. Figures 16 to 20 illustrate various such forms, namely a truncated square pyramid, a tetrahedron, a dodecahedron, a cylinder and a cone section. Each of these forms may be open or closed; for example the cylinder may be either with or without end faces, and the cone section may have a rear face, or not. The individual faces may be formed separately, and then joined, or groups of faces or even the whole structure may be integrally formed. As discussed in WO97/09842, good aspect ratios for an isotropic rectangular panel are 0.707:1 and 0.882 to 1. It is also possible to optimise acoustic devices according to the invention to maximise the distribution of resonant modes in frequency by adjusting the properties of the shell and to provide a good even coupling of a transducer to the modes by correctly locating the transducer on the panel. This may be done using the techniques discussed in various distributed mode patent applications. In particular, the use of an orderly approach to finding optimum aspect ratios and transducer locations to provide results that are as good as possible has been described in W099/41939, published 19 August 1999, in the names of New Transducers Ltd, etc. In order to find suitable properties for an open box, the first step that was carried out was to model the variation of the aspect ratio of the central panel in the open box of Figure 5. The aspect ratio was varied from 1 to 2.25, and the corresponding frequencies of the modes were calculated by finite element analysis . The root mean square central difference of mode frequencies plotted against aspect ratio (see Figure 21) . The central difference of mode frequency is, for the nth mode, the frequency of the (n+l)th mode plus that of the (n-l)th mode, minus twice the frequency of the nth mode. If the modes were equally spaced, this measure would equal zero. Accordingly, the root mean square (rms) central difference provides a Figure of merit for various aspect ratios. The smaller the rms central difference the better. From Figure 21 it may be seen that good results are obtained for aspect ratios between 1.6 and 2.2, with especially good results between 1.95 and 2.05. An aspect ratio of 2 was taken as a convenient value for further study. The next stage is to find the optimised drive point on the face. The velocity response as a function of frequency is calculated for several drive point positions. Figure 22 gives three examples, at the centre (22A) , at the standard drive point for a flat distributed mode panel (22B) , and at an optimum drive point (22C) . The standard deviation for graphs such as these is plotted as a function of position on Figure 23. The best results are those with the smallest deviation, shown in black. Note that the edges of the panel are not shown - these are poor drive points. As can be seen by inspecting the Figure the optimum drive point occurs in four regions, located around 30% of the distance along the long side and 30% of the distance along the short side, together with three other regions found by reflecting the first region about the central symmetry axis, to locations around 70% along the long axis and 30% along the short axis, 30% and 70% along the respective axes, and 70% and 70% along the respective axes. These positions are different from the optimum drive point for a simple rectangle, which occurs near centrally at coordinates around (3/7,4/9) expressed as a ratio of the distance along the sides. The positions are quite tolerant to variation and good results are obtained at positions from 14% to 42% along the long side and from 22% to 34% along the short side, together with reflections of these values. Although the above calculations are carried out without the influence of air being taken into account, they provide good indications of suitable aspect ratios and transducer locations even for a real device. Of course, features such as air coupling or slight anisotropy of the faces may move the optimum aspect ratios and drive positions slightly. The embodiments described above relate to loudspeakers, i.e. devices that convert electrical energy into sound. The methods of the present invention are equally applicable to microphones, in which incident sound energy is converted by a transducer to electrical energy.

Claims: Claims 1. An acoustic device, comprising a substantially continuous outer shell supporting bending waves, the shell being bent to at least partially enclose an air volume so that the bending waves couple to the volume to provide coupled resonant modes, and a transducer coupled to the shell for coupling an electrical signal in the transducer with the coupled modes and hence in turn to ambient sound. 2. An acoustic device according to claim 1 wherein the resonant modes span the shell . 3. An acoustic device, comprising a substantially continuous shell enclosing a volume, supporting a plurality of resonant modes coupling the shell and the enclosed volume, the resonant modes spanning the shell, and a transducer coupled to the shell for coupling an electrical signal in the transducer with the resonant modes and hence in turn to ambient sound. 4. An acoustic device according to any preceding claim wherein the transducer is an exciter for exciting resonant modes so that the acoustic device functions as a loudspeaker. 5. An acoustic device according to any preceding claims having a port in the outer shell. 6. An acoustic device according to claim 5, wherein the port includes a duct extending into the volume from the outer shell . 7. An acoustic device according to any proceeding claims wherein the outer shell comprises a plurality of faces. 8. An acoustic device according to claim 7 wherein each face has a natural resonant frequency and the natural 5 resonance frequencies have different values. 9. An acoustic device according to claim 8 wherein the different natural resonant frequencies are selected so that the ten to twenty lowest frequency resonant modes are at interleaved frequencies . 10 10. An acoustic device according to claim 7 wherein the outer shell includes a front face and the front face has an aspect ratio from 1.6 to 2.2. 11. An acoustic device according to claim 7 or 10 wherein the outer shell includes a rectangular front face and the 15 transducer contacts the front face at a location at between 14% to 42% from one edge along the long side and a distance of between 22% and 34% from one edge along the short side. 12. An acoustic device according to any of claims 7, 10 or 11 wherein the outer shell includes opposed front and 20 rear faces. 13. An acoustic device according to claim 12 wherein a first transducer is provided on the front faces, and a second transducer on the rear face. 14. An acoustic device according to claim 13 wherein the 25 first and second transducers are mechanically coupled. 15. An acoustic device according to claim 12 wherein a single transducer is coupled to both front and rear faces. 16. A method of driving an acoustic device comprising a substantially continuous outer shell supporting bending waves, the shell being bent to at least partially enclose an air volume so that the bending waves couple to the volume to provide coupled resonant modes, and two transducers coupled to the shell, including driving the two transducers in phase with a common electrical signal, so that the transducers drive the coupled modes of the shell and volume in a monopole configuration, and radiating sound energy from the coupled modes into the ambient air.

Inventors: HARRIS NEIL, HARRIS, NEIL
Applicants: HARRIS NEIL, NEW TRANSDUCERS LTD, HARRIS, NEIL, NEW TRANSDUCERS LIMITED
Application year: 1999
Main classification: H04R7
IPC codes: H04R1, H04R7, H04R9