Patent EA-002629-B1 (EA)
Title: Panel form acoustic apparatus using bending waves modes
Inventor: AZIMA HENRY
IPC: H04R7/04
Filed: 1999-09-01
Abstract: 1. An acoustic member in the form of a panel (1) capable of supporting bending wave vibration, wherein the bending stiffness and/or areal mass density in the panel varies over the area of the panel (1) to produce a range of coincidence frequencies having a ratio of maximum to minimum coincidence frequency of at least 1.2 to 1 so that bending waves in the panel (1) are coupled to the air more uniformly with respect to acoustic power and/or directivity as compared to an isotropic panel. 2. An acoustic device comprising an acoustic member in the form of a panel (1) according to claim 1 and a transducer coupled to the member. 3. A loudspeaker comprising an acoustic member in the form of a panel (1) according to claim 1 and an exciter (3) coupled to the panel (1) to excite bending waves in the member to cause an acoustic output, wherein that the effect of coincidence on the acoustic output of the panel is smoothed. 4. A loudspeaker according to claim 3 wherein the bending stiffness of the panel (1) varies over an area of at least 10% of the panel area. 5. A loudspeaker according to claim 3 or 4 wherein the bending stiffness has a maximum value and the exciter (3) is coupled to the panel (1) at an position having a bending stiffness at least 70% of the maximum value. 6. A loudspeaker according to any of claims 3 to 5 wherein the thickness of the panel varies over the area of the panel (1) to provide a range of bending stiffness and hence coincidence frequency. 7. A loudspeaker according to any of claims 3 to 6 wherein the bending stiffness has a maximum in the central region of the panel (1) and decreases towards the edges. 8. A loudspeaker according to claim 7 wherein the exciter (3) is coupled to the panel (1) near the maximum of bending stiffness. 9. A loudspeaker according to any of claims 3 to 8 wherein the exciter (3) is positioned at a maximum of bending stiffness of the panel (1). 10. A loudspeaker according to any of claims 3 to 7 wherein the panel stiffness has a minimum in the centre of the panel (1) and increases towards the edges of the panel (1). 11. A loudspeaker according to claim 10 wherein the exciter (3) is located near the centre of the panel (1) in a region of lower stiffness than the average stiffness of the panel (1). 12. A loudspeaker according to claim 10 or 11 wherein at least one of the edges of the panel (1) is clamped, and the bending stiffness of the panel (1) is maximal at least one clamped edge. 13. A loudspeaker according to any of claims 3 to 12 wherein the gradient of bending stiffness is concentrated near the exciter position. 14. A loudspeaker according to claim 13 wherein the gradient of bending stiffness is high close to the exciter (3) position and slowly reduces along lines extending outwards from the exciter position. 15. A loudspeaker according to any of claims 3 to 14 wherein the bending stiffness varies around the edge of the panel member. 16. A loudspeaker according to claim 15 wherein the bending stiffness is highest at the edges of the panel (1) and smoothly falls towards the interior of the panel (1). 17. A loudspeaker according to claim 15 or 16 in which at least one edge is clamped to a support. 18. A loudspeaker according to claim 17 wherein the bending stiffness at the edge of the panel (1) is such that the mechanical impedance of the panel (1) at its edge mismatches that of the support (13). 19. A loudspeaker according to any of claims 3 to 6 wherein the bending stiffness of the panel (1) varies in an undulating pattern so that the effect of coincidence on the acoustic output is smoothed. 20. A loudspeaker according to any of claims 3 to 19 wherein the panel (1) is a distributed mode panel having a plurality of resonant bending wave modes distributed in frequency. 21. An acoustical absorber comprising an acoustical member in the form of a panel (1) according to claim 1. 22. An acoustical resonator for reverberation control comprising an acoustical member in the form of a panel (1) according to claim 1. 23. An acoustical enclosure including an acoustical member in the form of a panel (1) according to claim 1. 24. A support for audio components, wherein the support comprises an acoustic member in the form of a panel (1) according to claim 1. 25. An acoustic device according to claim 2 wherein the transducer converts bending waves in the panel (1) to an electrical signal so that the acoustic device functions as a microphone. 26. A method of making a resonant mode loudspeaker system including selecting a panel material, panel size, and exciter type, selecting an initial exciter position on the panel, selecting an initial bending stiffness profile of the panel, iteratively varying the exciter position and panel profile to select a position and profile that smoothes the frequency and angular acoustic responses of the panel, providing a panel of the iteratively selected panel profile, and affixing an exciter thereto at the iteratively selected position. 27. A method according to claim 26 wherein in the step of iteratively selecting the exciter position and panel profile the distribution of resonant modes in the panel over frequency is also optimised. 28. A loudspeaker comprising a panel member (1) capable of supporting bending waves in the audio frequency range, an exciter on the panel member (1) for exciting  bending waves in the panel to produce an acoustic output, characterised in that the bending stiffness of the panel varies in the edge region of the panel and is relatively constant in the interior of the panel. 29. A loudspeaker according to claim 28, wherein the edges of the panel are clamped and the bending stiffness of the panel member (1) rises rapidly towards the edge of the panel.