Patent EA-002480-B1 (EA)
Title: Acoustic device
Inventor: AZIMA HENRY
IPC: H04R7/10
Filed: 1998-02-27
Abstract: 1. Acoustic device including a member extending transversely of its thickness and capable of sustaining bending waves causing consequential acoustic action by reason of areal distribution of resonant modes of natural bending wave vibration over its surface consonant with required achievable acoustic action of said member over a desired operative acoustic frequency range, wherein the member has a distribution of bending stiffness which varies over the area of the member for rendering said member favourable to said areal distribution of resonant modes for said acoustic action, said variation of bending stiffness including relatively higher and lower bending stiffness at different sides, respectively, of said location for bending wave transducer means and the centre of bending stiffness of the member is offset from the geometric centre of the member. 2. An acoustic device according to claim 1, wherein the centre of mass of the member is located at its geometric centre. 3. An acoustic device according to claim 2, wherein said variation of bending stiffness includes relatively higher and lower bending stiffness at different sides, respectively, of said location for bending wave transducer means. 4. An acoustic device according to any of claims 1 to 3, wherein said variation of bending stiffness includes relatively higher and lower bending stiffness at different sides, respectively, from said geometric centre of said element or said zone. 5. An acoustic device according to claim 3 or 4, wherein the centre of higher bending stiffness, where the initial stiffness moment is zero, is located from one side of the geometric centre of the member and the centre of lower bending stiffness, where the initial moment of reversed stiffness is zero, is located from the other side of the geometric centre of the member. 6. An acoustic device according to any preceding claim, wherein greater and lesser thicknesses of said member correspond to higher and lower stiffnesses, respectively of said distribution of bending stiffness. 7. An acoustic device according to any preceding claim, wherein said member has an additional mass or masses selectively provided having substantially no effect on desired acoustic action. 8. An acoustic device according to claim 7, wherein the or each additional mass is of sufficiently low mass that lower frequency acoustic action is substantially unaffected and has means of association with said member substantially effective to decouple the or each additional mass for higher frequency acoustic action. 9. An acoustic device according to claim 7 or claim 8, wherein the additional mass(es) are located such that the centre of mass of said member differs from the initial. 10. An acoustic device according to claim 9, wherein said desired position coincides with the geometric centre of said member. 11. An acoustic device according to claim 6, wherein said member is of sandwich structure having skins on a core having cell-defining walls extending through a varying thickness between said skins and defining cells of different cross-sectional size in order to provide the prescribed distribution of mass over said member. 12. An acoustic device according to claim 6, wherein said member is of sandwich structure having skins on a core having cell-defining walls extending through a varying thickness between said skins and in which the cell-defining walls are of different thicknesses in order to provide the prescribed distribution of mass over said member. 13. An acoustic device according to claim 11 or 12, wherein said prescribed distribution of mass is centred at the geometric centre of said member or said area. 14. An acoustic device according to claim 1, wherein said variation of bending stiffness includes at least one localised adaptation of the member being a relative weakening groove slot or cut into said member or any combination thereof. 15. An acoustic device according to any one of claims 1 to 5, wherein the member is of a structure having a skin and the bending stiffness variation is obtained by varying parameter(s) of the skin. 16. An acoustic device according to claim 15, wherein the thickness of the skin is a said skin parameter. 17. An acoustic device according to claim 15 or claim 16, wherein the Young's modulus of the skin is a said skin parameter. 18. An acoustic device according to any preceding claim, comprising acoustic transducer means having both bending wave and pistonic actions is located at a location which serves for bending wave transducer means to produce said acoustic action also serves for pistonic action acoustic transducer means. 19. An acoustic device according to claim 18, comprising acoustic transducer means producing bending waves and serving for pistonic action. 20. A loudspeaker drive unit as described according to any preceding claim, comprising: a chassis; a transducer supported on the chassis drivingly coupled to a panel diaphragm; a resilient edge suspension surrounding the diaphragm and mounting the diaphragm in the chassis, wherein the transducer is arranged to drive the diaphragm pistonically at relatively low audio frequencies to produce an audio output and to vibrate the diaphragm with bending waves at higher audio frequencies to cause the diaphragm to resonate to produce an audio output, the transducer being operatively coupled to the centre of mass and/or geometric centre of the diaphragm. 21. A loudspeaker drive unit according to claim 20, wherein the diaphragm is circular or elliptical in shape. 22. A loudspeaker drive unit according to claim 20 or claim 21, wherein the diaphragm comprises a lightweight cellular core sandwiched between opposed skins. 23. A loudspeaker drive unit according to claim 22 wherein one of the skins is extended beyond an edge of the diaphragm a marginal portion of the extended skin being attached to the resilient suspension. 24. A loudspeaker drive unit according to any one of claims 20 to 23, wherein the diaphragm is a distributed mode resonant panel. 25. A loudspeaker drive unit according to any one of claims 20 to 24, wherein the transducer is electromagnetic and comprises a moving coil mounted on a coil former the coil former being operatively coupled to the diaphragm. 26. A loudspeaker drive unit according to claim 25, comprising a second resilient suspension connected between the coil former and the chassis. 27. A loudspeaker drive unit according to claim 26, wherein one end of the coil former is connected to the diaphragm, the said second resilient suspension is disposed adjacent to the said one end of the coil former and a third resilient suspension is connected between the other end of the coil former and the chassis. 28. A loudspeaker drive unit according to any one of claims 25 to 27, wherein the end of the coil former adjacent to the panel diaphragm is coupled to drive the panel diaphragm substantially at one point. 29. A loudspeaker drive unit according to claim 28, comprising conical means connected between the coil former and the panel diaphragm. 30. A loudspeaker comprising an acoustic device as claimed in any one of claims 1 to 19 and a drive unit as claimed in any one of claims 20 to 29. 31. A stiff lightweight panel loudspeaker drive unit diaphragm adapted to be driven pistonically and to be vibrated to resonate with bending waves, the diaphragm having a centre of mass located at its geometric centre and a distribution of stiffness centred offset from its centre of mass. 32. An acoustic device according to any of claims 1 to 19, wherein said member has a bending wave transducer means to produce said acoustic action, at a location determined by said areal distribution of bending stiffness. 33. Loudspeaker drive unit actuator according to claim 30, comprising offset compliant and rigid drive coupling parts to drive a diaphragm pistonically centred on an axis and to provide offcentre resonant excitation to said diaphragm. 34. Loudspeaker drive unit actuator according to claim 33, comprising offset compliant and rigid drive coupling parts to drive a diaphragm pistonically centred on an axis including contribution by a least one said compliant part at lower frequencies and to provide offcentre resonant excitation to said diaphragm by at least one said rigid part at higher frequencies. 35. Loudspeaker acoustic drive unit actuator according to claim 33 or 34, comprising offset compliant and rigid drive coupling parts to drive a diaphragm pistonically at least at lower frequencies by way of at least one said compliant part and to excite vibration of said diaphragm by bending waves to cause distributed mode acoustic action at least at higher frequencies by way of at least one said rigid part. 36. Loudspeaker according to any of claims 33 to 35, wherein the rigid part(s) also contribute to the pistonic drive. 37. Loudspeaker according to any one of claims 33 to 36, wherein said parts are end-peripheral of a tubular member. 38. Loudspeaker according to claim 37, wherein the tubular member is secured to the diaphragm. 39. Method of making a panel member of or for an acoustic device according to any claims 1 to 19, the method including determining the nominal location of bending wave transducer means in the absence of bending stiffness variation and adjusting the areal distribution  of bending stiffness variations to displace said nominal location for bending wave transducer means to a desired location by providing relatively higher and lower bending stiffness on opposite sides of said desired actual location and also on opposite sides of said nominal location. 40. Method according to claim 39, wherein said relatively higher and lower bending stiffness are along extensions of a notional straight line through said desired and nominal locations. 41. Method according to claim 39 or 40, the method including  notionally superposing as a target geometry a desired or given configuration of the panel member and a subject geometry of a panel member