Acoustic device

1. A bending wave loudspeaker having an operating frequency range and a coincidence frequency which is above the operating frequency range, comprising a resonant panel having a main or major axis and a cross or minor axis and an aspect ratio of at least 2:1, a plurality of vibration exciters coupled to the panel to excite the panel into resonance along the cross or minor axis of the panel, and means restraining or preventing resonance along the main or major axis of the panel whereby the panel radiates an acoustic output which is of wide directivity along the cross or minor axis and of narrow directivity along the main or major axis of the panel.

2. A loudspeaker according to claim 1 , wherein the plurality of vibration exciters forms the means restraining or preventing resonance along the main or major axis.

3. A loudspeaker according to claim 2 , wherein the length of the plurality of vibration exciters along the panel is longer than the wavelength of sound in air at the lowest required frequency.

4. A loudspeaker according to claim 2 , wherein the plurality of vibration exciters comprises a line of discrete exciters extending along the main or major axis and operated substantially in phase.

5. A loudspeaker according to claim 4 , wherein the spacing between the exciters is not greater than half the wavelength in the panel at the highest operating frequency.

6. A loudspeaker according to claim 4 , wherein the line is rectilinear.

7. A loudspeaker according to claim 4 , wherein the line extends substantially from one end of the panel to the other end.

8. A loudspeaker according to claim 4 , wherein there are at east four exciters in the line.

9. A loudspeaker according to claim 4 , wherein the line of exciters is to one side of the median longitudinal axis of the panel.

10. A loudspeaker according to claim 9 , wherein the line is on the nodal line of the first lateral bending mode.

11. A loudspeaker according to claim 4 , wherein the exciters are equally spaced along the line.

12. A loudspeaker according to claim 4 , wherein the exciter spacing d in the line and the bending stiffness B and areal density μof the panel substantially conform to the formula:

B

μ

=

(

cd

π

)

2

.

13. A loudspeaker according to any one of claims 1 and 2 - 12 , wherein the panel is rectangular.

14. A loudspeaker according to any one of claims 5 and 7 to 12 , wherein the line is rectilinear.

15. A loudspeaker according to claim 14 , wherein the panel is rectangular.