Acoustic device

1. An acoustic device operable pistonically and in bending, comprising:

a panel-form acoustic radiator having a perimeter and a lowest natural frequency with an associated first nodal line of bending resonance;

a magnetic drive system including a voice coil on a tubular bobbin, the bobbin being directly coupled to the radiator to drive the radiator directly; and

a coupling device connected to the bobbin and to the radiator at a position between the bobbin connection to the radiator and its perimeter and at or near to said first nodal line of bending resonance, the acoustic device is arranged with the bobbin to drive the radiator directly and the coupling device to suppress the lowest natural frequency of the radiator.

2. An acoustic device according to claim 1 , in which the radiator is circular and the coupling device is a cone connected to the radiator at a circle at approximately ⅔ of the panel diameter.

3. An acoustic device according to claim 2 , in which the sides of the conical coupler are convex or concave.

4. An acoustic device according to claim 1 , in which the radiator is rectangular and the coupling device is connected to the radiator along at least one straight line at or near to the first nodal line of the radiator.

5. An acoustic device according to claim 1 , in which the coupling device is arranged to decouple from the radiator at a frequency slightly above the frequency of said first nodal line.

6. An acoustic device according to claim 1 , in which the coupling device is arranged to decouple from the radiator at a frequency slightly above the frequency which generates the first nodal line.

7. An acoustic device according to claim 6 , in which the coupling device is connected to the radiator through a compliant member which provides said decoupling.

8. An acoustic device according to claim 1 , wherein the radiator has an area and an operating frequency range, and the radiator being adapted such that it has resonant modes in the operating frequency range, and at least one mechanical impedance means coupled to or integral with the radiator, the positioning and mass of the at least one mechanical impedance means being such that the net transverse modal velocity over the area of the radiator tends to zero.

9. An acoustic device according to claim 8 , wherein the radiator is anisotropic in bending stiffness and has a symmetrically disposed axis of greater bending stiffness, and wherein the mass of at least one mechanical impedance means is positioned at two opposed locations substantially on the said axis of greater stiffness.

10. An acoustic device according to claim 9 , wherein the two masses are disposed at or near to the edge of the radiator.

11. An acoustic device according to claim 1 , wherein the acoustic device is a full-range device having a frequency range of at least seven octaves.

12. A loudspeaker comprising an acoustic device as claimed in claim 1 .

13. A loudspeaker drive unit comprising an acoustic device as claimed in claim 1 .

14. A method of improving the on-axis response of a loudspeaker having a panel-form radiator having a perimeter and a lowest natural frequency with an associated first nodal line of bending resonance and operable both pistonically and in bending, comprising driving the radiator by a directly connected tubular bobbin, and suppressing the lowest natural frequency of the radiator by providing a coupler connected from the bobbin to the radiator between the bobbin connection to the radiator and its perimeter and at or near to said first nodal line of bending resonance.

15. An acoustic device according to claim 1 wherein the coupling device has a thickness that tapers inwardly towards the bobbin.