Loudspeaker

1. A bending wave loudspeaker, comprising:

a transparent acoustic radiator adapted to support bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

at least one resonant element having a frequency distribution of modes in the operative frequency range, wherein parameters of the resonant element are such as to enhance the distribution of modes in the resonant element in the operative frequency range and wherein the distribution of modes in the resonant element has a density of modes which is sufficient for the resonant element to provide an effective mean average force which is substantially constant with frequency; and

a coupler mounting the transducer to the acoustic radiator.

2. A loudspeaker according to claim 1 , wherein the modes are distributed substantially evenly over the intended operative frequency range.

3. A loudspeaker according to claim 1 , wherein the resonant element is modal along two substantially normal axes, each axis having an associated fundamental frequency, and wherein the ratio of the two associated fundamental frequencies is adjusted for best modal distribution.

4. A loudspeaker according to claim 3 , wherein the ratio of the two fundamental frequencies is about 9:7.

5. A loudspeaker according to claim 1 , wherein the resonant element is plate-like.

6. A loudspeaker according to claim 1 , wherein the shape of the resonant element is selected from the group consisting of beam-like, trapezoidal, hyperelliptical, generally disc shaped, and rectangular.

7. A loudspeaker according to claim 6 , wherein the resonant element is plate-like.

8. A bending wave loudspeaker comprising:

a transparent acoustic radiator adapted to support bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

a plurality of resonant elements each having a frequency distribution of modes in the operative frequency range; wherein parameters of the resonant elements are such as to enhance the distribution of modes in the resonant elements in the operative frequency range and wherein the modes of the resonant elements are arranged to interleave in the operative frequency range whereby the distribution of modes in the transducer is enhanced, and

a coupler mounting the transducer to the acoustic radiator.

9. A bending wave loudspeaker, comprising:

a transparent acoustic radiator adapted to support bending wave vibration; the acoustic radiator having a first face and a second face.

an electromechanical force transducer mounted to the first face of the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, and

a mask mounted to the second face of the acoustic radiator to obscure the transducer, wherein the transducer has an intended operative frequency range and comprises:

at least one resonant element having a frequency distribution of modes in the operative frequency range; and wherein parameters of the resonant element are such as to enhance the distribution of modes in the resonant element in the operative frequency range and wherein the distribution of modes in the resonant element has a density of modes which is sufficient for the resonant element to provide an effective mean average force which is substantially constant with frequency, and

a coupler mounting the transducer to the acoustic radiator.

10. A loudspeaker according to claim 9 , wherein the modes are distributed substantially evenly over the intended operative frequency range.

11. A loudspeaker according to claim 9 , wherein the resonant element is modal along two substantially normal axes, wherein each axis has an associated fundamental frequency, and wherein the ratio of the two associated fundamental frequencies is adjusted for best modal distribution.

12. A loudspeaker according to claim 9 , further comprising:

a frame which at least partially surrounds the acoustic radiator; and

a suspension for mounting the acoustic radiator to the frame.

13. A loudspeaker according to claim 12 , wherein the frame acts as a baffle.

14. A telephone handset comprising:

a body supporting a microphone, at least one key, a display, and a window mounted over the display; and

a bending wave loudspeaker comprising:

a transparent acoustic radiator adapted to support bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

a resonant element having a frequency distribution of modes in the operative frequency range; wherein parameters of the resonant element are such as to enhance the distribution of modes in the resonant element in the operative frequency range, and wherein the distribution of modes in the resonant element has a density of modes which is sufficient for the resonant element to provide an effective mean average force which is substantially constant with frequency,

a coupler mounting the transducer to the acoustic radiator, and

wherein the window is operable as the acoustic radiator.

15. A telephone handset according to claim 14 , wherein the modes are distributed substantially evenly over the intended operative frequency range.

16. A telephone handset comprising:

a body supporting a microphone, at least one key, a display, and a window mounted over the display;

a bending wave loudspeaker comprising:

a transparent acoustic radiator adapted to support bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

a resonant element having a frequency distribution of modes in the operative frequency range; wherein parameters of the resonant element are such as to enhance the distribution of modes in the resonant element in the operative frequency range, and

a coupler mounting the transducer to the acoustic radiator, and

wherein the window is operable as the acoustic radiator;

the handset further comprising: a suspension which supports the window on the body and which prevents transmission of vibration from the window to the body.

17. A visual display unit comprising:

a body supporting a display unit and a window mounted over the display; and

a bending wave loudspeaker comprising:

a transparent acoustic radiator capable of supporting bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

a resonant element having a frequency distribution of modes in the operative frequency range, wherein parameters of the resonant element are such as to enhance the distribution of modes in the resonant element in the operative frequency range, and wherein the distribution of modes in the resonant element has a density of modes which is sufficient for the resonant element to provide an effective mean average force which is substantially constant with frequency; and

a coupler mounting the transducer to the acoustic radiator,

and wherein the window is operable as the acoustic radiator.

18. A visual display unit according to claim 17 , wherein the modes are distributed substantially evenly over the intended operative frequency range.

19. A bending wave loudspeaker comprising:

a transparent acoustic radiator adapted to support bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

a plurality of resonant elements each having a frequency distribution of bending wave modes in the operative frequency range,

at least one connector coupling the plurality of resonant elements together, and

a coupler mounting the transducer to the acoustic radiator,

wherein at least one of the parameters of the transducer is such as to enhance the distribution of bending wave modes in the resonant elements in the operative frequency range.

20. A bending wave loudspeaker according to claim 19 , wherein the at least one parameter of the transducer is selected from the group consisting of relative aspect ratios, relative bending stiffnesses, relative thicknesses and relative geometries of the plurality of resonant elements.

21. A bending wave loudspeaker according to claim 20 , wherein the at least one parameter of the transducer comprises the location of the at least one connector on each of the plurality of resonant elements.

22. A bending wave loudspeaker according to claim 21 , wherein the at least one parameter of the transducer comprises the location of the coupler on the transducer.

23. A bending wave loudspeaker according to claim 19 , wherein the at least one parameter of the transducer comprises the location of the at least one connector on each of the plurality of resonant elements.

24. A bending wave loudspeaker according to claim 19 , wherein the at least one parameter of the transducer comprises the location of the coupler on the transducer.

25. A bending wave loudspeaker comprising:

a transparent acoustic radiator adapted to support bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

at least one resonant element having a frequency distribution of bending wave modes in the operative frequency range and being modal along two substantially normal axes, and

a coupler mounting the transducer to the acoustic radiator, the coupler being attached to the resonant element at a position which is beneficial for coupling modal activity of the resonant element to the acoustic radiator,

wherein at least one of the parameters of the transducer is such as to enhance the distribution of bending wave modes in the resonant element in the operative frequency range.

26. A bending wave loudspeaker according to claim 25 , wherein the at least one parameter is selected from the group consisting of aspect ratio, bending stiffness, and thickness of the resonant element.

27. A bending wave loudspeaker comprising:

a transparent acoustic radiator adapted to support bending wave vibration; and

an electromechanical force transducer mounted to the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output, wherein the transducer has an intended operative frequency range and comprises:

at least one resonant element having a frequency distribution of bending wave modes in the operative frequency range, and

a coupler mounting the transducer to the acoustic radiator, the coupler being attached to the resonant element at a position which is away from the centre of the resonant element and which is beneficial for coupling modal activity of the resonant element to the acoustic radiator,

wherein at least one of the parameters of the transducer is such as to enhance the distribution of bending wave modes in the resonant element in the operative frequency range.

28. A bending wave loudspeaker according to claim 27 , wherein the shape of the resonant element is such as to provide an off-centre line of attachment which is generally at the centre of mass of the resonant element.

29. A bending wave loudspeaker according to claim 28 , wherein the resonant element is in the shape of a trapezium.

30. A bending wave loudspeaker according to claim 28 , wherein the resonant element is in the shape of a trapezoid.