Acoustic device
A method of improving the modal resonance frequency distribution of a panel ( 2 ) for a distribution resonant mode bending wave acoustic device involves analysing the distribution of the modal resonance frequencies of the panel, identifying a modal resonance frequency that is non-uniformly spaced relative to adjacent modal resonance frequencies, identifying a location on said panel that exhibits anti-nodal behaviour at said modal resonance frequency and changing the local impedance to bending wave vibration at said location ( 6 ). The method has particular application to distributed mode loudspeakers ( 1 ).
1. Method of improving the modal resonance frequency distribution of a panel for a panel-form distributed resonant mode bending wave acoustic device, the method comprising the steps of:
(a) analysing the distribution of the modal resonance frequencies of the panel;
(b) identifying a modal resonance frequency that is non-uniformly spaced relative to adjacent modal resonance frequencies;
(c) identifying a location on said panel that exhibits anti-nodal behaviour at said modal resonance frequency; and
(d) changing the local impedance of the panel to bending wave vibration at said location.
2. Method according to claim 1 , wherein the location identified in step (c) exhibits nodal behaviour at a second resonance frequency neighbouring said modal resonance frequency in addition to exhibiting anti-nodal behaviour at said modal resonance frequency.
3. Method according to claim 1 , wherein step (b) comprises identifying a plurality of modal resonance frequencies that are non-uniformly spaced relative to respective adjacent modal resonance frequencies; step (c) comprises identifying a plurality of locations on said panel that exhibit anti-nodal behaviour at respective modal resonance frequencies; and step (d) comprises changing the local impedance to bending wave vibration at one or more of said plurality of locations.
4. Method according to claim 1 , further comprising the step of iteratively changing said local impedance so as to improve the modal resonance frequency distribution of said panel.
5. Method according to claim 1 , further comprising the steps of changing said local impedance by various amounts; measuring the respective uniformity of modal resonance frequency distribution; and interpolating from the measured uniformity of modal resonance frequency distribution preferred values of local impedance change.
6. Method according to claim 5 , wherein the step of measuring comprises calculating the least squares central difference of mode frequencies.
7. Method according to claim 5 , wherein the step of interpolating comprises identifying values of local impedance change corresponding to a modal resonance frequency distribution that is better than that of a corresponding rectangular panel having isotropic material properties and an optimal aspect ratio.
8. Method according to claim 5 , further comprising the steps of changing said local impedance by various amounts; measuring the respective changes in modal resonance frequency distribution; and interpolating from the measured changes in modal resonance frequency distribution the optimal value of local impedance change.
9. Method according to claim 5 , wherein the step of changing the local impedance comprises changing the mass of the panel at said location.
10. Method according to claim 9 , wherein the step of changing the local impedance comprises attaching a discrete mass to the panel.
11. Method according to claim 10 , wherein the step of changing the local impedance comprises attaching the discrete mass to the panel by means of a member having compliance.
12. Method according to claim 11 , wherein the step of changing the local impedance comprises attaching the discrete mass to the panel by means of a member having damping.
13. Method according to claim 12 , wherein the step of changing the local impedance comprises attaching said discrete mass to the panel by means of a resilient foam member.
14. Method according to claim 5 , wherein the step of changing the local impedance comprises varying the stiffness of the panel at said location.
15. Method according to claim 5 , wherein the step of changing the local impedance comprises varying the damping of the panel at said location.
16. Method according to claim 5 , wherein step (b) comprises identifying a plurality of modal resonance frequencies that are non-uniformly spaced relative to respective adjacent modal resonance frequencies; step (c) comprises identifying a plurality of locations on said panel that exhibit anti-nodal behaviour at respective modal resonance frequencies; and step (d) comprises changing the local impedance to bending wave vibration at one or more of said plurality of locations.
17. Method according to claim 9 , wherein step (b) comprises identifying a plurality of modal resonance frequencies that are non-uniformly spaced relative to respective adjacent modal resonance frequencies; step (c) comprises identifying a plurality of locations on said panel that exhibit anti-nodal behaviour at respective modal resonance frequencies; and step (d) comprises changing the local impedance to bending wave vibration at one or more of said plurality of locations.
18. Method according to claim 9 , further comprising the step of iteratively changing said local impedance so as to improve the modal resonance frequency distribution of said panel.
19. Method according to claim 1 , wherein the step of changing the local impedance comprises varying the damping of the panel at said location.
20. Method according to claim 1 , wherein the step of changing the local impedance comprises varying, the stiffness of the panel at said location.
21. Method according to claim 1 , wherein the step of changing the local impedance comprises varying the damping of the panel at said location.