Damping for directional sound cancellation

Claim 2

Apparatus for the directional reduction of sound, comprising first and second electrically driven sound sources for positioning in the path of sound to be reduced with the first source nearer to the source of the said sound than the second source, signal-generating means for generating a drive signal to drive the first source, and processing means for so processing the drive signal and applying it to drive the second source that sound generated by the first and second sources tends to be in phase at all frequencies of interest on that side of the second source which is remote from the first source, and the signal-generating means and the processing means being such that the resultant generated sound on the said side of the second source tends to be in anti-phase with sound to be cancelled at all the said frequencies.

Claim 3

Apparatus according to claim 1 including a number of further sound sources, wherein the signal generating means is arranged to generate a respective drive signal for each further source, the drive signals being such that, at all frequencies of interest, the sound generated by each of the sources is in phase, on that side of the further source which is remote from the first source, with the resultant sound generated by other sources of the apparatus which are nearer to the source of the sound to be reduced.

Claim 4

Apparatus according to claim 1 wherein the signal generating means includes a detector for detecting the sound to be reduced, the detector being positioned nearer to the source of the sound to be reduced than the first source.

Claim 5

Apparatus according to claim 1 wherein the signal generating means includes a detector for detecting the sound to be reduced, the detector being positioned among the said sources of the apparatus.

Claim 6

Apparatus according to claim 4 including delay means which models the sound path between the second source and the detector, wherein the detector is coupled to a node at which the drive signal for the first source is added to the output signal of the detector and the output signal of the delay means is subtracted to form a resultant signal which is applied to an amplifier, the output signal of the amplifier forming the drive signal for the first source.

Claim 7

Apparatus according to claim 1 wherein at least one of the first and second sources comprises an array of sources driven in phase.

Claim 8

Apparatus according to claim 1 wherein the processing means comprises a delay circuit.

Claim 9

Apparatus according to claim 1 wherein the processing means comprises an adaptive filter.

Claim 10

Apparatus according to claim 1 arranged for the reduction of sound in a duct.

Claim 11

Apparatus according to claim 9 wherein the detector comprises an array of microphones positioned in the duct and coupled by means of appropriate delays to detect only sounds travelling along the duct away from the source of sound to be reduced.

Claim 14

Apparatus for the directional reduction of sound, comprising first and second electrically driven sound sources for positioning in the path of sound to be reduced with the first source nearer to the source of the said sound than the second source, signal-generating means for generating a drive signal to drive the first source, and processing means for so processing the drive signal and applying it to drive the second source that sound generated by the first and second sources tends to be in phase at all frequencies of interest on that side of the second source which is remote from the first source, and the signal-generating means and the processing means being such that the resultant generated sound on the said side of the second source tends to be in anti-phase with sound to be cancelled at all the said frequencies, said signal generating means including a detector for detecting the sound to be reduced, the detector being positioned among the said sources of the apparatus, said detector being coupled by way of means having a transfer function T D to the input of the first source to provide the said drive signal, and T D is equal to -1/(1-e -2i ωτ), where i is √-1, ω is angular frequency and τ is delay imparted by the processing means.

Claim 15

Apparatus for the directional reduction of sound, comprising first and second electrically driven sound sources for positioning in the path of sound to be reduced with the first source nearer to the source of the said sound than the second source, signal-generating means for generating a drive signal to drive the first source, and processing means for so processing the drive signal and applying it to drive the second source that sound generated by the first and second sources tends to be in phase at all frequencies of interest on that side of the second source which is remote from the first source, and the signal-generating means and the processing means being such that the resultant generated sound on the said side of the second source tends to be in anti-phase with sound to be cancelled at all the said frequencies, said signal generating means including a detector for detecting the sound to be reduced, the detector being positioned among the said sources of the apparatus, said detector being coupled by way of means having a transfer function T D to the input of the first source to provide the said drive signal, and T D is equal to -1/(1-e -2i ωτ), where i is √-1, ω is angular frequency and τ is delay imparted by the processing means.