Noise cancellation apparatus 



An array of independent sound cancellation units is arranged over a vibrating noise generating surface. Each unit includes an arrangement of acoustic transducers (sensors) positioned adjacent the surface to obtain an electrical average of the local acoustic noise generated by a predetermined zone of the surface. The summed average is changed in phase and gain by an active filter whose output drives an acoustic projector also positioned adjacent the surface and the acoustic output of which sums with the original noise signal in the acoustic far field, thus tending to cancel the noise. In essence, each vibrating surface zone and its associated sound cancellation unit tend to form an acoustic doublet. A signal indicative of the projector output is used as a feedback signal, with appropriate time delays, to cancel the effect of the projected output signal being picked up by the unit's transducers, and to cancel the effect of the output of other projectors of the array. 

Active acoustic attenuator 



An active attenuator is provided for the attenuation of a relatively broad band of lower frequency vibration such as sound waves from a given source, by the introduction of cancelling vibration having the mirror image amplitude and phase characteritics of the source vibration. The active attenuator herein includes an input sensor for detecting the source vibration, a cancelling speaker for generating cancelling vibration, an error sensor for sensing the combination of source and cancelling vibration, and an electronic controller coupled to the input sensor, cancelling speaker and error sensor. The electronic controller includes an adaptive cancelling filter which employs a deterministic algorithm operable to accommodate the propagation delays of the vibration sensed by the input and error sensors, and produce an output to activate and control the cancelling speaker for the production of cancelling vibration. 

Active attenuation of noise in a closed structure 



This invention is directed to a system for the active acoustic attenuation of noise produced in the interior of an enclosure by a source of noise which in one aspect comprises input sensing operable to sense the source noise to be attenuated, cancellor adapted to produce noise 180.degree. out of phase with the source noise and error sensor operable to sense the acoustic summation of the source noise and the noise produced by the cancellor. The input sensor, cancellor and error sensor are each preferably disposed at or immediately adjacent an area of high acoustic pressure within the enclosure. The system further includes a second input sensor disposed adjacent the source of noise, a second cancellor adapted to introduce noise 180.degree. out of phase with the source noise at a location at or immediately adjacent the enclosure wall and second error sensor disposed in the interior of the enclosure. Each set of input sensing means, cancellor and error sensor are adapted to be connected to an electronic controller means operable to process signals from the input sensor, produce outputs to drive the cancellor for the introduction of cancelling sound waves into the enclosure for combination with the source sound waves, and then adjust such outputs based on signals received from the error sensor. 

Attenuation of sound waves 



An active sound control system is described in which allowance is made in a relatively uncomplicated circuit for acoustic coupling between a sound generating system for generating a cancelling sound wave and a detector for sensing a sound wave to be cancelled. Unwanted sound from a source is detected by a microphone and cancelled by sound from a speaker connected by way of an amplifier to the microphone. The amplifier has a feedback processing system with a transfer function which takes account of acoustic feedback between the speaker and the microphone in deriving, with the amplifier, a signal to drive the speaker.