NOISE REDUCTION FOR PROPELLERS 



Method and device for reducing noise produced by propellers. Anti-sound generators (devices reproducing sound vibrations but of inverted phase) are placed in the region of the noise generation but at a spacing therefrom determined by a mathematical relationship such that the zone of noise cancellation will be at the desired zone with respect to the point of noise generation. When the noise suppression is to be through a substantial region, and variable pitch propellers, or other similar variations in the mechanics of noise generation will cause the above-mentioned spacing to vary, multiple units at varying distances from the noise source may be utilized. 

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.