Noise cancellation apparatus

Apparatus for cancelling acoustic noise radiated by a surface, comprising an array of sound cancellation units each said unit including:

• (A) means adjacent a predetermined zone of said surface for measuring the acoustic pressures emitted by said surface and to transform said pressures into an electric signal;
• (B) projector means responsive to said signals for transmitting a far field acoustic cancellation signal;
• (C) means for applying phase and gain corrections to said electric signal to compensate for near field measurements which are not the same as those assumed for far field measurements so that the acoustic output from said projector means and said zone cancel each other in the far field where the near field and far field are functions of the area of said zone and the highest frequency of interest to be cancelled;
• and
• (D) circuit means for reducing the effects of acoustical feedback from said projector means to said means for measuring.

Apparatus for cancelling acoustic noise radiated by a surface, comprising an array of sound cancellation units each said unit including:

• (A) transducer means positioned adjacent said surface and operable to provide an output signal indicative of said acoustic noise generated by a predetermined zone of said surface;
• (B) projector means operable to provide an acoustic output signal in response to an input signal;
• (C) an active network responsive to said output signal of said transducer means for inverting, and modifying, as a function of frequency, the phase and gain characteristics, of said signal;
• and
• (D) circuit means for cancelling that component of said output signal of said transducer means due to said transducer means detecting said acoustic output signal of said projector.