Apparatus for cancelling unwanted noise due to turbulent fluid flow past an acoustic sensor is disclosed. A plurality of sensors 8, 10, 12 are connected to an adaptive noise cancellation system. The cancellation system comprises a summator means 18, an input to which is connected to one of the sensors 10, and the remainder of the sensors 10, 12 are connected to inputs to the summator means 18 via controllable filter means 14, 16 which operate to delay and shift the respective monitored signals representative of each of the signals detected by the sensors 10, 12, such that a summation signal provided at an output of the summator means 18 is substantially devoid of noise. A restoring controllable filter 24, connected to the output of the summator means 18, operates to restore the frequency response of the summation signal, thereby providing an output signal devoid of turbulence-induced noise and representative of a wanted signal detected by the sensors 8, 10, 12. This apparatus can be applied to acoustic sensors in pipes or ducts, or mounted on vehicles or aircraft, and sonar sensors underneath ships or towed arrays, or mounted on submarines.
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