Ambient sea noise elimination method and apparatus

Abstract

An ambient sea noise elimination system is disclosed in which the outputs of a directional hydrophone and an omnidirectional hydrophone are filtered and summed such that the effective receiving pattern below approximately 300 Hz is a vertically oriented dipole pattern having a null in the horizontal direction and such that the effective receiving pattern above 300 Hz is a cardioid having its null either pointing upwardly or pointing downwardly in a vertical direction. The option of simple omnicoverage at the high frequencies can also be easily implemented. This system thus takes advantage of the discovery that ambient sea noise under 300 Hz primarily travels in a horizontal direction whereas ambient sea noise above 300 Hz generally travels in a vertical direction. Signal separation is accomplished by a low pass filter connected between the directional hydrophone and a first summing circuit whereas the cardioid receiving pattern is derived by summing the outputs of high pass filters interposed respectively, between the directional hydrophone and a second summing circuit, and between the omnidirectional hydrophone and this second summing circuit. The output of the second summing circuit is coupled as one input to the first summing circuit along with the output of the low pass filter such that the output of the first summing circuit provides acoustic signals having ambient sea noise substantially removed therefrom. For omnicoverage above 300 Hz, a simple high pass filter on the omnichannel is sufficient.