Ambient sea noise elimination method and apparatus

A method for minimizing ambient subsurface noise from acoustic signals propagating beneath the surface of the sea comprising the steps of

• detecting acoustic signals at a first location with a directional hydrophone having a null in a horizontal plane and providing a first signal corresponding in amplitude to the amplitude of acoustic signals detected by said directional hydrophone;
• detecting acoustic signals at a second location vertically spaced from said first location with an omnidirectional hydrophone and providing a second signal corresponding in amplitude to the amplitude of acoustic signals detected by said omnidirectional hydrophone,
• removing frequency components of said first signal above a predetermined frequency, so as to form a third signal,
• removing frequency components of said first signal below a predetermined frequency so as to form a fourth signal,
• removing frequency components of said second signal below a predetermined frequency so as to form a fifth signal,
• summing said fourth and fifth signals to form a sixth signal;
• and
• summing said third and sixth signals thereby to form an output signal in which both horizontal and vertical subsurface noise is minimized.

Apparatus for minimizing ambient subsurface noise from acoustic signals propagating beneath the surface of the sea comprising:

• means for detecting acoustic signals at a first location with a directional hydrophone having a null in a horizontal plane and providing a first signal corresponding to acoustic signals detected by said directional hydrophone;
• means for detecting acoustic signals at a second location vertically spaced from said first location with an omnidirectional hydrophone and providing a second signal corresponding to acoustic signals detected by said omnidirectional hydrophone;
• and
• means for summing said first and second signals thereby to produce a signal in which vertically travelling subsurface noise is minimized.

Apparatus for minimizing ambient subsurface noise from acoustic signals propagating beneath the surface of the sea comprising:

• means for detecting acoustic signals at a first location with a directional hydrophone having a null in a horizontal plane and providing a first signal corresponding in amplitude to the amplitude of acoustic signals detected by said directional hydrophone;
• means for detecting acoustic signals at a second location vertically spaced from said first location with an omnidirectional hydrophone and providing a second signal corresponding in amplitude to the amplitude of acoustic signals detected by said omnidirectional hydrophone, means for removing frequency components of said first signal above a predetermined frequency, so as to form a third signal;
• means for removing frequency components of said first signal below a predetermined frequency so as to form a fourth signal;
• means for removing frequency components to said second signal below a predetermined frequency so as to form a fifth signal;
• means for summing said fourth and fifth signals to form a sixth signal;
• and
• means for summing said third and sixth signals thereby to form an output signal in which both horizontal and vertical subsurface noise is minimized.

Apparatus for detecting underwater acoustic signals and for providing an output signal, in which the effects of ambient noise are minimized comprising:

• a directional hydrophone having a dipole receiving pattern;
• first amplifier means for amplifying the output signal from said directional hydrophone to produce a first signal;
• an omnidirectional hydrophone;
• second amplifier means for amplifying the output signal from said omnidirectional hydrophone to produce a second signal;
• a low pass filter;
• means for applying said first signal to the input of said low pass filter;
• a first high pass filter;
• means for applying said first signal to the input of said first high pass filter;
• a second high pass filter;
• means for applying said second signal to the input of said second high pass filter;
• first summing means;
• means for coupling the output signals from said first and second high pass filters to said first summing means;
• second summing means;
• and
• means for applying the output signal from said low pass filter and the output signal of said first summing meand to said second summing means.

Apparatus for use in detecting underwater acoustic signals comprising:

• a directional hydrophone having a receiving pattern with a null in a horizontal plane;
• first means for amplifying the output signal from said directional hydrophone;
• an omnidirectional hydrophone;
• second means for amplifying the output signal from said omnidirectional hydrophone;
• means for mounting said hydrophones in spaced vertical alignment;
• and
• means for summing the output signals from said first and second amplifying means, whereby ambient sea noise traveling in a vertical direction is eliminated from the output of said summing means.