In passive-sonar operations, the hydrophores pick up sounds generated by a multitude of sound sources. Sonarmen must differentiate between sounds generated by a target and interfering background noise. This process is best described in what is known as the passive sonar equation. The passive form of the sonar equation, like the active form, is written using several different symbols to represent the equation parameters. One form of the equation is as follows:

SIGNAL EXCESS. Signal excess has the same meaning in the passive equation that it does in the active equation.

SOURCE LEVEL. Source level pertains to target-radiated noise. It is the amount of sound energy generated by a target. The level of energy reaching the sonar receiver depends on the type of target and its mode of operation. Source level is a function of frequency, speed, depth, and target aspect. The latter refers to a targets orientation in relation to the sonar receiver.

RECOGNITION DIFFERENTIAL. RD has the same meaning as in the active sonar equation.

NOISE LEVEL. The definition for NL in the passive equation is the same as in the active equation. Passive sonars may be ambient-noise or self-noise limited. These sonars lessen the noise in certain frequency ranges, thereby permitting a target signal to be more readily detected.

Ambient Noise. Ambient noise is that part of the total background noise created by surface-ship traffic, wave action, precipitation, ice, and certain forms of marine life.

Self Noise. Self noise is that part of the total background noise attributable to the sonar equip-ment, the platform on which it is mounted, or the noise caused by the motion of the platform. The major classes of self-noise are machinery noise, propeller noise, and hydrodynamic noise. The latter results from the flow of water past hydrophores, supports, and the hull structure of the platform.

DIRECTIVITY INDEX. DI has the same meaning as in the active sonar equation.

PROPAGATION LOSS. PL has the same meaning as in the active sonar equation except that with passive sonar, the energy loss is one-way.

Fleet Oceanographic and Acoustic Reference Manual, RP-33, Naval Oceanographic Office, July 1986.

Operational Oceanography Module II, Acoustics and Sound Ray Theory, NOCF Bay St. Louis, Miss., 1988.

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