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.
Signal excess has the same meaning in the passive equation that it does in the active equation.
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 target s orientation in relation to the sonar receiver.