Having established the optimum equipment lineup, further noise reduction is achieved through ensuring the cavitation-free operation of pumps and piping. The background against which the sonar must detect and classify incoming target signals is made up of ocean ambient noise and noise which is generated by the ship and sonar system. That portion of the sonar background noise which is caused by own ship systems is self-noise. Propulsion and auxiliary machinery within the ship generates structureborne noise. Some of this energy is transmitted through the ship's structure to the hull where it is transferred into the water.
Ships can also radiate sonar energy in an effort to find submarines.
Structureborne noise and sonar noise combine to form the underwater radiated signature of the ship. Radiated noise is the acoustic signal detected by passive sonars. Radiated noise, therefore, provides a means for enemy units to detect your ship and also interferes with the sonars of escort ships. Self-noise is used when describing the effect a ship has on its own sonar. Radiated noise is used when describing the effect a ship has on off-board sensors.