Editing Sonar (section)

==Passive sonar==
{{More footnotes |section|date=October 2023}}
Passive sonar listens without transmitting.<ref name="usff-sound">{{cite web |title=Science of Sound |url=https://www.usff.navy.mil/Community-Outreach/US-Navy-Stewards-of-the-Sea/Science-of-Sound/ |website=www.usff.navy.mil |publisher=U.S. Fleet Forces Command |access-date=30 October 2023}}</ref> It is often employed in military settings, although it is also used in science applications, ''e.g.'', detecting fish for presence/absence studies in various aquatic environments – see also [[passive acoustics]] and [[passive radar]]. In the very broadest usage, this term can encompass virtually any analytical technique involving remotely generated sound, though it is usually restricted to techniques applied in an aquatic environment.

===Identifying sound sources===
Passive sonar has a wide variety of techniques for identifying the source of a detected sound. For example, U.S. vessels usually operate 60 [[Hertz]] (Hz) [[alternating current]] power systems. If [[transformer]]s or [[electrical generator|generators]] are mounted without proper [[vibration]] insulation from the [[hull (watercraft)|hull]] or become flooded, the 60&nbsp;Hz sound from the windings can be emitted from the [[submarine]] or ship. This can help to identify its nationality, as all European submarines and nearly every other nation's submarine have 50&nbsp;Hz power systems. Intermittent sound sources (such as a [[wrench]] being dropped), called "transients," may also be detectable to passive sonar. Until fairly recently,{{when|date=August 2012}} an experienced, trained operator identified signals, but now computers may do this.

Passive sonar systems may have large sonic [[databases]], but the sonar operator usually finally classifies the signals manually. A [[computer system]] frequently uses these databases to identify classes of ships, actions (i.e. the speed of a ship, or the type of weapon released and the most effective countermeasures to employ), and even particular ships.

===Noise limitations===
Passive sonar on vehicles is usually severely limited because of noise generated by the vehicle. For this reason, many submarines operate [[nuclear reactor]]s that can be cooled without pumps, using silent [[convection]], or [[fuel cell]]s or [[battery (electricity)|batteries]], which can also run silently. Vehicles' [[propeller]]s are also designed and precisely machined to emit minimal noise. High-speed propellers often create tiny bubbles in the water, and this [[cavitation]] has a distinct sound.

The sonar [[hydrophone]]s may be towed behind the ship or submarine in order to reduce the effect of noise generated by the watercraft itself. Towed units also combat the [[thermocline]], as the unit may be towed above or below the thermocline.

The display of most passive sonars used to be a two-dimensional [[spectrogram|waterfall display]]. The horizontal direction of the display is bearing. The vertical is frequency, or sometimes time. Another display technique is to color-code frequency-time information for bearing. More recent displays are generated by the computers, and mimic [[radar]]-type [[plan position indicator]] displays.

===Performance prediction===
Unlike active sonar, only one-way propagation is involved.  Because of the different signal processing used, the minimal detectable signal-to-noise ratio will be different.  The equation for determining the performance of a passive sonar is<ref>M. A. Ainslie (2010), Principles of Sonar Performance Modeling, Springer, p68</ref><ref name="ReferenceA"/>
:SL &minus; PL = NL &minus; AG + DT,
where SL is the source level, PL is the propagation loss, NL is the noise level, AG is the array gain and DT is the detection threshold.  The [[figure of merit]] of a passive sonar is
:FOM = SL + AG &minus; (NL + DT).