listens without transmitting. 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
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. Passive sonar
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 Hz alternating current power systems. If transformers or generators are mounted without proper vibration insulation from the hullor become flooded, the 60 Hz sound from the windings can be emitted from the submarine or ship.
This can help to identify its nationality, as most European
submarines have 50 Hz power systems. Intermittent sound sources (such
as a wrench being dropped) may also be detectable to passive sonar. Until fairly
recently, 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 even
particular ships. Publications for classification of sounds are
provided by and continually updated by the US office of naval defense
Passive sonar on vehicles is usually severely limited because of
noise generated by the vehicle. For this reason, many submarines
operate nuclear reactors that can be cooled without pumps, using silent convection, or fuel cells or batteries, which can also run silently. Vehicles' propellers
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 hydrophones
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 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.
Unlike active sonar, only one way propagation is involved. Because
of the different signal processing used, the minimum detectable signal
to noise ratio will be different. The equation for determining the
performance of a passive sonar is:
- SL − TL = NL − DI + DT
where SL is the source level, TL is the transmission loss, NL is the
noise level, DI is the directivity index of the array (an approximation
to the array gain) and DT is the detection threshold. The figure of merit of a passive sonar is:
- FOM = SL + DI − (NL + DT).