What Is Sonar? Seeing Underwater With Sound Because Light Will Not Do

Active sonar transmits a pulse and listens for echoes, timing the delay to calculate range since the speed of sound in seawater is known to within a small margin. It supplies range and bearing, works against objects that make no noise of their own, and announces the transmitter's presence to anyone listening, which is a decisive disadvantage in naval use. Passive sonar transmits nothing and listens, detecting the noise made by ships, submarines, marine animals and the sea itself, and it gives bearing but no range from a single sensor, requiring movement or multiple sensors to triangulate. Because it is silent, it is the primary mode for submarines, and the resulting technical competition drove enormous investment in quieting machinery, isolating propellers and shaping hulls, since any noise a vessel makes is a signal. Signal processing does most of the work in both modes, extracting weak returns from noise, and the classification of a contact by the character of its sound is a skilled task now heavily assisted by pattern recognition.

The ocean is a complicated acoustic medium and several effects dominate practical use:

• •Speed varies with temperature, salinity and pressure, so sound does not travel in straight lines but bends towards regions where it moves slower
• •The thermocline, a layer where temperature drops sharply with depth, refracts sound strongly and creates shadow zones where a vessel can hide
• •The deep sound channel, a depth at which the speed reaches a minimum, traps sound and allows low frequencies to travel across entire ocean basins with very little loss
• •Absorption rises sharply with frequency, so low frequencies travel far while high frequencies give better resolution over short ranges, which is the fundamental trade-off in every design
• •Reverberation from the surface, the seabed and scattering layers of marine organisms can mask returns entirely
• •Ambient noise from waves, rain, biological sources and shipping sets the floor below which nothing is detectable

Naval detection drove the development, with the first practical systems built during and after the First World War in response to submarine warfare, and the British work was known by an acronym that gave the technology its early name. Fishing fleets adopted echo sounders early and fish finders are now standard on vessels of every size, with the acoustic return from swim bladders making shoals conspicuous. Hydrographic survey maps the seabed, first with single beam echo sounders measuring directly below the ship and now with multibeam systems that sweep a wide swath and produce detailed bathymetry, which is how the modern maps of the ocean floor were made, although a large fraction of the seabed remains unsurveyed at high resolution. Side-scan sonar images the bottom obliquely and is the standard tool for finding wrecks, aircraft and objects, and it produced the images by which many famous wrecks were located. Sub-bottom profilers penetrate sediment. Medical ultrasound and industrial inspection use the same physics at higher frequencies in different media.

Introducing intense sound into an environment where animals depend on hearing has consequences that are now well established. Beaked whales are particularly affected by mid-frequency naval sonar, and several mass strandings have coincided closely in time and place with exercises, with the currently favoured explanation being a behavioural response involving rapid ascent that produces decompression injury rather than direct acoustic trauma. Seismic surveys for oil and gas use air gun arrays producing extremely loud repeated pulses that propagate across hundreds of kilometres, with documented effects on whale behaviour, on fish catch rates and on zooplankton. Mitigation measures include marine mammal observers, soft-start procedures that increase power gradually to allow animals to leave, exclusion zones, seasonal restrictions and in some jurisdictions litigation that has constrained naval training. The scientific difficulty is that establishing population-level effects rather than individual responses requires long-term data that is expensive to gather, so regulation has generally proceeded on precaution rather than on demonstrated population harm.

Light and radio fail underwater, so sound is the only practical signal, with active systems timing echoes and passive ones listening silently. Sound speed varies with depth, so it bends, creating shadow zones where vessels hide and a deep channel that carries low frequencies across ocean basins. Low frequencies travel far and resolve poorly, which is the central trade-off. Naval and seismic sound has been linked to whale strandings.