How Does Sonar Work? Listening to a Sea That Bends Sound

The two modes are genuinely different tools. Active sonar transmits a pulse and times the echo, which gives range directly from the travel time and the speed of sound, roughly 1,500 metres per second in seawater, a little over four times its speed in air. It works on objects that make no noise, which is its whole advantage, and it announces the transmitter's presence to everything within range, which is its whole disadvantage and the reason a submarine trying to hide will not use it. Passive sonar transmits nothing and simply listens, detecting the noise a target makes: machinery, propeller cavitation, flow over the hull. It gives a bearing to the source and, from the acoustic signature, frequently an identification of what class of vessel is making the noise, but it does not give range from a single measurement, so range must be inferred by manoeuvring and taking bearings over time, or by using a widely separated array.

The speed of sound in seawater depends on temperature, pressure and salinity, and because those vary with depth, sound rays refract continuously towards whichever region is slower. Temperature dominates near the surface and falls with depth, slowing sound, while pressure rises steadily with depth and speeds it up, so a typical sound speed profile has a minimum at some intermediate depth. That produces several consequences that dominate practical sonar:

• •The thermocline, a layer where temperature drops sharply, bends sound downward, so a target below it can be effectively invisible to a sensor above it, and submarines deliberately sit beneath the layer
• •A surface duct, formed when the top layer is well mixed, traps sound near the surface and carries it far along that channel while leaving a shadow zone beneath
• •The deep sound channel, at the speed minimum around a kilometre down, traps sound so effectively that a modest source is detectable thousands of kilometres away, which is how whale calls travel ocean basins
• •Convergence zones, where downward-bent rays are turned back up by pressure and resurface in rings tens of kilometres from the source, giving detection at long range with gaps in between
• •Bottom bounce paths, using reflection off the seabed to reach past a shadow zone, whose usefulness depends heavily on whether the bottom is rock or mud

Sonar range is set by a balance written out as the sonar equation, weighing the transmitted level and the target's reflectivity against everything that degrades the signal. Absorption removes energy from the wave and rises sharply with frequency, which is the central design trade: low frequencies travel far and give poor resolution, high frequencies give fine detail over short ranges, which is why a submarine search set works at hundreds of hertz and a medical or imaging sonar at megahertz. Spreading loss reduces intensity with distance. Ambient noise sets the floor and comes from breaking waves, rain, distant shipping and biological sources, with snapping shrimp loud enough to dominate in tropical shallows. Reverberation, the return of the transmitted pulse from the surface, seabed and suspended material, can swamp a real echo entirely and is the reason shallow water is far harder than deep. Target strength depends on size, shape, aspect and materials, which is why anechoic coatings on submarine hulls and careful hull shaping matter so much.

Most sonar has nothing to do with submarines. Echo sounders measure depth beneath a vessel and are the basis of all hydrographic charting, with multibeam systems fanning hundreds of beams across a swath to map the seabed in three dimensions, which is how the ocean floor has been surveyed and how wrecks and pipelines are found. Side scan sonar tows a fish that images the bottom obliquely, producing pictures that look like low-angle photographs and revealing texture and small objects. Fisheries acoustics identifies and counts fish schools by their echoes, distinguishing species partly by the resonance of their swim bladders. Sub-bottom profilers use low frequencies to penetrate sediment and reveal layering beneath the seabed for engineering and geology. Acoustic Doppler current profilers measure water movement by the frequency shift of scattering from particles. Underwater positioning systems locate divers and remotely operated vehicles, and acoustic modems carry the only practical wireless data links underwater, at data rates that make dial-up look quick.

Because the ocean is an acoustic environment, adding loud sound to it has consequences that have become difficult to dismiss. Powerful military sonar has been linked to mass strandings of beaked whales, with post-mortem findings consistent with a decompression-like injury thought to result from the animals changing their diving behaviour in panic, and several legal cases have restricted where and when high-power sets may be used. Seismic surveys for oil and gas use airgun arrays that are far louder and more widespread than sonar and have measurable effects on whale communication, fish catch rates and invertebrates. Chronic shipping noise has raised background levels across whole ocean basins, compressing the range over which whales can hear each other, a reduction documented directly when shipping fell sharply after September 2001 and again during the pandemic and stress hormone levels in right whales dropped. Mitigation practice now includes marine mammal observers, soft starts that ramp power gradually, seasonal and geographic restrictions, and quieter propeller and hull designs.

Sonar works because sound travels well in water while light and radio do not. Active sets transmit a pulse and time the echo, giving range but revealing themselves; passive sets only listen, giving bearing and identification but not range directly. Sound speed varies with depth, so rays bend, producing thermoclines that hide targets, surface ducts, shadow zones, convergence zones and a deep channel carrying sound for thousands of kilometres. Low frequencies reach far with poor resolution and high frequencies do the reverse.