The device that uses sound waves underwater is called sonar, which stands for Sound Navigation and Ranging. Sonar works by either sending sound pulses through the water and listening for echoes, or by passively picking up sounds already present in the ocean. It is the primary technology for detecting objects, measuring depth, mapping the seafloor, and communicating beneath the surface, where radio waves and light travel only a few meters before being absorbed.
Sound travels roughly four times faster in seawater than in air, at about 1,500 meters per second. That speed shifts with temperature, salinity, and depth, but the core principle stays the same: sound moves efficiently through water, making it the most reliable way to “see” underwater.
How Active Sonar Works
Active sonar sends a pulse of sound into the water from a device called a transducer. When that pulse hits an object, it bounces back as an echo. The system measures how long it took the echo to return and how strong the returning signal is, then calculates the distance, direction, and size of whatever reflected the sound. This is the technology behind everything from submarine detection to the depth finder on a fishing boat.
Scientists use active sonar to develop nautical charts, locate underwater hazards like rocks and reefs, search for shipwrecks, and map the ocean floor in fine detail. Military forces rely on it to find submarines and mines. The concept is straightforward, but the engineering behind modern active sonar systems allows them to distinguish between types of objects and build detailed pictures of the underwater environment.
How Passive Sonar Works
Passive sonar does not emit any sound at all. Instead, it simply listens. An array of sensitive receivers picks up noises already traveling through the water, whether from a ship’s engine, a submarine’s propeller, or a whale call. Because it stays silent, passive sonar is especially valuable for military vessels that don’t want to reveal their own position, and for researchers who want to monitor marine life without disturbing it.
The tradeoff is that a single passive sonar device cannot tell you how far away a sound source is. To pinpoint a location, you need multiple passive receivers spaced apart. By comparing the tiny differences in when the sound reaches each receiver, a technique called triangulation, the system can estimate where the sound originated.
Echo Sounders and Fish Finders
An echo sounder is a specific type of active sonar pointed straight down. It sends a sound pulse toward the seabed, times the echo, and converts that into a depth reading. This is the most common system on commercial and recreational vessels for avoiding underwater rocks, reefs, and other hazards.
Fish finders are a consumer-friendly version of the same technology. They use sound pulses tuned to detect not just the bottom but also objects in the water column, like schools of fish. Modern fish finders use a technique called CHIRP, which sweeps through a range of frequencies in each ping rather than relying on a single tone. This gives a much clearer, more detailed picture of what’s below the boat, separating individual fish from vegetation or the seabed itself.
Side-Scan Sonar for Seafloor Mapping
Where a standard echo sounder looks straight down, side-scan sonar fans sound pulses out to the sides, painting a wide image of the seafloor as the device moves forward. The returning echoes create something like an acoustic photograph, revealing texture, debris, geological features, and man-made objects resting on the bottom. Search-and-rescue teams use side-scan sonar to locate wreckage or evidence, and marine archaeologists use it to find shipwrecks.
Because multiple overlapping passes can be combined, side-scan sonar can produce maps at resolutions even higher than what the sensor itself captures in a single sweep. Researchers have mounted these systems on autonomous underwater vehicles that follow pre-programmed routes, building detailed mosaic images of large areas of seafloor without a human pilot.
Acoustic Doppler Current Profilers
An Acoustic Doppler Current Profiler, or ADCP, measures the speed and direction of ocean currents using a principle familiar from everyday life: the Doppler effect. Just as an ambulance siren sounds higher-pitched as it approaches and lower as it moves away, sound waves bouncing off tiny particles drifting in the water shift in frequency depending on which direction those particles are moving.
The ADCP sends out pings at a known frequency. As those pings ricochet off suspended particles carried along by the current, the reflected sound returns at a slightly different frequency. Particles moving toward the instrument send back higher-frequency waves; particles drifting away return lower-frequency waves. By measuring both the frequency shift and the return time, the ADCP can calculate current speed at many different depths simultaneously with each series of pings. Oceanographers rely on these instruments to study everything from tidal flows to deep-ocean circulation patterns.
Underwater Acoustic Communication
Sound waves are also the only practical way to send data wirelessly over long distances underwater. Acoustic modems convert digital information into sound signals, transmit them through the water, and decode them on the other end. These devices connect underwater sensors, remotely operated vehicles, and seafloor observatories to surface stations or to each other.
The technology works, but it’s slow compared to anything on land. Conventional commercial acoustic modems have been limited by low data rates and inflexible designs. Newer experimental platforms have pushed speeds up to about 150 kilobits per second, roughly double the fastest commercial systems available. For context, that’s thousands of times slower than a typical home Wi-Fi connection. Still, in an environment where radio and optical signals fade within meters, acoustic communication remains the only viable option for long-range underwater data transfer.
Acoustic Positioning Systems
Tracking the exact location of a diver, a remotely operated vehicle, or an autonomous submarine requires underwater acoustic positioning. These systems use sound pulses exchanged between transponders to calculate position, much like GPS uses satellite signals in the air. GPS signals cannot penetrate water, so acoustics fill the gap.
There are several configurations. Long Baseline (LBL) systems use transponders spread far apart on the seafloor, providing the highest accuracy, with bearing precision better than one degree. Ultra-Short Baseline (USBL) systems mount a compact array of receivers on a surface vessel and track a single transponder on the underwater vehicle. USBL setups are simpler to deploy but less precise at longer ranges. The choice depends on how much accuracy the job demands and how much infrastructure can be placed on the seafloor.
Effects on Marine Life
Underwater acoustic devices are powerful tools, but they introduce artificial noise into an environment where many animals depend on sound to navigate, hunt, communicate, and find mates. Marine mammals like whales and dolphins are particularly sensitive. Intense sound exposure can cause temporary or permanent shifts in hearing ability, and prolonged noise can alter behavior, driving animals away from feeding or breeding grounds.
In the United States, NOAA publishes acoustic thresholds that predict when underwater sound is loud enough to risk injuring a marine mammal’s hearing. These thresholds guide federal agencies and industry operators when planning activities like sonar exercises, seismic surveys, or construction that generates underwater noise. Both the Marine Mammal Protection Act and the Endangered Species Act set limits, covering not just marine mammals but also fish and sea turtles. Operators may be required to use lower power levels, limit the duration of sound emissions, or shut down when protected species are detected nearby.