People and marine animals use sound in the sea to accomplish many tasks. Light travels only a few hundred meters in the ocean before it is absorbed or scattered. Since light travels relatively short distances in the ocean, sound is often used by animals for such basic activities as finding food or a mate, navigating, and communicating.

In a very general sense, there are two ways that sound is used: passively and actively. In passive acoustics, the organism or user makes no sound of their own, but listens to sounds that are being made by animals, natural phenomena, or people. The ocean, particularly in coastal environments, is a very noisy place. By listening to the sounds, much can be learned about the environment.

Two blue whales swim through Channel Islands National Marine Sanctuary. Image credit: NOAA

The second way that sound is used underwater is called active acoustics. In this case, sound is created by a source and then received by a receiver. Some active acoustic systems, such as underwater telephones, have a separate source and receiver that are located in different places (bistatic sonar). Other active acoustic systems have the source and receiver located at the same place (monostatic sonar). In this case, sound goes out from the source, bounces off an object, and returns to the receiver. The sound returning to the receiver is called backscatter. By analyzing the sound received (the return echo), much information can be learned about the object, such as its distance, size, and composition.

Echosounders calculate water depth by measuring the time it takes for the acoustic signal to reach the bottom and the echo to return to the ship. Image Credit: DOSITS.

There are a multitude of uses of sound in the ocean by both people and animals, as illustrated on the DOSITS pages in the Animals and Sound Section and the People and Sound Section. One example of people and animals using sound in a similar manner is sonar and echolocation. People use sonar to explore the ocean and animals use echolocation to find food and identify objects.

Sonar

Sonar (SOund Navigation And Ranging) is the generic name of the technology that is used to locate objects underwater. Sonar systems are of two basic types – active and passive. In active sonar, the system emits a pulse of sound and then the operator listens for echoes. In passive sonar, the operator listens to sounds emitted by the object one is trying to detect, identify, or locate.

Active Sonar

When a sound signal is sent into the water, part of it will be reflected back if it strikes an object or “target.” The distance to the object can be calculated by measuring the time between when the signal is sent out and when the reflected sound, or echo, is received. For example, if the source and receiver are in the same location and four seconds elapse between the emission of the outgoing sound and the return of its echo, the sound has taken two seconds to travel to the object and two seconds to return. The average speed of sound in seawater is 1,500 meters per second. If it takes two seconds for sound to reach the object, the object is estimated to be 2 sec x 1,500 m/sec or 3,000 meters away. The sound may reflect off anything in the water, such as a submarine or the seafloor, where sonar can be used to determine the depth of the ocean.

Sonar systems generally use highly directional beams of sound. When looking for objects in the water, these beams help to determine the direction to the target, as well as the distance. The echoes heard by active sonar systems can also be very distinct. Experienced sonar technicians are often able to tell the difference between echoes produced by a submarine, a rock outcrop, a school of fish, or a whale.

Active sonar example. Copyright University of Rhode Island
This is a recording of an active sonar that is tracking a close target. Sound courtesy of J & A Enterprises.

Passive Sonar Systems

People, marine mammals, fishes, and invertebrates can make sound underwater, and this sound can be used to locate them with passive sonar systems, in much the same way humans use their ears to locate someone speaking in a room. Passive sonar systems used by people can be arrays of hydrophones towed behind a ship or submarine, or a fixed system of hydrophones cabled to shore or attached to a mooring.

Echolocation

Marine mammals use sound to obtain information about their surroundings and to find food. One way they do this is by producing sounds or sonar clicks that are reflected back when they strike an object. This is called echolocation. Echolocation is important to marine mammals because it allows them to navigate and feed in the dark at night and in deep or murky water where it is not easy to see. Toothed whales, including beluga whalessperm whales, dolphins, and porpoises are known to echolocate.

Animation showing a dolphin emitting an echolocation sound and the sound reflecting off a school of fish and returning to the dolphin.
Animation illustrating echolocation by a dolphin. Copyright University of Rhode Island.

The echolocation signals produced by animals provide information about what is in the environment. Toothed whales that use echolocation produce high frequency click sounds. The sounds bounce off distant objects, and the echoes are received by the animal that produced them. The animal that produced the original echolocation clicks can determine the distance to an object based on the time an echo takes to return. The farther away the object is, the longer it takes for the echo to return. Echolocating animals typically do not produce a second click until they hear the echo from the first click.

As an echolocating animal gets closer to its target, the rate at which it produces clicks gets faster and faster. The series of echolocation clicks leading up to a capture attempt of a prey is called a click train. As the interval between the clicks gets shorter, the click train starts to sound like a buzz.

Sperm whale “creak,” an echolocation signal used during foraging.
Sound courtesy of Marc Lammers. Released under Creative Commons License, non-commercial attribution.

The returning echoes sound different than the original click produced by the animal. The differences between the sound of the original click and the returning echo provide the echolocating animal with information about the size, shape, orientation, direction, speed, and even composition of the object. Dolphins have an amazing ability to detect targets as small as the size of a golf ball almost a football field away. That distance is much farther than a dolphin can possibly see underwater. The beam of the echolocation clicks is also very directional and can be moved with a slight turn of the animal’s body.

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