The ocean is filled with sound. Underwater sound is generated by a variety of natural sources, such as breaking waves, rain, and marine life. It is also generated by a variety of man-made sources, such as ships and military sonars. You can listen to examples of many undersea sounds in the Audio Gallery.

Some sounds are present more or less everywhere in the ocean all of the time. The background sound in the ocean is called ambient noise. The primary sources of ambient noise can be categorized by the frequency of the sound.
In the frequency range of 20-500 Hz, ambient noise is primarily due to noise generated by distant shipping. Even after removing any noise generated by ships close to the receiver, distant ships can be detected. The amount of noise is greater in regions with heavy shipping traffic. There tend to be fewer ships in the southern hemisphere, and low-frequency ambient noise levels are substantially lower as a result. Noise generated by shipping has increased as the number of ships on the high seas has increased (Andrew et al., 2002).

In the frequency range of 500-100,000 Hz, ambient noise is mostly due to spray and bubbles associated with breaking waves. It increases with increasing wind speed.

At frequencies greater than about 100,000 Hz, the noise generated by the random motion of water molecules, called thermal noise, dominates. This noise sets the ultimate limit to the minimum sound levels that can be measured.

The background sounds in the ocean can be summarized in a graph showing typical sound levels at different frequencies. The sound levels in this graph are in dB relative to 1 µPa in a 1 Hz wide frequency band, which is usually written dB re 1 µPa²/Hz.

 

Noise that interfered with the ability to hear underwater sounds had been a problem since the early 1900s when the Submarine Signal Company first installed underwater bells near lighthouses to warn mariners of navigational hazards. However, the systematic study of ambient noise did not begin until World War II. During WWII, acoustic mines were developed that were triggered by the sound of a passing ship. Accurate knowledge of ambient noise levels was needed in order to set the sound levels at which acoustic mines would be triggered, so that they would explode only when a ship was present. This need helped stimulate development of the calibrated receiving systems needed to measure ambient noise levels. Under the direction of Vern Knudsen, the director of the University of California Division of War Research, ambient noise measurements were made in coastal waters and harbors for frequencies from 1000 Hz to 50,000 Hz. They found that the noise level increases with increasing wind speed and wave height. For a given wind speed, the noise level decreases with increasing acoustic frequency. The curves summarizing their results became known as the "Knudsen curves".

The dependence of ambient noise on wind speed at frequencies between roughly 1000 Hz and 50,000 Hz implied that measurements of ambient noise might be used as a tool to determine wind speed over the ocean. This principle was exploited many years later in oceanographic instruments given the name WOTAN, for "Wind Observations Through Ambient Noise."

Unlike ambient noise discussed above, which is almost always present, some sounds are intermittent or only occur in limited regions of the ocean. There are a large number of intermittent sources of sound in the ocean, including natural physical processes, marine life, and man-made sources.

For an Advanced Topic that describes the contributions of these underwater sound sources, click here.

Physical processes that intermittently generate sound in the ocean include rain, lightning striking the sea surface, cracking sea ice, undersea earthquakes, and eruptions from undersea volcanoes. Some of these phenomena generate extremely loud sounds, such as lightning strikes, which can have source levels of up to 260 underwater dB at 1 m. Even such a common occurrence as heavy rain can increase noise levels by up to 35 underwater dB across a broad range of frequencies extending from several hundred Hertz to greater than 20,000 Hz.

The source levels for some specific natural sources of underwater sound are given in the following table. The source levels of sound producers are given in underwater dB at 1 m. Source levels are defined as if the receiver was one meter from the sound source. (Sound levels decrease as sound moves away from the source.) These are broadband source levels, which mean that they are the sum of the acoustic energy over all of the frequencies generated by the source. The sound levels given in the graph above are ambient noise levels in a 1 Hz wide bandwidth, meaning that the acoustic energy was only summed over 1 Hz. Because of this difference, they are therefore not directly comparable with the source levels given in the following table.

 

The sounds produced by marine life are many and varied. Marine mammals, such as humpback whales and bottlenose dolphins, produce sounds over a wide frequency range, from less than 10 Hz to greater than 200,000 Hz, depending on the species of marine mammal. Certain types of fish, such as the toadfish and croaker, and marine invertebrates, such as snapping shrimp, also produce sounds. Examples of marine animals sounds can be heard in the Audio Gallery.

Marine animals use sound to obtain detailed information about their surroundings. They rely on sound to communicate, navigate, and feed. Marine mammals, such as dolphins, use sound to locate and identify objects such as food, obstacles, and other whales. By emitting clicks, or short pulses of sound, and listening to the echo, dolphins can detect individual prey and navigate around objects underwater.

Marine mammal calls can actually increase ambient noise levels by 20-25 underwater dB in some locations at certain times of year. Blue and fin whales produce low-frequency moans at frequencies of 10-25 Hz with estimated source levels of up to 190 underwater dB at 1 m. The ambient noise levels at frequencies of 17-20 Hz increase off coastal California during the fall and winter months due to blue and fin whale calls.

Snapping shrimp inhabit shallow tropical and semi-tropical waters having a bottom of rock, shell, or weed that offers the animals some concealment. Colonies of snapping shrimp generate sounds with broad spectral peaks at frequencies of 2,000 - 15,000 Hz. Individual snaps can have peak-to-peak source levels as high as 189 underwater dB at 1 m. Sound generated by colonies of snapping shrimp can dominate other sources of background noise in shallow tropical waters.

It is likely that many more species of marine life make and use sounds than we currently know about.

The source levels for some sounds generated by marine life are given in the following table.

 

Sounds generated by human activities are an important part of the total ocean acoustic background. Undersea sound is used for many valuable purposes, including communication, navigation, defense, research and exploration, and fishing. However, some sounds are just a by-product of another activity, such as the noise generated by ships and by offshore industrial activities, including oil drilling and production.

Sounds generated by human activities cover a wide range of frequencies, from a few Hz up to several hundred kHz, and a wide range of source levels.

The source levels for some sounds generated by human activities are given in the following table.

It is likely that many more species of marine life make and use sounds than we currently know about.

The source levels for some sounds generated by marine life are given in the following table.

 

Source level tables with specific references in table