Frequently Asked Questions
1. What are common underwater sounds?
The ocean is filled with sound. Underwater sound is generated by a variety of natural sources, such as breaking waves, rain, lightening strikes, cracking sea ice, undersea earthquakes, eruptions from undersea volcanoes and marine life. It is also generated by a variety of man-made sources, such as ships, sesmic devices, and sonars. Some sounds are present more or less everywhere in the ocean all of the time.
Background sound 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.
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.
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Examples of source levels of common underwater sound producers:
underwater dB at 1 m
|seafloor volcanic eruption||255|
|humpback whale song||144-174|
|sperm whale click||236|
|mid-frequency naval sonar||235|
2. Why is sound important to marine animals?
Sound travels far greater distances than light under water. Light travels only a few hundred meters in the ocean before it is absorbed or scattered. Even where light is available, it is more difficult to see as far under water as in air, limiting vision in the marine environment. In addition to sight, many terrestrial animals rely heavily on chemical cues and the sense of smell for important life functions (such as marking territorial boundaries). Olfactory cues are restricted in the marine environment. Therefore the sense of smell is much less important to some marine species. Underwater sound allows marine animals to gather information and communicate at great distances and from all directions. Many marine animals rely on sound for survival and depend on adaptations that enable them to acoustically sense their surroundings, communicate, locate food, and protect themselves under water.
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3. How does sound in water differ from sound in air?
Sound in water and sound in air are both waves that move similarly and can be characterized the same way. Sound waves can travel through any substance, including gases (such as air), liquids (such as water), and solids (such as the seafloor). Because liquids and gases have different properties such as density, they also have different sound speeds. This is one of the reasons that decibel levels in air can’t be directly compared with decibel levels in water.
Confusion also arises because there is a different scientific convention for measuring sounds in water and air. Scientists have arbitrarily agreed to use the intensity of a sound wave with a pressure of 1 microPascal (µPa) as the reference intensity for underwater sound. In air, scientists have agreed to use the intensity of a sound wave with the higher pressure of 20 µPa as the reference intensity. Scientists selected this value because sounds in air at a frequency of 1000 Hz and with a pressure of 20 µPa can just barely be heard by most people.
This is similar to reporting the temperature. To simply say that it is 50 degrees outside is confusing because 50 degrees Fahrenheit is equal to 10 degrees Celsius, whereas 50 degrees Celsius is equal to 122 degrees Fahrenheit – quite a difference! To make sure there is no confusion, we indicate what temperature scale we are using. It is the same thing with dB scales in air and in water. To avoid confusion, you need to specify that sounds in water, a denser medium, were measured relative to 1 µPa and that sounds in air were measured relative to 20 µPa. To make the distinction clear for the reader, the Discovery of Sound in the Sea resources use “underwater dB” for underwater sounds.
Sound waves in water and air have relative intensities that differ by 61.5 dB. This amount must be subtracted from relative intensities in water referenced to 1 µPa to obtain the relative intensities of sound waves in air referenced to 20 µPa.
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How does Sound in air differ from sound in water?
4. What sounds can marine mammals hear underwater?
Every species hears a particular range of frequencies. Some species have hearing ranges similar to people but many have good hearing at higher or lower frequencies we cannot hear at all. Most marine animals can hear lower frequencies moderately well, but some animals, like dolphins and porpoises, cannot hear low frequencies well, but, like bats, they can hear ultrasonic (high frequency) sounds very well.
All species of mysticetes (baleen whales), such as blue, fin, and humpback whales produce low frequency sounds (less than 1 kHz). There are no direct measurements of hearing thresholds for baleen whales, but anatomical evidence and the fact that they commonly produce sounds at low to infrasonic frequencies imply that many mysticetes hear best at frequencies below 10 kHz. Many behavioral and physiological studies have been performed on a number of odontocete (toothed whale) species including bottlenose dolphins, beluga whales, and harbor porpoises. All species of toothed whales tested have been found to hear best at higher frequencies (10 kHz to 110 kHz). Pinnipeds (seals and sea lions) also have hearing ranges that vary by species. Most have moderately good. low frequency hearing and, like whales and dolphins, some can hear at very low frequencies while others have some hearing into the ultrasonic range (but not as high as most odontocetes). A few pinnipeds that were tested in air vs. water appear to have somewhat different sensitivities in each medium.
There are also measurements of hearing in some fish species. The majority of species, including salmon, tuna, sturgeon, and most others, can detect sound from below 50 Hz up to 500 or 1,000 Hz. A smaller number of species, such as catfish, some squirrelfish and some croakers, as well as herring, hear sounds from below 50 Hz to 3 or 4 kHz and are considered to be among the best hearing fish. Additionally, a few species, all related to menhaden and shads, actually can detect sound to over 200 kHz, an adaptation to detect echolocation clicks of odontocetes in order to avoid being eaten.
5. How do animals use sound underwater?
Marine animals use sound to sense their surroundings, communicate, locate food, and protect themselves underwater. They generate sounds to attract mates, defend territories, and coordinate group activities. Marine mammals use sound to maintain contact between mother and offspring, for reproduction, and to display aggression. Fishes produce various sounds that are used to attract mates as well as to ward off predators. Some marine invertebrates, such as spiny lobsters, are thought to produce sound in order to scare away predators.
One of the best-known examples of animals that use sound over long distances for reproduction is the song of the humpback whale. Male humpback whales produce a series of vocalizations that collectively form a song. These songs can be heard miles away. Humpback songs are complex in structure and long in duration. Whales have been known to sing the same song for hours.
Reproductive activity, including courtship and spawning, accounts for the majority of sounds produced by fishes. Croakers are renowned for their sound producing ability. During the spawning season, these fish form large groups that vocalize for many hours. These vocalizations often dominate the acoustic environment in which they occur.
Some marine mammals also use sound to locate food and navigate through water. Toothed whales use echolocation to find prey and avoid obstacles. These whales send out sounds that are reflected back when they strike an object. Echolocation functions just like active sonar systems. The echoes provide information about the size, shape, orientation, direction, speed, and even composition of the object. Dolphins have an ability to detect and identify a target the size of a golf ball at a distance of 100 meters (more than the length of a soccer field).
6. How do people use sound underwater?
People use sound in the ocean for a wide variety of purposes, employing both active and passive acoustics. A primary use of active acoustics is to locate objects in the ocean, including rocks on the seafloor, marine animals, submarines, and shipwrecks. Active acoustics is also used to map and characterize ocean sediments, explore for oil and gas, and determine suitable locations for structures such as wind turbines.
People commonly use sound to determine the depth of the ocean. The most common tool for measuring water depth as well as preventing collisions with unseen underwater rocks, reefs, etc., is the echosounder, an active sonar. Fishermen also use a version of an echosounder, called a fish finder, to locate and identify fish. Scientists also use active acoustics to measure fish and plankton.
Active acoustics also helps scientists to study the physical characteristics of the ocean such as temperature, salinity, and currents. Scientists use tools such as Acoustic Doppler Current Profilers (ADCPs) to determine the speed and direction of ocean currents by measuring how the frequency of a sound changes as it reflects from a moving object. These tools can also be used to measure waves in the surf zone.
Passive acoustics can be used to investigate undersea earthquakes and volcanoes. Passive acoustics is also used to estimate marine animal abundance and study their distributions. For example, each species of whale and dolphin produces distinctive sounds including songs, moans, clicks, sighs, and buzzes. Scientists can listen for these sounds and track different marine mammal species, sometimes even individual animals, while they are producing sound. Passive acoustic sensors have proven to be an effective complement to visual marine mammal surveys.
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People and Sound
7. How does sound travel relatively long distances underwater?
Sound travels approximately 1500 meters per second in seawater. That’s a little more than 15 soccer fields end-to-end in one second! Sound travels much more slowly in air, at approximately 340 meters per second, only 3 soccer fields a second. The speed of sound in seawater is not a constant value, and although the variations in the speed of sound are not large, they have important effects on how sound travels in the ocean.
A sound channel exists in the ocean that allows low-frequency sound to travel great distances. This channel is called the SOund Fixing And Ranging, or SOFAR, channel. Sound bends or refracts towards the region of slower sound speed, creating this sound channel in which sound waves can travel long distances.
In the spring of 1944, ocean scientists, Maurice Ewing and Joe Worzel, departed Woods Hole, Massachusetts, aboard the research vessel R/V Saluda to test a theory that predicted that low-frequency sound should be able to travel long distances in the deep ocean. A deep receiving hydrophone was hung from R/V Saluda. A second ship dropped 4-pound explosive charges set to explode deep in the ocean at distances up to 900 miles from the R/V Saluda’s hydrophone. Ewing and Worzel heard, for the first time, the characteristic sound of a SOFAR (SOund Fixing And Ranging) transmission, consisting of a series of pulses building up to its climax.
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The SOFAR Channel
8. How do you determine if a sound affects a marine animal?
The process for considering if and how much a sound source is likely to affect marine animals is called ecological risk assessment. The first step of this scientific process is to identify the problem. The next stage involves estimating the probability of being exposed to the problem and, based on that exposure, determining the types of ecological effects that are expected. Then the risk can be estimated.
This general model can be used to determine if a specific sound source might affect a particular species by answering the following questions:
- What is the level of sound at different distances and depths as sound travels away from the source?
- Where are marine animals likely to be located relative to the source?
- What are the sound levels and durations to which the animals are likely to be exposed?
- Can the animal sense these sounds?
- What effects might these sound levels have on the animals?
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Determine if a sound affects a marine animal
9. What do we currently know about the effects of sound on marine animals?
Research suggests that increased background noise and specific sound sources might impact marine animals in several ways. Certain sounds may cause marine animals to change their behavior, prevent marine animals from hearing important sounds (masking), cause hearing loss (temporary or permanent), or strandings (in the case of marine mammals).
10. What has been scientifically proven about sound and marine mammal strandings?
The term stranding refers to an aquatic animal, especially a marine mammal, landing on a beach or in shallow water, dead or sometimes alive, and probably in distress. Some animals strand live and are returned to sea. Others die at sea or on shore. Animals may strand singly or in groups. When 3 or more animals strand together in time and place, it is called a mass stranding.
Observations as far back as Ancient Greece show that marine mammals have been stranding for millennia. There are many identified causes of strandings, including disease, ship strike, pollution exposure, etc. One controversial and unresolved issue is how the use of military sonar relates to strandings, particularly strandings of some species of beaked whales. In four well-documented cases worldwide, there is sufficient information about military sonar operations, the times and locations of the strandings, and the injuries to the animals to associate the strandings with sonar use. In these four events, less than 50 animals are known to have stranded. In comparison, about 1,000 cetaceans and 2,500 pinnipeds strand annually in the U.S. alone.
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Marine Mammal Strandings
11. How can we mitigate the effects of sound on marine animals?
Actions may be taken to reduce effects on marine life. If it is not possible to eliminate the sound source, it may be possible to change the frequency or amplitude of the sound source. Gradually increasing the sound source level (“ramp-up”) or using bubble screens or barriers around stationary sources are other approaches that have been used. Another obvious way to mitigate the effects of anthropogenic sound is to avoid concentrations of marine animals.
Federal laws such as the Endangered Species Act, Marine Mammal Protection Act, and National Environmental Policy Act that aim to protect animals from harassment (including impact from sound sources) have motivated studies of marine animals and the development of mitigation techniques and alternative technologies. The extent to which many commonly used mitigation measures are effective has not been determined.
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How can we moderate or eliminate the effects of human activities?