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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Importance of Sound

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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?

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Marine organisms that use sound have variable sensitivity to sounds of different frequencies. Generally they will hear well in the frequencies of sounds that they generate. Marine mammals hear best at the frequencies that they use and have thresholds of hearing that differ for each species. The threshold of hearing is the minimum intensity where, with normal hearing, a sound can be heard.

All species of baleen whales (Mysticetes), such as blue, fin, and humpback whales, produce low frequency sounds. Anatomical evidence and vocalizations strongly suggest that they are adapted to hear low frequencies. Direct studies have been performed on seven toothed whale species including dolphins, beluga whales and harbor porpoises. All species of toothed whales (Odontocetes) hear best in the high-frequency range (10,000 to 50,000+ Hz). Pinnipeds (seal and sea lions) have increasing sensitivity from low to high frequencies.

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What sounds can we hear?
How do marine animals hear sounds?
What are the effects of anthropogenic sound on marine mammals?

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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 football field).

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How do marine animals communicate using sound?
How do marine animals use or make sound when feeding?
How do marine animals use sound to navigate?

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People use sound in the ocean for a wide variety of purposes. Many important everyday activities, such as fishing, depend on sound for success. A primary use of sound is to locate objects in the ocean, including rocks on the seafloor, marine animals, submarines, and shipwrecks. Sonar (Sound Navigation and Ranging) is a technology that uses sound waves to identify objects and their locations in the ocean.

People commonly use sound to determine the depth of the ocean. The most common system for measuring water depth, and preventing collisions with unseen underwater rocks, reefs, etc., is the echo sounder, a form of active sonar. Fishermen use a version of echo sounding technology called a fish finder to locate fish.

Sound is used to study marine mammal distributions by listening to the sounds animals make. Different species of whales and dolphins produce different sounds, such as songs, moans, clicks, roars, whistles, and sighs. Each species is unique in its vocalizations. Scientists can listen for these sounds and track the different marine mammal species, and sometimes even individual animals, while they are producing sound.

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 football field).

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People & Sound

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Sound travels approximately 1500 meters per second in seawater. That's a little more than 15 football fields end-to-end in one second! Sound travels much more slowly in air, at approximately 340 meters per second, only 3 football 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

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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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How do you determine if a sound affects a marine animal?

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Research suggests that increased background noise and specific sound sources might impact marine animals in several ways. The potential impacts include sounds that cause marine animals to alter their behavior, prevent marine animals from hearing important sounds (masking), cause hearing loss (temporary or permanent), or damage tissue. In at least a few well-documented cases there is a relationship between the use of mid-frequency sonar and the stranding of cetaceans, particularly beaked whales.

Behavioral responses to sound vary greatly. In order to understand how anthropogenic sounds may impact marine life, the animal's reaction to known sounds must first be measured. Observations of normal behavior, "control" or "baseline" data, provide the reference points for measuring any changes occurring during or after sound exposure.

An animal's behavioral response depends on a number of factors, such as hearing sensitivity, tolerance to noise, exposure to the same noise in the past, behavior at the time of exposure, age, sex, and group composition. Some marine animal responses to sound are momentary inconsequential reactions, such as the turn of a head. Other responses are short-term and within the range of natural variation in these behaviors. In other cases, more significant changes in behavior have been observed. Some of the strongest reactions occur when the sounds are similar to those made by predators.

Just as it can be difficult to hear someone talking at a loud party, elevated noise levels in the ocean may interfere with marine animals' ability to hear important sounds. Masking occurs when a loud sound drowns out a quieter sound or when noise is at the same frequency as a sound signal. Masking is also influenced by the amount of time that the noise is present. The potential impacts that masking may have on individual survival, what things marine animals may do to avoid masking, and the energetic costs of changing behavior to reduce masking are poorly understood. However, because of the widespread nature of anthropogenic activities, masking may be one of the most extensive and significant effects on the acoustic communication of marine organisms today.

Exposure to loud sounds can cause hearing impairment or loss. Hearing loss depends on the intensity and frequency of the sound, and the duration of the animal's exposure to the sound. Just as humans exposed to extremely loud sounds for short periods of time (e.g. rock concerts) experience temporary or permanent hearing impairment (called temporary threshold shift or TTS and permanent threshold shift or PTS, respectively), marine mammals and fishes might also experience hearing loss from exposure to anthropogenic sounds. Hearing damage can also be caused by exposure to moderate levels of noise over long periods of time. Hearing loss due to noise does not occur if the frequency of the sound to which the animal is exposed is outside the range that the animal can hear.

Stranding events involving multiple beaked whales have been reported that coincided closely in time and space with military activities using sonar. There are many causes of marine mammal strandings, some natural and some related to human activity. In a small number of well-documented cases, there is sufficient information about the sonar operations, the times and locations of the strandings, and the animalsŐ injuries to associate the strandings with sonar use.

The mechanism by which the sonars might have caused the strandings is still a mystery. Much more scientific research is needed to understand why there is a relationship in time and location between the beaked whale strandings and the use of multiple, mid-frequency sonars in nearshore areas. At present, it is uncertain whether stranding events are limited to beaked whales and near shore areas.

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What are the effects of anthropogenic sound on marine animals?

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Marine mammals are known to strand, and unfortunately many stranded animals die. Strandings occur worldwide. Approximately 40,000 stranded marine mammals were reported in the United States alone by the National Marine Fisheries Service stranding network over the decade 1990-2000, with an average of 3,600 strandings per year (1). Investigating the cause of a stranding is very difficult, because scientists must try to reconstruct what happened after the event, based on limited information. As a result, it is often impossible to draw firm conclusions about the cause of a particular stranding.

One controversial issue is the extent to which there is a relationship between the use of military sonar and the stranding of cetaceans, particularly beaked whales. Beaked whale strandings are relatively rare, with 17 beaked whale strandings reported in the U.S. in 1999 and 5 strandings in 2000 (1). Strandings of more than one beaked whale at the same time are very uncommon. Stranding events involving multiple beaked whales have been reported that coincided closely in time and space with military activities using sonar (2).

In three well-documented cases, there is sufficient information about the sonar operations, the times and locations of the strandings, and the injuries to the animals to associate the strandings with sonar use. These events occurred in Greece (1996), the Bahamas (2000), and the Canary Islands (2002). There are currently only limited scientific publications describing and discussing these strandings, and most of these publications have not undergone independent scientific review. Although these strandings are closely related in time and space to the operation of military sonars, the mechanism by which the sonars might have caused the strandings is still a mystery. These three stranding events, and ways in which scientists think that the use of sonars might have resulted in the strandings, are described below.

In May, 1996, eleven Cuvier's beaked whales stranded along 38 kilometers of Greece's coastline in the Mediterranean Sea. This stranding coincided with a nearby military exercise conducted by the North Atlantic Treaty Organization (NATO). The exercise used sonar at frequencies of 450-700 Hz and 2.8-3.3 kHz. This incident is described in both a North Atlantic Treaty Organization report (3) and in a published scientific paper (4). The stranded whales were too decomposed to reveal the cause of death.

Fourteen beaked whales and two minke whales mass stranded in the Northeast and Northwest Providence Channels of the Bahamas Islands on March 15 and 16, 2000. Six beaked whales are known to have died. The strandings were clustered in time (within a 36-hour period) and in space (along a 240-km arc), and were strongly correlated with the passage of five U. S. Navy ships taking part in an exercise and using mid-frequency (1-10 kHz) sonars. The incident has only been described in a preliminary report issued jointly by the U.S. Navy and the National Marine Fisheries Service (5). The stranded animals that died were examined for injuries by scientists. Four of the beaked whales examined had unusual hemorrhages near and around the ears that occurred before death.

In September, 2002, a mass stranding of fourteen beaked whales occurred in the Canary Islands. These strandings began about four hours after the start of a nearby international naval exercise using mid-frequency sonar. The details of the sonar transmissions that occurred are not available. Ten of the stranded animals were examined. Gas bubbles and tissue damage were found in several organs (6 and 7).

One hypothesis that has been proposed to explain the internal hemorrhaging that was observed in the Bahamas stranding is that tissue damage can occur when resonance from loud sounds causes air- or fluid-filled organs (such as the lungs or swim bladder) to vibrate at very large amplitudes. As the organs vibrate, the tissues surrounding the organs might become damaged. NOAA held a workshop in 2003 to discuss resonance in cetaceans (for more information see Report of the Workshop on Acoustic Resonance as a Source of Tissue Trauma in Cetaceans). The workshop concluded that acoustic resonance was not likely the cause of the Bahamas stranding for several reasons. One reason was that the mid-frequency (1-10 kHz) sonars did not operate at the lung's resonant frequencies (8).

A hypothesis that has been proposed to explain the gas bubbles and tissue damage observed in the stranding in the Canary Islands is that they were consistent with decompression sickness (6 & 7). The scientists suggested that beaked whales might have changed their diving pattern in response to the sounds and come to the sea surface faster than normal, causing bubbles to form in the tissues. This hypothesis is still being debated (9, 10) and more research is needed to develop conclusive answers. The gas bubbles and tissue damage that have been observed could have resulted from many causes, some that are not related to sound (3). A recent report has found degeneration in the bones of sperm whales specimens obtained over the last 111 years (11). The scientists hypothesize that the degeneration is due to bubble formation associated with decompression sickness that is unrelated to sound. These hypotheses about decompression sickness, bubble growth, and degeneration in the bones of sperm whales have not been tested, and they should not be used as scientifically accepted explanations until they are. They are basically ideas that scientists are now testing and may or may not be correct.

Another hypothesis to explain the cause of the tissue damage is that sound causes bubbles to form or grow in tissues that are supersaturated with nitrogen. One way this could happen is through a process called rectified diffusion (12). In this hypothesis, sounds cause small bubbles that normally exist in the blood and tissues to grow larger. It is unlikely that this process caused the tissue damage observed in the Bahamas stranding because the sound exposures were too short. However, if sound caused bubbles to form or grow, they would continue to grow by static diffusion as long as the tissues remained supersaturated, which could resulting in tissue damage. Whether or not this hypothesis is plausible for marine mammals is still being debated (3, 13, 14).

Much more scientific research is needed to understand why there is a relationship in time and location between the beaked whale strandings and the use of multiple, mid-frequency sonars in nearshore areas (3). At present, it is uncertain whether stranding events are limited to beaked whales and near shore areas. Science is an evolving process and future work may help us further understand what we are observing.

See the list of references for this FAQ entry

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What are the effects of anthropogenic sound on marine mammals?

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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?

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