Sounding out the secrets of the sea

Key text

Sound under water is very important for animals. It allows them to navigate, to hear approaching predators and prey, and is a way of communicating with other members of the same species.

Humans too make use of sound to explore the underwater world. Sound is used for many reasons including geological and biological surveys and to locate oil and gas fields. To understand how the human use of sound can impact on animals we need to take a closer look at the use of sound by animals in the ocean.

How do animals hear under water?

Sun light does not penetrate water very well and visibility can be poor, even in shallow water. The properties of sound make it an ideal way to communicate under water (Box 1: Comparison of the properties of sound in air and water), so many animals use sound to communicate with each other and to ‘observe’ objects in a marine environment.

Marine animals have evolved a variety of ways to detect and make sound in water. Most fish, apart from sharks and stingrays, have sensory hair cells lining a small cavity in the ear that is filled with viscous liquid. Attached to the hairs (technically known as stereocilia) and suspended in the liquid of each ear, there is a small stone of calcium mineral called an otolith or ‘ear stone’. When a sound wave passes through the water and the body of the animal, the otoliths tend to stay still, relative to the movement of the fish. The inertia of the otoliths stimulates the stereocilia to convey a message to the brain.

Related site: What is an otolith? Explains what otoliths are and how they are used to determine the age of fish. (Bedford Institute of Oceanography, Canada)

Some fish have a lateral line of pores that open to a continuous canal along the length of the body. The canal contains structures called neuromasts that detect sound waves. The motion of water by the sound moves the hairs a tiny amount, and the supporting cells transmit a message to connecting nerve cells.

Another method uses enclosed pockets of air: either the lungs of mammals such as dolphins and whales or swim bladders in fish. The air in the swim bladder is easily compressed by sound pressure waves, which are converted to vibrations, allowing the fish to detect sound as well as vibrations. The sensitivity of fish to noise and vibration differs depending on the proximity of the swim bladder to the inner ear in different species.

How animals make sounds under water

Crustaceans such as crabs and lobsters which have an exoskeleton make sounds by hitting or scraping one part of their body against another to make it vibrate, similar to the way insects make sounds. The ‘snapping’ sound made by many such animals gives a series of sharp sound pulses that can be heard at a great distance. Because there are so many of these small creatures, they produce much of the background noise in oceans.

Related site: Listen up Provides recordings of sea animals including shrimp, whales and herrings. (Ocean Link, Canada)

Fish that have soft skin cannot produce sound in this way and must actively vibrate some part of their body. Some vibrate the air in their swim bladders to make sounds which then radiate into the surrounding water.

Another way of making sound is typified by whales and dolphins. They generally make sounds by moving air from one body cavity to another through some sort of valve with a vibrating ‘lip’. Because the density of the flesh of a sea animal is very similar to that of water the sound radiates efficiently, is quite loud and travels long distances.

Echo, echo, echolocation

Some animals such as whales and dolphins have evolved to use sonar (SOund NAvigation and Ranging) or echolocation to produce and detect sound. This compensates for the lack of visual information available in the ocean. The animal produces a very short high frequency ‘click’ by passing air through vibrating ‘lips’ in their head. The sound wave is directed mainly forward like a sonic headlight, being focussed by an organ in the head that contains fats. The sound is partly reflected by objects such as a rock or a fish, and is then transferred to the ear drum via their lower jaw, which includes an area filled with fats. The time delay for the pulse journey is only about 1.5 milliseconds per metre travelled, but this is long enough for the animal to determine the position of the object. Repetitive pulses help the animal to reduce the influence of background noise or clicks from other animals, and the exact frequency of the returning click gives information about the movement of the object.

Related site: Echolocation Explains how marine mammals use echolocation. (Ocean Link, Canada)

Depending upon the size of the target, sonar is useful over tens of metres or even more. Some animals have developed remarkably sensitive powers of discrimination: a dolphin can detect the difference between a solid and a hollow metal ball the size of a baseball at a distance of 20 metres.

Oceans of noise

Background noise in the ocean is produced by breaking waves, wind, rain and by the huge number of small crustaceans and other animals. A typical background noise level is about 100 decibels (dB), which is about the same in energy terms as 40 dB in air. Wind and waves in storms, and choruses from fish and invertebrate can increase this level to about 120 dB (Box 2: Measurement of sound levels).

Related site: A collection of sounds from the sea Provides recordings of ships, airguns, earthquakes, volcanoes and whales. (Ocean Explorer, National Oceanic and Atmospheric Administration, USA)

Measurements show that the Pacific Ocean is still relatively quiet and that most of its background noise is produced by wind and by marine creatures. This is in contrast to the Atlantic Ocean, where most of the background noise is from the churning propellers of ocean-going ships.

Relative noise levels from man-made and animal sources

The diagram below shows the frequency distribution of pressure levels for natural and man-made sources of sound in Australian waters. The decibel figures are low because they show the sound pressure levels in individual bands only one hertz wide at each frequency. The contributions of all these bands must be added together to get the total sound pressure.

Within one metre of a typical sonar transducer the sound power level can be as high as 180 dB, which is comparable to the peak level for animal calls. But human sonar transmitters can be arranged in long lines so that the signal maintains a high level of power at considerable distances (Box 3: Use of sonar in the sea).

The calls of the loudest animals in the ocean, such as seals and whales, have levels as high as 190 dB at a distance of one metre, which is about the same sound power level as a loud human shout in air at the same distance. Some echo-locating clicks can reach peak levels as high as 230 dB, though for only very short times.

Other sounds in the ocean, including undersea earthquakes and seafloor volcanic eruptions, have been recorded at levels that exceed that of close echo-locating clicks, reaching levels beyond 240 dB over very large areas.

Is the use of sonar by man harmful to ocean animals?

The potential impacts of sonar on marine animals are similar to those of humans exposed to noise and include:

• behavioural changes;
• temporary or permanent hearing loss or tissue damage; and
• physiological stress responses.

The species affected by the sound depends on the frequency and sound pressure level. For example sound sources such as airguns produce more noise in the hearing range of baleen whales than mid-frequency echo sounders, which produce more noise in the ranges used by seals and toothed whales. Sounds that disturb one species appear to have no effect on others.

The potential risks to animals posed by the use of sound by humans result from a combination of the energy level, frequency and local acoustical effects due to the underwater ‘landforms’. Instruments of sufficiently low power and high frequency pose a minor risk. The equipment with the highest risks are airgun arrays and low frequency, high-power transducers with wide beam angles.

How can we tell if an animal is affected by sound?

Related site: Effects of sound: How do you determine if a sound affects a marine animal? Lists a series of questions to be answered to determine whether a sound affects an animal. (University of Rhode Island, USA)

It is not always easy to determine whether an animal is harmed by exposure to a sound. Biologists and engineers have developed a new digital tag that provides information about whale behaviour, including how deep they dive, what they hear, and what sounds they make to communicate. The tags make it possible to do controlled experiments rather than one-off incidental observations of animal behaviour. CT scans and 3-D imaging are also being used to study the structure of the ears of marine mammals and how they might be injured by exposure to sound sources (Box 4: Disturbing beaching events).

Safe and sound

Several studies have been done to investigate the potential impact of the human use of sonar for research purposes, especially in the naturally quiet waters around Antarctica. International guidelines have been developed for the use of sonar, particularly when long arrays are used, to minimise the possibility of damage or confusion to sea animals. Strategies include using the minimum sound energy level required, providing an escape route for animals in the area, and minimal use at times when animals are more sensitive to disturbance, such as breeding or mating times. Researchers record details of acoustic activities to allow retrospective assessment of the cause of any future changes to the distribution, numbers or breeding patterns of animals. Until we know more about the impact on marine life of the use of sound, caution should prevail.

Box 1: Comparison of the properties of sound in air and water

Some properties of sound in air

Sound is generated by rapid vibrations that compress the air locally. The compressions, or sound waves, move away from the source in all directions at about 340 metres per second. The sound gets weaker with distance because it spreads more and more, but it is also gradually absorbed by the air. When a sound wave reaches our ears, the pressure vibrations cause movement in our eardrums and these vibrations are transmitted to nerve cells in our inner ear (the cochlea) by small bones in the middle ear.

Some properties of sound in water

Sound is produced and transmitted under water in much the same way as it is in air. One important difference is that water is 1000 times denser and is 30,000 times less easily compressed than air, so sound under water travels at about 1500 metres per second. There are other reasons why sound travels further in water. Sound in water is absorbed less quickly than sound in air and the waves spread in a circle rather than a sphere because the sea is shallow compared with the propagation distance, allowing it to travel long distances. Water layers that are more salty or colder than the surrounding water tend to confine the sound and this also helps its long-distance propagation. The combined effect is that sound in the ocean can propagate and be detected over hundreds of kilometres.

Box 2: Measurement of sound levels

The level of noise energy in the atmosphere is called the sound pressure level, measured in decibels (dB) above a reference level or 20 micropascals, which is about the threshold of human hearing. Every rise of 10 dB increases the sound power by a factor 10. More subtle measurements, given as dB(A), allow for the variation of human hearing ability with frequency. Noise in a quiet suburb at night is about 30 dB(A), noise beside a busy street in the city about 80 dB(A), and a rock concert about 110 dB(A). Above about 120 to 130 dB(A) the listener experiences severe discomfort. Levels above about 85 dB(A) can damage hearing if maintained continuously, and higher levels can do the same in a much shorter time.

Sound is measured in decibels under the sea too, but the reference level is one micropascal, rather than 20. When the sound pressure is converted to sound power there are other ‘adjustments’ to be made, because water is 1000 times as dense as air and the speed of sound in water is about five times greater than in air. Sound levels quoted for the ocean are generally about 60 dB higher than those in air for the same actual sound power.

Related sites:

• Sounds in the sea: How does sound in air differ from sound in water? (University of Rhode Island, USA)

• How loud is loud? (Cornell Laboratory of Ornithology, USA)

Box 3: Use of sonar in the sea

Marine researchers and animals use sonar in many different ways, but all methods fall into one of two categories: active or passive.

Passive sonar

Passive sonar simply listens to sound in the ocean environment using an array of receivers. Background sound provides ‘acoustic daylight’, but sources such as ships or calling whales are easily detected.

Active sonar

Active sonar uses underwater electronic sound generators spaced in an array to produce a beam of pulsed sound that can be swept from side to side like a searchlight: a similar array of receivers detects the echo signals. These echoes can be from the sea floor, from objects such as ships, shipwrecks or submarines, or from animals. Using computers it is then possible to produce maps of the sea floor and to highlight particular sorts of echoing objects, such as rocks, sand, or vegetation.

Applications of sonar

These sonar techniques are being used for many purposes around Australia, producing maps of the sea floor to determine areas to be set aside as marine park reserves, for studies of the behaviour of marine creatures, to search for ancient wrecks and to assess the numbers or variety of fish in an area for harvesting. Both passive and active sonar are also important in ship navigation and in the surveillance operations of the Australian Navy.

Related sites:

• Sounds in the Sea; How do people and animals use sound in the sea? (University of Rhode Island, USA)

• Deep sea mapping (Australia Advances, CSIRO, Australia)

• Found, maybe! Captain Cook’s Endeavour (News in Science, 17 May 2006, Australian Broadcasting Corporation)

• Underwater sonar (Quantum, 22 February 2001, Australian Broadcasting Corporation)

• SEA 1555 – Minehunter Coastal Project (Defence Materiel Organisation, Australia)

• Mapping our marine resources (Science and Technology in Action, Ireland)

Box 4: Disturbing beaching events

Could the extensive use of sonar for scientific or military purposes have effects on the health of sea animals? One well documented case in 2002 involved the use of medium frequency sonar by the US military and the beaching of beaked whales. Examination of the whales showed internal haemorrhaging and gas bubbles present in organs, evidence for tissue damage similar to that observed in divers with ‘the bends’. The study suggested that the observations were consistent with the effects of exposure to sonar and called for a limit to future military activities. Exposure to sublethal levels of sound that cause hearing impairment would also eventually be lethal to animals because they are unable to locate food or avoid predators.

The use of explosives under water can also be extremely harmful to sea animals. Explosives produce shock waves of very high intensity sound that can cause physical damage to the whole bodies of sea animals, not just their hearing, and either kill them or leave them severely injured.

Other beaching events

Not all beachings coincide with the use of sonar by the military. In many cases the reason for beaching is not identified. Could this be due to confusion induced by competing sonar signals or is there some other explanation?

Some animals that beach themselves are simply in poor health, suffering from ear infections for example, providing a clear link with their perception of sound and the stranding. It has been suggested that other beachings are the result of anomalies in ocean temperature or salinity in the region that make the land edge ‘invisible’ near smooth shallow beaches. The effect of chemical contamination on the ability of animals to hear sounds is also being investigated.

Australian researchers are looking for patterns in the beaching behaviour. They have found that seasonal fluctuations in the water temperature and nutrients can affect the abundance and location of prey. Peaks in the abundance of prey can be used to predict beaching events in Tasmania. Some areas experience more beachings than others, suggesting that localised features in the sea floor or shoreline increase the likelihood of events. When the presence of these features are combined with the behaviour of animals that have strong social bonds, distressing beaching events occur.

Related sites:

• Bends may be culprit in whale strandings (News in Science, 9 October 2003, Australian Broadcasting Corporation)

• Whale strandings (The Science Show, 6 November 2004, Australian Broadcasting Corporation)

• A whale strandings database and network is developing (Ecos, Issue 123, 2005, page 7)

• The fatal shore (School of Physics, University of Western Australia)

Activities

• Discovery School (Discovery Education, USA)
  • The phenomenon of sound: Waves – students learn that sound is a form of energy that travels in waves; that sound waves can travel through different mediums; and make observations about the behaviour of sound waves.

• Project Oceanography (University of South Florida, USA)
  • Introduction to marine mammals and acoustics – introduces basic acoustic principles and the movement of sound through air and water. Students gain an understanding of the importance of the study of acoustic oceanography and of why and how marine mammals use sound.
  • Sound production and reception – students learn how sound is generated and how it is interpreted in the human and marine mammal ear.
  • Sound use by marine mammals – students measure frequency, wavelength and the speed at which sound travels, why sound is emitted at different frequencies and appreciate the importance of sound to marine mammals.
  • Equipment and sounds people use to explore the oceans – students will gain knowledge and appreciation for research vessels and the equipment found on them.
  • Noise pollution – students learn what causes hearing damage in humans and animals, and that noise pollution is more than loud noises.
  • Recording sounds from wild marine mammals – students gain an understanding of the technology used by researchers to study underwater sound.

• Discovery of Sound in the Sea (University of Rhode Island, USA)
  • Classroom activities – provides a number of activities on the use of sound by marine mammals including: ‘Are we hearing the same thing?’; ‘On the trail of a whale’; ‘How do dolphins sense their environment?’ and ‘Thinking inside the box’.

• The Educator's Reference Desk (Information Institute of Syracuse, USA)
  • Seeing sound and sonar – introduces students to ocean floor mapping.

• SeaWorld/ Busch Gardens (USA)
  • Now hear this – students investigate their hearing range.

• Strike a Chord (The National Science and Technology Centre, Australia)
  • What did you say? – a series of activities to help students understand how the ear works.
  • Marco Polo is for whales – this activity uses the concept of the swimming pool game Marco Polo to introduce the principles of sonar.

• Otolith Research Laboratory (Bedford Institute of Oceanography, Canada)
  • A fun project to do in class – students dissect a fish head to identify otoliths.

Further reading

Useful sites

Discovery of sound in the sea (University of Rhode Island, USA)

Provides information about the properties of sound, animals and sound, audio and technology galleries, and the use of sound in water by man.
http://www.dosits.org/

An ocean of sound: An exploration of underwater acoustics (Oceanlink, Canada)

Provides an introduction to underwater sound, describes echolocation and the vocal abilities of some whales and seals.
http://oceanlink.island.net/oinfo/acoustics/acoustics.html

Sounding out the ocean's secrets (Beyond Discovery, National Academy of Sciences, USA)

Discusses the history and development of sonar.
http://www.beyonddiscovery.org/includes/DBFile.asp?ID=88

Killer whales: Senses (SeaWorld Busch Gardens, USA)

Looks at the senses of the killer whale, using diagrams to explain how they hear and use echolocation.
http://www.seaworld.org/animal-info/info-books/killer-whale/senses.htm

Looking at the sea: Physical features of the ocean (Ocean’s Alive, The Museum of Science, USA)

Describes common geographical features found under the sea off the coast of continents and the measurement of ocean depths using sonar.
http://www.mos.org/oceans/planet/features.html

Impacts of marine acoustic technology on the Antarctic environment (Scientific Committee on Antarctic Research, Australia)

A technical summary of the use of marine acoustics in Antarctica, its potential impact on animals and strategies to reduce the impact.
http://www.geoscience.scar.org/geophysics/acoustics_1_2.pdf

Glossary

CT scans. A series of X-ray images of the body. The body is X-rayed from many directions and the results are analysed by a computer. The computer generates images of cross-sections (slices) of the body. CT scans show details of the shape and location of soft tissues, as well as bones and blood. Other names for this technique are computerised tomography, CAT scan and computerised axial tomography. For more information see CAT scans (University of Colorado at Boulder, USA).

dB(A) or A-weighted decibels. Decibels with the sound pressure scale adjusted to conform with the frequency response of the human ear. A sound level meter that measures A-weighted decibels has an electrical circuit that allows the meter to have the same sensitivity to sound at different frequencies as the average human ear. There are also B-weighted and C-weighted scales, but the A-weighted scale is the one most commonly used for measuring loud noise.

decibel (dB). A logarithmic scale used to denote the intensity, or pressure level, of a sound relative to the threshold of human hearing. A step of 10 dB is a ten-fold increase in intensity or sound energy and actually sounds a little more than twice as loud.

The quietest sound we can hear is 0 dB; a soft whisper has about 100 times more sound energy and so is about 20 dB. A power lawn-mower has a factor of 10⁹ more sound energy and is about 90 dB. A rock band may be as high as 110 dB. Above 120 dB the sound produces discomfort and even pain. The scale is often adjusted to take account of the reduced sensitivity of human hearing to high and low frequencies and is then specified as dB(A). On this adjusted scale (the A-weighted scale), the range of human hearing is about 3 to 140 dB(A). Since decibels measure relative power level, a reference must be specified, which is normally 20 micropascals, or 10^-12 watts per square metre.

For more information see What is a decibel and what is the loudest sound I can listen to before it hurts my ears? (How Stuff Works, USA); What is a decibel? (University of New South Wales, Australia); and Sound properties and their perception – intensity and the decibel scale (The Physics Classroom, USA).

frequency. A measure of how frequently a wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – one oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is one million hertz – one million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).

newton (N). The SI unit of force. One newton gives an acceleration of one metre per second per second to a mass of one kilogram. The weight you feel when you hold an average size apple in your hand is about one newton (mass is about 0.1 kilogram and gravitational acceleration of about 10 metres per second per second). It is named after Sir Isaac Newton.

pascal (Pa). The SI unit of pressure equivalent to one newton acting uniformly over an area of one square metre (newton per metre squared).

radar. The use of reflected radio waves to determine the location of an object and its speed if it is moving. It is an acronym derived from radio detecting and ranging. For more information see How radar works (How Stuff Works, USA).

root mean square. The square root of the average of the squares of a set of numbers.

sound pressure. The pressure deviation from the ambient pressure caused by a sound wave. Sound pressure underwater is measured using a hydrophone. The unit for sound pressure is the pascal (Pa). The reference sound pressure in air is 20 micropascals (root-mean-square). In water, the reference sound pressure is one micropascal (root-mean-square).

sound pressure level (SPL). A logarithmic measure of the root mean square pressure (force/area) of a particular noise relative to a reference noise source. It is measured in decibels.

sound wave. A wave that is transmitted through a solid, liquid, or gas as a result of mechanical vibrations of particles in the medium. The direction of motion of the particles is parallel to the direction of propagation of the wave.

transducer. A device that converts one form of energy into another. For example, a microphone converts sound energy into electricity. A loudspeaker converts electrical energy into loudspeaker sound energy.