5.12: Transmission of Sound

Electromagnetic radiation travels both through a vacuum and through substances such as air and water. Sound is fundamentally different because it cannot be transmitted through a vacuum. The sound effects in space movies are pure fantasy, because sound cannot travel across the vacuum of space. Sound is transmitted as a vibration in which adjacent molecules are compressed in sequence. One molecule is pushed into the next, increasing the “pressure” between the two molecules. The second molecule pushes on a third, increasing the pressure between the second and third molecules and relieving the pressure between the first two molecules, and so on. Hence, sound waves are pressure waves transmitted through gases, liquids, and solids. The pressure waves can range from very high frequencies, which the human ear cannot hear (ultrasound), to low frequencies below the audible range that we sometimes feel as vibrations from our surroundings.

Sound waves are absorbed, reflected, and scattered as they pass through water much less than electromagnetic radiation is. Therefore, sound travels much greater distances in water, and sound waves are the principal tools of communication and remote sensing for both oceanographers and marine animals.

Sound Velocity

The speed of sound is about four times greater in water than in air. In seawater, the speed of sound increases with increases in salinity, pressure, and temperature (Fig. 5-21a,b). However, the changes due to salinity variations are relatively small within the range of salinities in ocean waters. Thus, sound velocity generally decreases with depth in the upper layers of the oceans, where the temperature change is large, but then increases again with depth in the deep layers, where temperature variations are small and pressure changes are more important (Fig. 5-21c). Sound velocity in the oceans can be determined if the salinity, temperature, and depth (pressure) are known. 

Figure 5-21. The velocity of sound in ocean water varies with temperature, pressure, and salinity. Sound velocity increases with (a) increasing temperature or salinity and (b) increasing pressure. Salinity is comparatively less important in determining sound velocity than temperature or pressure. (c) Variations of pressure and temperature with depth in the oceans produce changes in the sound velocity that are sometimes complex. Generally, sound velocity decreases with depth until it reaches a minimum at about 1000 m (1 km) and then increases as depth increases.

Sonar

Sound velocity is important because sound waves are used to measure distances in water. Sound pulses sent through water bounce off objects and the seafloor. If the sound velocity at all points within the sound path is known and the time it takes for the sound to travel to and from an object is measured, the distance to the object (most often the seafloor) can be calculated. This is the principle of sonar.

Sonar systems send and receive sound pulses to measure ocean floor depth or to measure the distance and direction of objects that reflect sound, such as submarines. Schools of fish, concentrations of tiny animals called zooplankton on which fishes feed, and concentrations of suspended sediment can also be detected with sonar.

Different frequencies are employed for different applications. For example, low frequencies are used for deep-ocean sounding. Higher frequencies allow smaller particles (or animals) to be detected, but higher-frequency sound is more effectively absorbed by water. Therefore, higher frequencies are used in applications involving small objects in shallow water, such as finding fishes and tracing plumes of particles from river discharges or ocean dumping. Dolphins use their natural sonar to locate objects such as fishes, and they can tune the sound frequencies they use: low frequencies for long distance, and higher frequencies for more detailed echo “vision” at shorter distances.

Sound Refraction and the Sound Channel

Just as light is refracted as it passes between air and water, sound waves are refracted as they pass between air and water (Fig. 5-22a), or through water in which the sound velocity varies. At any specific ocean depth, the parameters that determine sound velocity are fairly constant over great distances (Chap. 10). Hence, sound waves that travel horizontally are little affected by refraction. Sound waves that travel vertically are also not affected significantly by refraction, because they pass perpendicularly to the horizontal layers of varying sound velocity. In contrast, sound waves that pass through the ocean at any angle other than horizontal or vertical are refracted in complicated curving paths. The paths are determined by the initial direction of the sound waves and their varying velocity in the waters through which they pass (Fig. 5-22b).

Figure 5-22. (a) Sound of certain wavelengths may be reflected when it encounters an interface between two water layers in which the sound velocity is different. Sound of other wavelengths can pass through such an interface and be refracted. (b) Sound can be reflected and refracted in complex ways. A sonar pulse sent from the surface can be refracted so that there is a shadow zone within the depth range around the sound velocity minimum. Because sound emitted by a surface vessel cannot enter the zone, submarines can hide in this shadow zone. (c) A sound generated at the depth of minimum sound velocity can be refracted back and forth within the low-velocity sound layer as it travels out from the source. This layer is called the “sound channel.” Sound can be transmitted very long distances horizontally within the sound channel with little loss of intensity. (d) Outside the sound channel, sound spreads spherically, and its intensity is reduced in proportion to the square of the distance from the source. Within the sound channel, sound spreads cylindrically and loses intensity only in proportion to the distance from the source.

Two important consequences of sound refraction in the oceans are of interest. First, sonar sound pulses sent out by a vessel at any downward angle not close to the vertical are refracted away from a zone called the “shadow zone” (Fig. 5-22b). The zone starts at a depth where sound velocity is at a maximum, being slower at both shallower and deeper levels. Such a maximum is present within submarine operating depths in many parts of the oceans. Hence, submarines can hide from sonar detection if they can remain in the shadow zone. This is one reason why the navies of the world continuously monitor changes in the temperature and salinity of the upper few hundred meters of the ocean throughout their operating areas. Small changes in these characteristics can change the location and effectiveness of the shadow zone. Because sound travels in complicated curved paths between the sources and echoing objects, knowledge of salinity and temperature distributions is essential in determining the sound velocity distribution and estimating the location and depth of an object detected by sonar.

The second important consequence of sound refraction in the oceans is that sound emitted at or close to  the depth of a sound velocity minimum is focused in what is called the “sound channel” (Fig. 5-22c). With the exception of sound waves traveling out vertically (or nearly so) from such a source, all sound waves are refracted back and forth as they pass alternately up and down across the depth of the velocity minimum. A sound velocity minimum, and therefore a sound channel, is present at a depth of several hundred meters throughout most of the world’s oceans. Sound normally spreads out spherically (in all directions), and the loss in intensity (attenuation) due to the spreading is proportional to the square of the distance from the source (Fig. 5-22d). In contrast, sound is focused in the sound channel and spreads cylindrically, so that the attenuation is proportional only to the distance from the source. Hence, sound can travel very large distances within the sound channel with relatively little attenuation. For example, sounds of small underwater explosions in the sound channel near Australia have been detected as far away as Bermuda in the North Atlantic Ocean.

Acoustic Thermometry

The vast distance over which sound can travel in the sound channel is the basis of an experiment, begun in 1991, that provides a sensitive means to determine average ocean temperature. 

Determining whether the Earth’s average surface and/or atmospheric temperature has recently changed or is now changing as a result of greenhouse effect enhancement is very difficult. Changes of only one-tenth of a degree per year in the global average temperature could drastically change the Earth’s climate in just a few decades. However, because of the high temporal and spatial variability of temperatures in the ocean, land surface, and atmosphere, the best measurements currently possible from conventional thermometers or satellites are not accurate enough to identify changes in the global average temperature of less than about a tenth of a degree. In contrast, changes of only a few thousandths of a degree per year in the average temperature of ocean water can be identified by measurement of the travel times of sounds over distances of thousands of kilometers in the sound channel. This method of monitoring is known as “acoustic thermometry.”

The principle of acoustic thermometry is simple and was proven with a preliminary experiment, the Acoustic Thermometry of Ocean Climate (ATOC) program, in 1991. A large underwater transducer (a sound producer like a loudspeaker) is lowered into the sound channel. In the preliminary experiment, the transducer was placed in the ocean near Heard Island in the southern Indian Ocean (Fig. 5-23). The transducer transmits low-frequency sound pulses into the sound channel. At several locations tens of thousands of kilometers away, sensitive hydrophones detect the incoming sound and precisely record its arrival time. The travel time measured to within a fraction of a second is used to compute the average speed of sound between transducer and receiver.

Figure 5-23. The Heard Island experiment. Sound pulses were emitted from a source in the sound channel at Heard Island, and the time of travel to the various receiving sites (shown by the black dots) was measured very precisely. Because the travel time of sound would change if the average temperature of the ocean water in the sound channel changed, arrays like this could be used to monitor changes in the average temperature of ocean waters at the depth of the sound channel.

The speed of sound in seawater increases with increasing temperature. Therefore, if the travel time between a transducer and a listening point in the system decreases from year to year, it indicates that the average temperature within the sound channel between those two points has increased (unless the average salinity or average depth of the sound channel has changed). An experimental acoustic array with the sound source located just north of the Hawaiian Islands and receiving stations surrounding the Pacific Ocean basin at distances of 3000 to 5000 km from this source has demonstrated that average ocean temperature measurements can be made with a precision of about 0.006°C. Results from this program have revealed surprisingly large seasonal and other short-term variations of average temperatures. The program operated between 1996 and 2006 but did not continue long enough to reveal any long-term trends. The program was abandoned, at least in part, because of controversies concerning the possible effects of the sounds produced on marine species, especially whales, dolphins, and other marine mammals.  

Recently the idea of monitoring ocean temperatures by acoustic thermometry has been revived by studies of acoustic waves that are created by earthquakes and that travel in the sound channel. Results obtained using this technique have identified temperature fluctuations in the East Indian Ocean on timescales of 12 months, 6 months, and approximately 10 days, and have enabled an estimate of the decadal warming trend that is consistent with but significantly higher than the trend estimated from Argo float data. As with all environmental measurements, the existence of two independent techniques for measuring trends and assessing  temporal variability promises to substantially improve the accuracy and detail of ocean water temperature change monitoring studies. 

Ocean Noise

Scuba divers know that the undersea world is not silent. Besides the sounds of breathing regulators and boat propellers, an observant scuba diver hears a variety of other noises. They often sound like a well-known snap-crackle-pop cereal. The natural sources of sound in the oceans are many and varied, but they generally fit into three categories: noises from breaking waves and bursting air bubbles, noises from vessels and other human mechanical equipment, and biological noises. Each of the sources produces sounds across a wide range of frequencies.

Biological sounds are produced by many species and through various means. For example, whales and dolphins use sound to locate objects, such as prey, and to communicate with each other. Dolphins may also use intense bursts of sound to confuse or stun their prey. Certain crustaceans (shrimp, crabs, and lobsters) make clicking noises as they close their claws, and certain fishes make sounds by inflating and deflating the swim bladder (a gas sac used to control buoyancy).

Natural sounds and the sounds of human activities, both of which are always present in the ocean, constitute a background noise above which the sonar sound probes used by oceanographers or navies must be strong enough to be heard. Submarines can be detected easily if they send out sonar signals to locate vessels that are searching for them, so instead they use sensitive directional microphones to detect the noise of ships’ propellers.

Because sound can travel very long distances in water, sensitive microphones can pick up sounds made hundreds or even thousands of kilometers away. Using sensitive microphone arrays, navies obtain data that are analyzed by supercomputers to separate the sounds of submarine or ship engines and other ship noises from background noise. This technology has become highly sophisticated. In some cases, the listening arrays can identify a specific submarine or surface vessel by its own specific sound “fingerprint,” determine the vessel’s location precisely, and track its movements across oceans. This capability has been extended so that the system can now identify and track some species in the oceans including whales. Individual whales can even be identified and tracked as each whale makes its own unique sounds.