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The ocean is sometimes characterized as a noisy place, though it may seem silent to scuba divers in the open ocean because most of the sounds are not discernible to human ears. Sound levels do vary considerably in the sea in the horizontal and vertical planes, and only a part is generated by the activities of humans. Detection of sound and other vibrations is a sensory modality shared by virtually all oceanic species. Some pelagic species, bottlenose dolphins, for example, use echolocation to locate prey, just as a bat does.

Sound levels do not vary predictably in the ocean except in a general sense. Increasing distance from the crashing waves of a rocky shore will decrease the levels of ambient sound, as will increasing depth and distance from the wind‐induced turbulence of surface waters. However, the properties of sound do vary predictably, and a presentation of some basic concepts now will help with later discussions of hearing and mechanoreception in open‐ocean fauna. The physics of sound is quite complex; only the most rudimentary aspects will be covered in this book.

“Sound is a longitudinal mechanical wave that propagates in a compressible medium” (Rogers and Cox 1988). A mechanical wave results from the displacement of an elastic medium from its original position, causing it to oscillate about an equilibrium position (Figure 1.19a; Halliday and Resnick 1970). The trick here is to recognize that the disturbance, or mechanical wave, moves through the medium with no resulting movement in the medium itself. A good visualization of the passage of a mechanical wave is to think of a cork bobbing while a surface wave passes underneath it. The cork moves up and down as the wave passes by, transferring some energy to it, but the cork does not follow in the wave’s path.

A longitudinal wave is propagated in a back and forth motion along the direction of propagation (Figure 1.19a). This is in contrast to a transverse wave (such as an electromagnetic wave – see the treatment of light as follows), which yields a displacement at right angles to the axis of propagation. Visualize the propagation of sound through a medium by imagining the movement of a rapidly oscillating piston in a tube. As the piston moves back and forth, it creates regions of higher and lower pressure, areas of compression and rarefaction (Figure 1.19b). Tiny volumes of water (water particles) oscillate in place as the areas of compression, or wave fronts, of the sound wave propagate past them. The hair‐like sensory elements of open‐ocean fauna are designed to detect the water motion, or vibration of sound, as it moves past them.

Figure 1.19 Mechanical wave propagation. (a) Transverse wave. Particles displaced perpendicular to the direction the wave travels; (b) Longitudinal wave. Particles displaced parallel to the direction the wave travels.

Source: Halliday and Resnick (1970), figure 16.1 (p. 301). Reproduced with the permission of John Wiley & Sons.

The speed of sound in a medium is a function of the medium’s compressibility: the stiffer the medium, the faster sound will propagate through it. That is why the speed of sound in water is very much faster (4.3 times faster) than it is in air. However, to know if the speed of sound varies with depth in the ocean, we need to know a little more than that. We already know that the density of water does not increase much with increasing pressure. The ratio of the change in pressure on a volume of water (Δp) to the resulting change in volume of that water (−ΔV/V) is known as its bulk modulus of elasticity (“B”, Halliday and Resnick 1970, Denny 1993). B is positive because an increase in pressure results in a decrease in volume (or increase in density).

(1.9)

where Δp is the change in pressure, ΔV is the change in volume, p is the ambient pressure, and V is the volume at the original pressure. Put in a more empirical way, the same equation can be expressed as (Denny 1993):

(1.10)

where p is the ambient pressure, p₀ is the pressure at 1 atm, V is the volume at pressure p, and V₀ is the volume at 1 atm. The bulk modulus of water is about 2 × 10⁹ Pa depending on the temperature, which is a very considerable pressure. As mentioned earlier, the Challenger Deep at about 11 km of depth would yield a pressure of about 10⁹ Pa, not nearly enough to double the density of water.

The speed of sound through water is equal to the square root of the ratio of its bulk modulus to its density (Denny 1993).

(1.11)

where c is the speed of sound, ρ is the density, and B is the bulk modulus.

The declining temperature and increasing pressure with increasing depth in temperate and tropical regions act to change the speed of sound such that there is both a maximum and a minimum velocity in the top 1500 m (Figure 1.20). At the bottom of the mixed layer, the pressure has increased very slightly, increasing the B also very slightly, but the density has remained the same. The result is a maximum sound velocity at the bottom of the mixed layer. The minimum speed results from the influence of declining temperatures over the permanent thermocline. The increase in density with depth in the denominator is not enough to offset the decline in bulk modulus with depth resulting from the declining temperatures, producing a minimum speed near the bottom of the permanent thermocline. Once the bottom of the permanent thermocline is reached, the pressure continues to increase, but the temperature changes little. Pressure then becomes the main factor governing the speed of sound (Eqs. 1.10 and 1.11), which continues to increase with depth. Refraction of sound at the depths of maximum and minimum velocity has importance in submarine warfare, where echoes from pulses of high‐energy sound (SONAR) are used to locate enemy submarines.

Before leaving the properties of sound in the ocean, there is one more item of importance: the relationship between speed, wavelength, and frequency. Simply put,

Figure 1.20 Velocity of sound in seawater as a function of depth. Maximum velocity at the bottom of the mixed layer. Minimum velocity at the base of the permanent thermocline.

(1.12)

where c is the speed of sound (m s⁻¹), f is the frequency (cycles per second or Hertz (Hz)), and λ is the wavelength (m). An important thing to note here is that the wavelength of sound in water is 4.3 times that of sound in air at equal frequency.