The Physics of Sound in Water

From "SONAR Technology for Fish Finders" compiled by Nolan Laxamana

It is clear to anyone who has immersed himself or herself in a lake or ocean that sounds can be heard underwater. The sounds of waves, power boats, and other bathers can be heard with remarkable clarity, even at considerable distances. In fact, sounds move quite efficiently through water, far more easily than they do through air. As an example, whales use sound to communicate over distances of tens or even hundreds of kilometers. The ability of sound to travel over such great distances allows remote sensing in a water environment. Devices that use sounds in such an application fall under the family of instruments known as sonars. To understand sonars, you must first understand sound. In particular, you must understand how sound moves in water.

Sound travels in water in a moving series of pressure fronts known as a compressional wave. These pressure fronts move (or propagate) at a specific speed in water, the local speed of sound. The local speed of sound can change depending on the conditions of the water such as its salinity, pressure, and temperature, but it is independent of the SONAR Technology for Fish Finders characteristics of the sound itself all sound waves travel at the local speed of sound. In a typical ocean environment, the speed of sound is in the neighborhood of 1500 meters per second (m/s).

The physical distance between pressure fronts in a traveling sound wave is its wavelength. The number of pressure fronts that pass a stationary point in the water per unit time is the frequency of the wave. Wavelength, if measured in meters (m), and frequency, if measured in cycles per second (Hz), are related to each other through the speed of sound, which is measured in meters per second (m/s):

speed of sound = frequency ´ wavelength

When a sound wave encounters a change in the local speed of sound, its wavelength changes, but its frequency remains constant. For this reason, sound waves are generally described in terms of their frequency. As a sound wave propagates, it loses some of its acoustic energy. This happens because the transfer of pressure differences between molecules of water is not 100% efficient some energy is lost as generated heat. The energy lost by propagating waves is called attenuation. As a sound wave is attenuated, its amplitude is reduced.

Sound waves are useful for remote sensing in a water environment because some of them can travel for hundreds of kilometers without significant attenuation. Light and radio waves (which are used in radar), on the other hand, penetrate only a few meters into water before they lose virtually all of their energy. The level of attenuation of a sound wave is dependent on its frequency high frequency sound is attenuated rapidly, while extremely low frequency sound can travel virtually unimpeded throughout the ocean. A sound wave from a typical sonar operating at 12 kHz loses about half of its energy to attenuation traveling 3000 meters through water.

While acoustic energy travels well in water, it gets interrupted by a sudden change in medium, such as rock or sand. When a moving sound pulse encounters such a medium, some fraction of its energy propagates [absorbed] into the new material. The energy that is not transmitted [absorbed] into the new material must go back into the original medium the water as sound. Some amount of it is reflected off the surface of the material essentially it bounces off in a direction that depends on the angle of incidence (surface). The remainder is scattered
SONAR Technology for Fish Finders in all directions. How much energy goes into reflection and how much goes into scattering depends on the characteristics of the material and the angle of incidence. The energy returned to the water (in other words, the energy that is not transmitted into the new medium) is called an echo. The echo maintains the frequency characteristics of the source wave.

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