As sound energy leaves a sound source it travels in waves. The sound waves expand as they move away from the source. See figure 2-2-2. A sound waves path of travel is dependent on its speed and any matter in its path. Sound, like light, is refracted, reflected, and scattered.

REFRACTION. As a sound wave moves through the sea, it travels along a curved path. The path is curved, because sound speed varies along the wave front. Sound waves bend (are refracted) in the direction of the slower sound speeds. This is the fundamental principle of sonar-range prediction and is derived from Snells law. Snells law states that a sound ray propagating through a region with one sound speed will change direction (be refracted) on entering a region having a different sound speed.

The degree of refraction is proportional to the sound-speed gradient. The greater the change in speed over a given distance or depth, the greater the refraction. The gradient is a function of speed versus depth or distance. For example, in a layer of water where sound speed decreases rapidly with depth (a strong negative-velocity gradient), sound waves bend sharply downward. Sound rays refract upward if sound speed increases with depth (a positive-velocity gradient). Figure 2-2-3 illustrates the five basic sound-transmission patterns. The BT sounding and SVP which bring about these paths accompany each pattern.

Straight Rays. Sound rays travel in straight lines only where the speed is everywhere constant (isovelocity); no change in velocity with depth. Straight sound rays occur when the temperature profile is slightly negative (a decrease of about 1C per 30 meters of depth). Long sonar ranges are possible when this type of profile exists. 

Figure 2-2-3.-Representative sound patterns based on temperature and sound-velocity gradients. 

Rays Curved Downward. A negative-temperature gradient (temperature decreasing with depth) produces a negative-velocity gradient. The sound rays leave the sonar and are bent downward, thereby limiting sonars to very short ranges. For example, a decrease in temperature of .56C in the first 10 meters causes the sound beam to miss a shallow target at a range of 1 km.

This is a common occurrence in the near-surface layer. Beyond the range of the downward bending sound rays, sound intensity is negligible.

Rays Curved Upward. A positive-temperature gradient causes sound speed to increase with increasing depth, and sound rays to refract upward. Longer ranges are attained with this type gradient, especially if the sea is relatively smooth. As the rays bend upward and strike the sea surface, they are repeatedly reflected to longer ranges.

Split-beam Pattern. A split-beam pattern occurs when the temperature gradient in the near-surface layer is isothermal, and negative below. Sound rays from a sonar split at the depth of the gradient change. Part of the sound rays are refracted upward toward the surface, and part are refracted downward toward the bottom. At the point where the rays split, a shadow zone exists. A submarine operating at the split depth improves its chances of avoiding detection. 

Sound Channel. A sound channel occurs when a negative-velocity gradient overlies an isovelocity or positive-velocity gradient. The depth where the velocity gradient changes from negative to positive is the axis of the sound channel. The axis is the level of minimum sound speed. The sound rays on both sides of the axis travel faster than the rays in the center. And since sound refracts toward slower sound speeds, the faster rays are continually refracted toward the axis.

REFLECTION. Sound waves that strike solid surfaces have all or a portion of their energy redirected or absorbed. The surface or object struck determines if the sound energy is reflected, scattered, or absorbed.

Reflected sound energy can be good or bad. The type or quality of reflected sound is dependent on the surface from which the sound bounces. For example, a smooth hard surface is a good reflector. Sound waves bounce off such surfaces specularly (like a mirror) and lose little of their energy. On the other hand, an irregular hard surface is not a good reflector. The sound waves are reflected in many different directions and lose a good deal of their energy. This type of inflective energy loss is known as scattering.

Sound energy in the sea is scattered by the sea surface, sea floor, and suspended matter. Because the sea surface is rarely smooth, it is more apt to scatter sound than to reflect it specularly. A rough or rocky bottom also disperses or scatters sound energy.

In contrast to these rough surfaces, a smooth rock ocean bottom is perhaps the best reflector of sound in the sea. A smooth sand bottom also reflects sound very effectively. The sea surface, if it is calm, is also a good reflector.

REVERBERATION. Reverberation is noise or interference at a sonar receiver, which makes target detection very difficult. This interference is caused by scattered sound energy being reflected back to the sonar receiver. There are three types of reverberation: surface, volume, and bottom.

Surface Reverberation. Surface reverbera-tion is a product of surface wave action. At short ranges, surface scattering increases with wind speeds between 7 and 18 knots. Above 18 knots, a further increase in the surface-reverberation level is prevented by a sound screen of entrapped air bubbles. The air bubbles form near the surface and are caused by the wave action.

Volume Reverberation. Volume reverbera-tion is caused by scatterers or reflectors in the water such as fish, marine organisms, suspended solids, and bubbles. Volume scatterers are not uniformly distributed in depth, but tend to be concentrated in a diffuse layer known as the deep-scattering layer.

The deep-scattering layer is found in tropical waters at depths between 100 and 400 fathoms.

The intensity of the scattering is a function of sonar frequency (some sonar frequencies are affected to a greater degree than others) and the density of the organisms in the layer. In the Northern Hemisphere, the maximum volume reverberation occurs in March and the minimum in November.

Bottom Reverberation. Bottom composition and roughness govern the degree of reverberation that contributes to the masking of target echoes. In theory, the amount of bottom reverberation is directly related to the roughness and com-position of the sea floor. However, the problem of bottom reverberation is a bit more complicated. Scientists consider the ocean floor to be a two-dimensional volumetric scattering surface. In other words, sound is not only reflected off the sea floor but also from formations of rock beneath the sea floor.

Also, bottom roughness can be slight or great, and the wavelength component of the reflected sound can range from microns to miles. The following conclusions were drawn from a Russian study (Jitkovskey and Volovova, 1965): (1) When bottom roughness is large compared to the wavelength of the sound being bounced off it, the amount of sound energy scattered back to the receiver (back scattering) is independent of frequency and (2) when the bottom roughness is small compared to the wavelength of the transmitted sound, scattering strength expands with increasing frequency.

Another problem created at the ocean bottom is one of absorption. When the bottom is composed of soft mud, sound energy is absorbed. Absorption also occurs as sound propagates through the sea, and the energy is converted to heat.

ATTENUATION. Attenuation is the energy loss that occurs in propagated sound waves due to scattering and absorption.

Learning Objective: Differentiate between active and passive sonar; define the modes of active-sonar search; and describe the propagation paths used with each mode.

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