In physics, the word propagate means to cause (e.g., a wave) to move through a medium. In our study of sound, we learned that the medium controls sound. In this lesson, we will look at the effect of the sea on sound waves as they move through it.

Learning Objective: Define sound velocity, and describe the effect of temperature, pressure, and salinity on sound.

Sound velocity takes into account the speed and direction of sound rays. The direction or path that sound energy takes as it moves through the water is primarily a function of sound speed.

The speed of sound in the sea is a function of water temperature, pressure, and salinity. Of these three variables, temperature is the most important. It is the primary controller of sound speed, and therefore direction, in the upper 300 meters (1,000 feet) of seawater. In general, sound speed increases 2.4 m/sec for every 1C increase in temperature.

The effect of pressure on sound speed is a function of depth. The greater the depth, the greater the pressure; the greater the pressure, the greater the sound speed. Sound speed increases approximately 1.7 m/sec per 100 meters of depth. Pressure is the dominant sound speed controller below 300 meters, because below 300 meters, the temperature is relatively constant.

The effect of salinity on sound speed is slight in the open sea, because salinity values are pretty much constant. The affect of salinity on sound speed is greatest where there is a significant influx of fresh water or where surface evaporation creates high salinity. A one part per thousand (1%) increase in salinity increases sound speed 1.4 m/see.

SOUND-VELOCITY PROFILE (SVP). A sound-velocity profile is simply a graphic representation of speed versus depth. See figure 2-2-1. Sound-velocity profiles are con-structed from sound-speed nomograms based on temperature, depth, and salinity. They can also be constructed from bathythermograph soundings by computing the sound speed at significant and mandatory depths. An SVP provides surface sound speed, depth of maximum sound speed (sonic-layer depth), and layers where sound travels great distances (ducts and sound channels).

Sonic-Layer Depth (SLD). The sonic-layer depth is the depth of maximum sound speed. In most instances, the SLD is the same as the

mixed-layer depth (MLD). The SLD can be determined from a BT trace. A negative-temper-ature gradient (temperature decreasing with depth), within certain limits, compensates for an increase in sound speed with depth due to pressure; this results in a constant sound speed with depth. These gradient limits per 30 meters of depth are as follows:

Temperature gradients that are more negative than those listed (temperature decreases at a greater rate) result in decreasing sound speed with depth. Gradients that are more positive result in increasing sound speed with depth. Of course, sound speed increases with depth when the water temperature is constant because of the increasing pressure.

The SLD can be determined from a BT trace by considering the following criteria:

1. If the maximum temperature is at the surface, and the gradient is more negative than the limits listed, the SLD is zero. Its at the surface.

2. If the BT trace is isothermal or has a slight negative gradient (less than the stated limits) and then becomes more negative, the SLD is at the bottom of the isothermal or slightly negative gradient layer. 

3. If the maximum temperature occurs at a depth other than the surface, this is the SLD, unless the gradient below depth of the maximum temperature is less than the stated limits.

When predicting sonar ranges, in-layer and below-layer ranges are computed. The term

while below-layer pertains to the layer beneath the SLD. 

Learning Objective: Explain why sound propagates along more or less curved paths, and deseribe the five basic sound ray patterns and their attendant temperature and sound velocity profiles.

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