Refraction of Sound 
Maximum ranges are usually limited by the refraction 
of sound produced by variations of temperature and pressure with 
depth. In an isothermal wind-stirred layer the sound rays curve 
gradually upwards. Where the temperature decreases with dept 
the rays. curve downward. 
The accompanying diagrams indicate the refraction 
pattern under varying temperature conditions for a sound source 
at a depth of 15 feet. The sound source can be either the pro- 
jector of echo-ranging equipment or the high frequency component 
of ship noises as in listening. The temperature decrease be- 
tween the surface and 300 feet is assumed to be 10°F and the 
change of temperature with depth is indicated to the left of 
each diagram. The depth scale is greatly amplified but the 
angle at which each ray leaves the source is indicated. The 
Spread of the sound beam is taken to be 12°; the wider angle 
rays do not achieve a significant horizontal distance. Since 
each ray is reversible, the echo from a target in the sound beam 
returns to the projector along the same path by which the sound 
reaches the target. 
Figures 1, 2, 3 and 4, which are for deep water, show 
the change of the sound beam when the depth of the isothermal 
layer is continually decreased and demonstrate the transition 
between GOOD and VERY BAD echo-ranging or listening conditions. 
The shading denotes the zone from which no echoes can be re- 
turned unless the sound is reflected from the sea surface. Note 
that in no case will the shadow zone be extended or diminished 
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Reflection of Sound 
When a sound beam hits a large area of the bottom, 
the sound is partly absorbed, partly reflected, and partly scat-— 
tered. The scattered sound goes out in all directions; some of 
it goes back to the echo-ranging gear and produces reverbera- 
tion. The reflected sound behaves like light reflected from a 
mirror; it is this reflected sound which makes extended ranges 
possible when the refraction is downward. 
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by dotted lines. The Shading indicates the region into which 
the sound reflected only once from the bottom cannot usually 
penetrate. It is evident from these figures that when the bot- 
tom is an efficient reflector, echo ranges can be at least 
doubled, and listening conditions may also be improved according- 
jy. in fact, under many conditions bottom reflected sound may 
improve listening ranges even more than echo ranges. 
It is fairly certain that SAND bottoms do not absorb 
very much sound, and either reflect or scatter a considerable 
fraction of the sound which falls upon them. The ratio of scat- 
tering to reflection is not known, and may vary widely for dif- 
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