Introduction 
Over the continental shelf the acoustical qualities of 
the bottom frequently play an important part in underwater sound 
transmission. This is especially true when downward refraction 
prevails, as when the temperature decreases with depth. Under 
such conditions a sound beam is bent down to the bottom a short 
distance from the source, and if no reflection of sound from the 
bottom were possible the- maximum range of detectign would be 
Small. if the bottom is an efficient reflector, as is the case 
with a smooth, SAND, bottom, and if also the bottom is not too 
deep (roughly 100 fathoms or less) the range at which a signal 
can be detected may be greatly increased. A soft, MUD, bottom 
absorbs sound, and over such a bottom the range will not be in- 
creased by bottom-reflected sound. With very rough and espe- 
cially ROCK bottoms, strong reverberation may Sometimes make 
echo-ranging very difficult. A brief summary of the probable a- 
coustical effects of various types of bottom is given in the Key 
below 
THE EFFECT WHICH DIFFERENT TYPES OF BOTTOM HAVE ON ECHO RANGING AND LISTENING 
|. With Downward Refraction 
Type of Bottom Probable Acoustic Effects of Bottom 
SAND Firm, relatively smooth bottom, 
shells and washed gravel included. 
Long extension of range commoniy obtained. 
MUD AND SAND Relatively firm, smooth 
bottom. 
Moderate to poor extension of range. 
Sound commonly absorbed and little extension 
possible. 
MUD Soft, smooth bottom. 
ROCK Rough broken bottom, includes 
bedrock outcrops and areas covered by 
boulders. 
STONY Hard bottom, commonly rough. 
Predominantly cobbles and Pebbles. Vary- 
ing amounts of sand and mud commonly 
present. 
CORAL Hard bottom with sandy patches 
irregular to smooth. Includes various marine 
forms which secrete masses of lime cover- 
ing the bottom. 
Strong reverberation tends to mask echoes. 
Extension of range unlikely with either echo 
ranging or listening. 
Extension variable depending on local condi- 
tions. Reverberations may be strong. 
Extension variable depending on local condi- 
tions. Reverberations may be strong. Noise 
level may be high due to presence of noise 
making animals living on the coral. 
Under these conditions, maximum ranges are usually long, and not dependent on the type of bot- 
tom. Over ROCK bottoms, however, strong reverberations may be encountered. 
With Upward Refraction 
