ferent types of SAND bottom. SAND bottoms are the best reflec- 
tors known, however, and over such bottoms extended ranges may 
frequently be expected. 
MUD bottoms are known to reflect less sound than 
SAND bottoms. When the mud is very soft, as near the mouth of 
Ceci Vcr VciIny llGEle SOumd 1S cetlected and mo extension of 
range may be expected. Firmer muds may in some circumstances 
reflect enough sound to extend maximum listening or echo ranges 
somewhat; such extension of range is thought to be much less 
frequent over MUD bottoms, however than over SAND bottoms. 
Similarly, the reverberation found over a MUD bottom will usu- 
ally be less intense and, when the refraction is downward, also 
of shorter duration, than over a SAND bottom. 
The properties of MUD AND SAND bottoms should be in- 
termediate between SAND and MUD bottoms although the information 
is lacking in this respect. 
Although STONY, CORAL, and ROCK bottoms do not absorb 
much sound, they are likely to show such irregularities that 
most of the sound is scattered, producing strong reverberation 
with little extension of range. Of these three types STONY is 
likely to be the most regular and the best reflector. Occasion- 
al CORAL reefs will produce high reverberation, although the 
CORAL sand in between the reefs is a good reflector. as eviden- 
ced in the extended ranges obtained in the Key West area. ROCK 
bottoms give probably the highest reverberations, and in some 
cases the numerous false echoes obtained over a ROCK area may 
make submarine detection very difficult. 
Research on these problems is now in progress. When 
further information becomes available, it should be possible to 
specify more exactly the probable acoustic effects to be expect- 
ed in different areas. 
Background Noises 
Crackling background noises resulting from the 
activities of snapping shrimps have been shown to be pre- 
alent over rocky, coral or shell bottoms which offer suit- 
able habitats for these animals in tropical and subtropical 
waters of less than 30-40 fathoms in depth. 
