24 
Type Il tails are even more evident (38 c/s). The overlap 
of the pulses shown in this Sonagram is typical and suggests 
the probability that the pulses are from multiple sources. 
Sounds in the 20 c/s region pose difficulties not only 
in on-the-spot study, but also in detection and monitoring. 
The main problem is that below 100 c/s human audition is 
relatively insensitive and gets rapidly worse as frequency 
goes down. However, an increase of speed (and frequency) 
of recorded samples by a factor of four results in an 
auditory presentation suitable for monitoring the 20 c/s 
Pulse activity without losing perspective relative to the 
other sounds. 
Figure 8 shows a series of 20 c/s Short Pulses at 
time 0, 8, 15, and 20 seconds. The Short Pulse charac- 
teristically shows downward frequency shift and ends close 
to 20 c/s. Except for the average frequency and uniformly 
short duration, it is not greatly different from what has 
been called the Low-Frequency Groan. Although the pulse 
at time 24 seconds has the characteristic shortness of a 
20 c/s Short Pulse, it is twice as high in frequency, and 
for the purposes of this survey is considered to be a Low- 
Frequency Groan. 
Characteristically, 20 c/s Short Pulses occur 
clustered in time. Sometimes the pulse intervals within a 
cluster are very regular and the clusters can be considered 
as pulse trains. Typical intervals range from about 20 
seconds to 10 seconds. Sometimes the pulse intervals are 
very irregular and also not readily interpreted in terms of 
multiple sources. Usually, a pulse cluster begins abruptly 
and ends abruptly. The 20 c/s Short Pulses found in this 
survey generally had very low signal-to-noise ratio and 
some were almost completely masked by the 20 c/s Long 
Pulses. For this reason an attempt to establish the num- 
ber of sources and pulse interval would not be worthwhile. 
Figure 9 shows a mixture of 20 c/s Short and Long 
Pulses as well as other biological sound. In figure 9A 
Short Pulses can be seen at 0 and 13 seconds, while the 
