was closely interwoven with the other sounds typical of the 
sea lion in particular and pinnipeds in general, but was 
considered separately because of its possible echo-location 
function. The author has heard a sea lion making a very 
similar sound in air at the San Diego Zoo. The sound con- 
sists of a pulse train lasting several seconds and usually is 
preceded by a guttural vocalization as shown in figure 5 at 
time 0.5 second. The pulse repetition rate is about 30 
per second. 
Sound 6, the Low-Frequency Groan (fig. 6), is a low- 
frequency tonal sound, beginning at from 60 to 100 c/s and 
ending 1 or 2 seconds later at from 20 to 40 c/s. At least 
some groans, such as that shown in figure 6, consist of two 
components, the fundamental and a strong second harmonic 
which dominates because of the positive correlation between 
frequency and aural response below 1000 c/s in man. 
Sonagrams A and B show the same groan, but with different 
frequency and time scale combinations, as received at the 
shallow hydrophone. The third Sonagram shows what was 
received at the deep hydrophone. It is matched with the 
shallow hydrophone presentation shown above it, and the 
pair illustrates a case in which the difference in time of 
arrival indicates that the source was seaward from the 
deep hydrophone (the time difference approximated the 
maximum possible with the 4-mile separation of the 
hydrophones), 
Figures 7 and 8 illustrate with Sonagrams the two 
kinds of 20 c/s Pulses, the Long and the Short. Such 20 c/s 
pulses, particularly short ones of approximately 1-second 
duration, have received much attention during the last 8 
years. Beene ee SPB Oth types were prominent in the San 
Clemente recordings, especially the 20 c/s Long Pulses 
which were detected in all the recorded samples. These 
Long Pulses are of two types, which have already been 
designated Types I and II. + Samples of each are shown in 
figure 7. Examples of Type I occurred at times 1! 30'' and 
3' 45" and a good example of Type II occurred at time 0. 
The second harmonic content of the Type Il Pulses is 
evident at 43 c/s and the second harmonic traces of the 
11 
