single sound sample, which shows relative 3-octave level 
versus frequency; and an oscillographic photo, which shows 
voltage amplitude versus time. Included in each spectrum 
plot is a rough approximation of the relative ambient 
background level, indicated by a dashed-line curve. When 
the spectrum curve rises well above the dashed curve, the 
difference is due to the biological sound plus or minus the 
error of positioning of the background curve. However, a 
peak at 60 c/s due to equipment hum is sometimes present. 
The names assigned to the various sounds represent 
an attempt to be as descriptive as possible, yet brief. No 
means was available for connecting any sound positively to 
any particular source; so, for example, the barking was 
not called ''sea lion barking'' even though the source of the 
barking in all probability was the sea lion. 
Sound 1, the cyclic Click Chorus (fig. 1), has pre- 
viously been described.*’*’°'* The individual click could 
just as well be called a snap or a clap or a knock and is 
very similar to what is often heard by Navy sonarmen and 
commonly referred to as the ''carpenter'’ sound. The 
carpenter sound, however, does not have the cycling or 
surging characteristic of this chorus and usually has a 
wider frequency range. This sound during daylight hours 
may consist of single trains of pulses at a pulse repetition 
rate of 1.5 per second and comes in for only a few seconds 
during a 5-minute sampling period, for example. During 
nighttime hours, however, it always sounded like the result 
of hundreds of individuals knocking in concert typically for 
20 seconds or so and then tapering off as the result of one 
individual after another momentarily stopping. During the 
next few seconds the number of individuals knocking would 
have decreased enough to reduce the ambient noise intensity 
by up to 10 dB. Sometimes the number clicking was much 
smaller than usual and the clicking part of the cycle was 
shorter. On the other hand, during the 2000 sample period, 
which was one hour after sunset, the apparent number of 
individuals clicking was at a maximum, and the density and 
intensity of the sound were at a constant high level without 
the characteristic cyclic variation. 
