238 Lecture 13 
1. Frequency. The fundamental frequency is seven cycles per week, i.e., 
one cycle per day. The fundamental period agrees closely with the solar 
day. 
2. Amplitude. Amplitudes are relatively small in most cases, the over- 
all change in level being 1.5 to 5 db, and the phenomenon is often masked 
by other sounds such as ship traffic noise. However, changes of 10 to 
20 db at one location during the summer solstitial period were observed. 
This is an extreme Case. 
3. Phase. Maxima occurred at approximately midnight, local-zone stand- 
ard time, and did not shift from day to day as do the tidal maxima. 
4, Waveform. The harmonic content varies with location. A second har- 
monic (14 cycles per week) was relatively strong and so phased as to 
give a second maximum at approximately noon, local time. In some 
cases, aS many as seven harmonics were found. 
The periodic variation has been observed each season of the year. No marked 
seasonal dependence was evident except at one location. Here the variation was 
a maximum at approximately the time of the summer solstice. 
The periodic variation has been observed at six locations, including sites 
in both deep and shallow water, both near to and remote from shore, and extend- 
ing over three time zones (45° longitude). 
Wenz has no explanation for this variation, although he has apparently ex- 
plored every possibility that comes to mind. He has considered wind-speed time 
patterns, tidal changes, variation in system sensitivity of performance, ship 
traffic noise, biological noise, and seismic activity. None of these seem to pro- 
vide any logical clues. The close agreement of the period of these variations 
with the period of the earth's rotation suggests the possibility of an extra- 
terrestrial connection. For example, the intensity of cosmic radiation has a 
diurnal variation with a maximum near noon. But what mechanisms can account 
for an increase in noise level whenthere is a decrease in the intensity of cosmic 
radiation ? 
13.1.3. Shallow Water—Man-Made and Biological Noise 
In contrast with the deep-water ambient noise levels, which are compara- 
tively well defined, the ambient noise levels in coastal waters vary widely: For 
this reason, only very rough predictions of expected ambient levels can be made, 
However, the deep-water levels do define a lower limit for the shallow-water 
levels. 
Except for those that also occur in deep water, the two most important 
sources of noise in coastal waters are soniferous marine life and man-made 
disturbances, such as those caused by ships and industrial installations on the 
adjoining shores. 
The noise produced by marine organisms has been studied extensively since 
the beginning of World War II. More recently, the occurrence and acoustic char- 
acteristics of soniferous life in the Atlantic and Pacific oceans have been studied 
and the results summarized by M.D. Fish [5, 6]. Many types of marine or- 
ganisms are known to produce sound, but it has not been possible to associate 
