236 Lecture 13 
important at high frequencies in deep water, since it limits the hydro- 
phone threshold above about 50 kc 
Sea-surface noise associated with waves is the dominant source of am- 
bient noise in open-sea deep water in a frequency range from about 100 
cps to 50 kc and varies in level with sea state 
3. Biological noise caused by snapping shrimp and other soniferous sea 
creatures occurs locally in shallow water when these organisms are 
present 
4, Man-made noise, including that from distant ships and from industrial 
sources in and near busy harbors, is often the dominant source below 
1 ke 
5. Rain noise in and near storms 
6. Shore noise produced by surf on coasts or reefs 
7. Flow noise caused by current flow over rocky bottoms and hydrostatic 
pressure changes produced by waves 
8. Terrestrial noise caused by earthquakes, volcanoes, microseisms, and 
distant storms 
to 
Noises from sources (7) and (8) are normally of very low frequencies. 
13.1.2. Deep Water—Thermal and Surface Noise 
Since there exists such a variety of sources of ambient noise, one would 
expect a considerable variability in level as a function of frequency and con- 
ditions related to the production of the ambient noise and indeed this is the case. 
It is found that in any given region of the spectrum, one or more sources are 
dominant and the output of the remainder is so low by comparison as to be en- 
tirely insignificant. Thus, for example, the thermal agitation due to molecular 
motion in the water provides a lower limit for the ambient noise at all fre- 
quencies, but it is dominant only in the upper frequency region, above about 50 
to 200 kc, depending upon the sea state. The thermal noise level that sets this 
limit has been shown by R.H. Mellen [1] to be 
L=-115 +30 logf 
in db relative to 1 d/cm? in a 1-cps band at a temperature of about 15°C, and at 
a frequency f in kc. In the complete absence of all other sources of noise, this 
is the equivalent noise pressure that would be detected by an omnidirectional 
hydrophone with an efficiency of 100%. 
At lower frequencies, roughly 1to 50 kcandhigher, depending upon sea state 
the most important source of noise in the ocean is sea-surface noise, which ap- 
pears to depend upon the speed of the wind and on sea state. The first extensive 
measurements of ambient noise over this frequency region appear to have been 
made during World War II. The results of the measurements made during these 
studies were summarized in a series of curves known as the Knudsen curves 
[2]. In these curves, shown in Fig. 13.1, the spectrum of deep-water noise is 
plotted as a function of sea state and frequency. An extrapolation of the experi- 
mental data extends them to the thermal noise limit, which is also indicated. 
The principal characteristic of deep-water ambient noise is its variation 
with sea state and wind speed. At all sea states it is noted that the spectrum 
level decreases about 5 db/octave. It is also found that the intensity of ambient 
