LECTURE 13 
AMBIENT NOISE IN THE SEA AND ITS MEASUREMENT 
P.M. Kendig 
Ordnance Research Laboratory 
The Pennsylvania State University 
University Park, Pennsylvania 
U.S.A. 
13.1 AMBIENT NOISE 
13.1.1 Introduction 
It has been found that acoustic waves afford the best means for detecting 
underwater objects in the ocean. However, in any detection system there is an 
interfering source of noise or signal that sets a lower limit to the level of the 
signal it is desired to detect. 
There are several different types of interference. Very often this limitation 
is the self-noise of the detection system, which may be caused by the hydro- 
phone, the electronic system, or the vehicle with which the observations are 
made. In an active sonar detection system, the underwater object is detected 
by means of an echo due to transmitted acoustic energy reflected from the ob- 
ject. However, some of the transmitted acoustic energy is returned to the 
listening hydrophone, even if the sound waves do not encounter a target, be- 
cause sound is scattered back to the hydrophone from the surface, the bottom, 
and even from scatterers in the medium itself. This may be, and often is, the 
principal source of intez ference. 
There is still a third general type of interference, the only one to be dis- 
cussed here, known as the ambient noise. This is the interfering noise that is 
due to natural conditions or sources inthe ocean. It will be considered a property 
of the medium itself at the time and place of observation, irrespective of the 
hydrophone and the platform used to observe it. It is the composite noise from 
all sources present in a given environment; desired signals and noise inherent 
in the measuring equipment and platform are excluded. The ambient noise level 
is expressed in terms of the level of an "equivalent" isotropic noise field at the 
observing hydrophone. Such an equivalent field is one that would produce, at the 
output of the measuring system, a response equal to that produced by the noise 
actually present. Following is a list of the commonly recognized sources of 
ambient noise: 
1. Thermal noise due to molecular agitation of the medium is especially 
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