THE QUIET PLATFORM, KEY TO SUCCESSFUL OCEANOGRAPHIC 
ACOUSTIC RESEARCH 
by A, DONN COBB, Electronic Engineer 
U.S. Navy Underwater Sound Laboratory 
Fort Trumbull, New London, Connecticut 
ABSTRACT 
This paper discusses the surface vessel as a plat- 
form from which to conduct acoustic research. It 
points out the main problems attendant to such a plat- 
form and discusses techniques for control and for 
elimination of many types of acoustic self-noise. An 
attempt is made in this paper to bring cause and effect 
together in the light of available corrective measures to 
provide a foundation for valid measurements of acous- 
tic data. 
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In any electroacoustic investigation which is predi- 
cated on the detection of low-level signals especially 
when they have a broad spectral distribution, the 
signal-to-noise ratio is of great importance. This is 
particularly true in acoustic survey work such as the 
mapping of a muddy bottom or the recording of sounds 
of biological origin. A quiet platform is essential to 
this fype of work; when a ship is used for the surveys, 
every effort should be made to keep interfering noise 
at a minimum. 
NOISE SOURCES 
It is the intent of this paper to describe a method 
of determining the principal noise sources of avessel 
and to suggest remedial action. Let us first consider 
the types of noise that may be encountered. Assuming 
the presence of a signal of some finite level, the inter- 
fering noise may be divided into three broad types. 
These are: 
1. Electrical noise. 
2. Sea state noise. 
3. Self-noise of the carrying vessel. 
For the purpose of this article it will be assumed 
(1) that electrical noise can be controlled, and (2) that 
sea-state noise can be kept at a predetermined level 
by the selection of the time and place for the conduct 
of the experiment. We are then left with the self-noise 
of the vessel, which will be the subject of this article. 
The acoustic self-noise of a vessel dead in the 
water is usually composed of noises generated by main 
propulsion machinery, auxiliary machinery, ventilating 
system, etc. , and when under way, by all these plus 
the hydrodynamic flow noise of water. 
To determine the charateristics of the machinery 
noises, a comprehensive machinery noise survey should 
be conducted. To this end a calibrated broad-band, 
omni-directional hydrophone should be installed at, or 
as near as is feasible to, the location of the research 
transducer. A word of caution seems in order here: 
inasmuch as the machinery noise to be measured is 
generated to a large extent as structural vibration and 
then transferred to the water surrounding the ship's 
hull, a suitable vibration isolation mounting for the 
hydrophone must be provided in order to prevent direct 
vibrational excitation. The simple expedient of sus- 
pending the hydrophone with shock cord is often very 
effective; where more permanence is desired, a 
mounting consisting of several flanges alternately of 
brass and rubber can be used. 
EQUIPMENT 
Since accurate analysis equipment is generally too 
large to take to sea,it is usually necessary to record 
data on magnetic tape for later analysis in the labora- 
tory. An idealized system for such recording would 
include all battery-operated equipment, with a pre- 
amplifier between the hydrophone and the tape recorder. 
A word here about the preamplifier will help 
anyone interested in conducting this type of survey. 
Typical acoustic noise, when plotted as a curve of 
amplitude versus frequency, displays about a 6-db-per- 
octave negative slope. If we consider the 10 octaves 
of frequency between 20 cycles and 20 kilocycles as 
the observational band, we find that our tape recorder 
must be able to accept a dynamic range of amplitude of 
60 db. Even the most ambitious sellers of tape record- 
ing equipment do not claim this capability. If, however, 
6-db-per-octave pre-emphasis is added to the preampli- 
fier, it becomes apparent that theoretically all portions 
of the spectrum from 20 cycles to 20 ke will have the 
same amplitude; consequently, very unsophisticated 
recording equipment will suffice. In practice, the 
noise to be observed is not precisely random, and the 
deviation from the mean noise level may cause as 
much as +10 db of signal amplitude difference through 
this system. This 20-db dynamic range is well within 
the capabilities of most tape-recording equipment. 
