Some provision should also be made for a low- 
level calibrate signal at the input of the preamplifier. 
Before commencement of the noise survey, a suitable 
number of discrete calibration frequencies should be 
recorded on the tape to provide a means of deter- 
mining the recording system gain throughout the spec- 
trum of interest. These levels, along with the cali- 
bration of the hydrophone, will provide means for 
reducing the recorded noise data to equivalent plane- 
wave sound pressure in the water. 
RECORDING 
Once the equipment is set up and properly checked 
out, the vessel should proceed to deepwater, i.e. , 
more than 100 fathoms, where Sea State 2 or less 
prevails. On arrival, the vessel should stop and 
secure all machinery. A sample of this noise field 
should be recorded; this is the base line or quietest 
condition attainable. Then each item of machinery 
should be operated individually, insofar as possible, 
with samples recorded under each condition. If the 
basic acoustic research to be conducted will require 
the vessel to be under way, a noise-versus-speed 
observation should be made by recording data at one- 
or two-knot intervals throughout the speed range 
desired. 
CORRECTIVE ACTION 
If the analysis of the sea-test data indicates that 
the vessel is too noisy (as it probably will), there are 
several potential ''fixes" available: 
1. Quite often relocation of the research trans- 
ducer away from known high-intensity noise sources 
can be effective. 
2. Isolation mounting of machinery observed to be 
noisy may solve the problem. There are many fine 
isolation mountings available. 
3. If the machinery to be isolated is too large or 
for some other reason cannot be isolated, a visco- 
elastic damping material applied to the machine and/or 
the hull in the vicinity of the acoustic research trans- 
ducer may effectively dissipate this energy. 
4. If the research to be conducted does not require 
the research transducer to see in all directions, 
baffles can be arranged in the desired blind spots as 
well as above the transducer. 
In the preparation of the research transducer for 
the under-way portion of any research project, care 
should be exercised to provide a suitably vibration 
damped and isolated streamlined enclosure. Any 
smoothing of hull discontinuities ahead of the research 
transducer location should assist in hydrodynamic 
noise reduction especially at high speeds. Generally 
speaking, the achievement of a quiet platform requires 
some or all of the previously mentioned devices. 
If a truly quiet platform is to be achieved, it may 
be necessary to repeat the sea test several times 
after various remedial measures have been taken. 
CONC LUSION 
In conclusion, it should again be emphasized that 
the end result of good interference-free data more than 
justifies the effort required to achieve a quiet working 
platform. 
