Research ora Underwater Sound Transmission „ 



The chief problem of transmission studies is the measure 

 of the sound, field of any source as a function of oceanographic 

 conditions o When the sound is complex, such measurement re- 

 quires a number of analyses of Intensity and spectrum as func- 

 tions of range and hydrophone depth,, When the source can vary 

 in depth,, as does a submarine, that variation must also be 

 measuredo Time variations further complicate transmission 

 studies, since a single pulse will sometimes travel by differ- 

 ent paths to arrive at a hydrophone as a series of discrete or 

 interfering pulses „ Even when the source emits a steady tone 

 of constant intensity, a hydrophone more than a few yards away 

 receives a sound of fluctuating intensity,, 



In measuring the effect of oceanographic conditions on 

 sound transmission., obviously 9 complete control of experiments 

 is impossible and the separation of independent variables is 

 exceedingly difficult Experiments must be planned with great 

 care because the subsurface "weather" cannot be planned,, Large 

 quantities of data must be collected in the hope that eventually 

 all combinations of variables will occur,, 



In the Underwater Sound Transmission program, it was, of 

 course, impossible to bring the sea to the laboratory, so the 

 laboratory had to be put to sea Studies were made in water 

 of varying depth over bottoms of different character, in dead 

 calms and near-gales, at various frequencies, at different 

 times of day and night and in different seasons „ Under such 

 conditions it is not to be wondered at that it required months 

 of experimentation to establish the effects of any one variable 

 upon sound transmission,, 



Studies were not limited to the sound emitted by specially 

 constructed transducers and standard echo-ranging projectors „ 

 Studies were made of the transmission of sound from ships and 

 propellers, the output of various types of noisemakers and of 

 explosions,, 



The general results obtained in the NDRC transmission 

 studies may be explained on the basis of a few simple prin- 

 ciples o 



Sound travelling outward from a source always weakens 

 with distance o Even though it travels through a perfect 



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