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SOUND TRANSMISSION STUDIES 



It is evident from the preceding discussion of sound ranging that for the 

 use of acoustics as a survey tool, it is essential to understand the details of 

 sound transmission in the area to be surveyed. Sound transmission studies are 

 also valuable as a means of understanding the physical properties and structure 

 of the sea water itself and of its principal boundaries, the surface and the ocean 

 floor. The subject is involved and has been pursued on a number of levels of 

 complication for various purposes. For instrumentation, attention must be 

 paid to the receiving equipment and the sound source; and for the experimental 

 procedure one must consider the location of both source and receiver in both 

 horizontal separation and depth. Again the instrumentation described under 

 passive listening serves as the receiving equipment in all instances. The par- 

 ticular facet of the transmission problem to be studied determines to a large 

 extent the type of source employed. A great deal of use has been made of un- 

 derwater explosions as sound sources for studying transmission properties and 

 they are the workhorse of several experienced groups in this field. They have 

 the advantage that the initial sound consists of a very short transient whose pres- 

 sure-time curve is approximately an exponential and whose spectrum, though 

 far from white, nevertheless contains a considerable amount of energy over a 

 very wide frequency range. Hence TNT is a very important instrument in un- 

 derwater acoustics. Explosives research during the war, particularly at the 

 Underwater Explosives Research Laboratory at Woods Hole has made a consid- 

 erable contribution to the art of underwater acoustics by supplying a great deal 

 of detailed information about the properties of shock waves in water. Much of 

 this information is available in the book entitled "Underwater Explosions" by 

 Robert Cole (1948). 



In addition to explosives, transducers which are similar to the hydro- 

 phones we have already been discussing can be powered by an electronic oscil- 

 lator and driver to produce intense sound at a single frequency. These are very 

 useful for studying the effect of the environment on transmission in a restricted 

 frequency range. Whenever the needs of the problem at hand are either ade- 

 quately or best served by single frequency transmissions there are an excellent 

 array of transducers available to cover a wide range of frequencies. However, 

 I feel that the use of single frequencies in a scientific research program on 

 sound transmission is not nearly as fruitful as the use of a source having a com- 

 plex spectrum. This is because the present need in this field is for an under- 

 standing of the relative importance of different environmental factors as a func- 

 tion of frequency and for knowledge of how their effect may vary geographically 

 and seasonally. For this work, explosives or other sound sources having a 

 complex spectrum with appreciable energy distributed over a wide frequency 

 range are direct and economical tools for the job. There is no single complex 

 sound source which is completely satisfactory over this whole range for one rea- 

 son or another, with the possible exception of explosives. There are various 

 devices such as underwater sirens, electrical sparks discharged under water, 

 certain fuels which can be burned under water and various mechanical devices 

 which expend energy acoustically by a hammer driven by an electric motor 

 beating on a steel diaphragm. All of these devices are moderately intense 

 sound sources and can be used for sound transmission studies. At present 

 there is little published information about complex sound sources. A discussion 

 of some of them in Beranek's "Acoustical Measurements" (Chapter 9) indicates 

 their general properties. 



It is important to note in connection with sound transmission studies, 

 particularly where they involve complex sound, that the complete transmission 

 system includes the receiving electronics, the amplifier, the recorder and also 



