Chapter 10 

 SUMMARY 



RESEABCH ON SOUND transmission during World 

 War II was concerned almost exclusively with 

 the investigation of sound fields which were operation- 

 ally important. More than half of the experimental 

 work was devoted to the sound field of standard echo- 

 ranging transducers operating at frequencies around 

 24 kc. The purpose of the work was primarily to pro- 

 vide information which could be used to increase the 

 effectiveness of Navy gear already in use on sub- 

 marines and antisubmarine vessels. The instrumen- 

 tation used for research differed as little as possible 

 from standard operational gear; what modifications 

 were made usually represented the minimum neces- 

 sary for quantitative evaluation of the data obtained. 

 Questions which did not seem important opera- 

 tionally, such as the physical cause of the observed 

 attenuation of supersonic sound in the sea, received 

 scant attention in these studies. 



In the sections which follow, the essential results 

 of these experiments on underwater sound trans- 

 mission are summarized. Section 10.1 lists the defi- 

 nitions of the most important quantities used in 

 describing underwater sound fields. Sections 10.2 and 

 10.3 summarize what is known concerning the aver- 

 age transmission of supersonic and sonic sound in the 

 sea. In Section 10.4, data on the fluctuation and varia- 

 tion of seaborne sound are summarized. Finally, 

 Section 10.5 provides a brief discussion of probable 

 trends in future research on sound transmission. 



10.1 



10.1.1 



BASIC DEFINITIONS 



Sound Pressure and Sound 

 Field Intensity 



A sound wave in a fluid can be described con- 

 veniently in terms of the pressure disturbance which 

 arises in the vicinity of a sound source, travels 

 through the fluid, and is finally received by a hydro- 

 phone. The instantaneous sound pressure is the dif- 

 ference between the instantaneous value of the pres- 

 sure at a chosen location and the mean or equilibrium 



pressure at the same point. The rms value of the 

 instantaneous sound pressure is usually called the 

 rms sound pressure. Usually, the average is carried 

 out over a time interval which is long compared with 

 the periods of the principal frequencies making up the 

 sound signal. In the case of single-frequency sound, 

 the average is extended over one period (or an inte- 

 gral number of periods). Unless specified otherwise, 

 "sound pressure" as used in the technical literature 

 is short for rms sound pressure. Except in the case 

 of standing waves, the rms sound pressure is an ex- 

 cellent measure of the energy carried by the sound 

 wave. At the present time, sound pressure values 

 are uniformly I'eported in units of dynes per square 

 centimeter. 



The sound field intensity is defined as the averaged 

 power carried by a sound wave per unit cross section 

 of a wave front. The units in present use are watts 

 per square centimeter. If the radii of curvature of 

 the wave fronts are large compared with the wave- 

 length, then the rms soimd pressure and the sound 

 field intensity are connected in excellent approxima- 

 tion by the formula 



I = 10-' 



pc 



(1) 



in which p is the rms sound pressure, p is the density 

 of the fluid in grams per cubic centimeter, c is the 

 sound velocity in the fluid in centimeters per second, 

 and / is the sound field intensity. 



10.1.2 



Sound Level 



The sound field intensity is usually reported on a 

 logarithmic scale. The most common scale for this 

 purpose is the decibel scale. The quantity L, 



L = 20 log p (2) 



in which the rms sound pressure p is expressed in 

 units of dynes per square centimeter, is called the 

 sound pressure level or simply the sound level. As de- 

 fined by equation (2), L is the sound level in decibels 

 above a standard which corresponds to a sound pres- 



236 



