Chapter 4 

 EXPERIMENTAL PROCEDURES 



THE PRECEDING chapter was concerned chiefly with 

 the development of the ray-tracing technique, the 

 earliest theoretical approach which led to practical 

 results in the prediction of maximum ranges. This 

 method was, however, only partially successful. Its 

 chief accomplishment was the prediction of the 

 shadow zone boundary in the presence of pronounced 

 negative gradients at the surface. 



Predicted maximum echo ranges computed by ray- 

 tracing methods agreed with the available observed 

 range data to a fair degree of accuracj', but it was 

 clear that these prediction methods were too simple. 

 The evidence relating maximum observed ranges to 

 temperature conditions was too incomplete to be 

 analyzed with a view to improving range-prediction 

 methods. Navy vessels could not often be made avail- 

 able for range determinations under carefully con- 

 trolled conditions, and the scattered observations 

 made in the course of routine operations were incon- 

 clusive. It was decided, therefore, to initiate a pro- 

 gram in which the sound field produced with standard 

 Navy echo-ranging gear would be measured in much 

 greater detail than before. It was contemplated that 

 this study would place the prediction of sound ranges 

 on a firmer basis and in general would lead to a better 

 understanding of the basic factors important in trans- 

 mission of sound through the ocean. Subsequently, 

 this program was broadened to include sound of fre- 

 quencies between 100 and 60,000 c, and to cover 

 situations somewhat different from those encountered 

 in routine operation of standard gear. Only such a 

 broad experimental investigation of the propagation 

 of sound under various conditions can possibly fur- 

 nish an adequate insight into the mechanisms deter- 

 mining the sound field in the sea. 



This chapter deals with the experimental methods 

 which have been developed in connection with the 

 sound field program. The results obtained will be 

 discussed in Chapters 5 and 6. 



4.1 QUANTITIES CHARACTERIZING 

 TRANSMISSION 



Before launching into a detailed discussion of these 

 experimental methods, it will be necessary to review 

 briefly the principal quantities which characterize the 

 transmission of sound energy in the sea. In general, 

 sound power is transmitted at a particular frequency 

 or in a specified frequency band ; all statements in this 

 section concerning power, intensity, and sound level 

 refer to the frequency or frequency band once speci- 

 fied. 



Let F denote the power output per unit solid angle 

 on the axis of symmetry of the sound source; at a 

 moderate distance r from the source, the sound in- 

 tensity on the axis therefore equals F/r^. The power 

 output per unit solid angle in any other direction will 

 be given by hF, where 6, the -pattern function defined 

 in Section 2.4.4, is a function of the direction; by 

 definition, h equals unity for the direction of the 

 projector axis. 



Since decibels are commonly used in soimd field 

 measurements, we shall transform F into a more con- 

 venient quantity, the source level S. At a point on the 

 axis at a distance of 1 yd from a point source, the 

 sovmd intensity h will be proportional to F. The 

 source level S is defined as this sound intensity at 1 yd 

 in decibels above a suitably chosen reference in- 

 tensity Ir'. 



S= 10 logi 



©• 



(1) 



The reference intensity h is iisually chosen as that 

 corresponding to an rms pressure of 1 dyne per sq cm. 

 Actual sound sources, such as a battleship gen- 

 erating propeller and machinery noise, frequently 

 have large spatial extensions, and the sound level 1 yd 

 from the source is not well defined. However, at dis- 

 tances large compared with the linear dimensions of 



69 



