70 



EXPERIMENTAL PROCEDURES 



200 



150 200 



VELOCITY ANOMALY 

 IN FT PER SEC 



o tn 



oo 

 — u 

 en Q 



60 3« 



?? 



in y 

 — o 



1 = 



a: 



K 



80 



100 



4000 

 RANGE IN YARDS 



6000 



8000 



Figure 1. Transmission loss (H) and transmission anomal}' (A). 



such a source, the sound field intensity drops off hke 

 that of a point source producing similar sounds; in 

 other words, for intensity calculations at long ranges, 

 the actual source may be replaced by an equivalent 

 point source. The reported source level of an ex- 

 tended source is nothing but the sound level of the 

 equivalent point source at a range of 1 yd. For echo- 

 ranging transducers, the sound level is often meas- 

 ured at a distance of a few yards and then extra- 

 polated to a distance of 1 yd by means of the inverse 

 square law. 



Consider now the sound field intensity I at some 

 specified location, presumably at a fair distance from 

 the sound source. This intensity is commonly ex- 

 pressed as a sound pressure level L in decibels above 

 an rms pressure of 1 dyne per sq cm (decibels of a 

 pressure level are defined as twenty times the loga- 

 rithm of the ratio between the rms acoustic pressure 

 and 1 dyne per sq cm). If the sound source is highly 

 directional, like an echo-ranging projector, it is 

 usually understood that the projector is trained, that 

 is, rotated about its vertical axis, toward the point 

 at which the transmission loss is to be determined. 

 But in the absence of a tilting device, the axis ray 



leaves the projector in a horizontal direction and may 

 then be refracted to a depth different from that of the 

 recording hydrophone. The difference S — L will 

 therefore depend, in general, on the directivity pat- 

 tern of the transmitter. As long as the distribution of 

 acoustic pressure does not approach the conditions 

 of explosive sound, S — L will be independent of the 

 absolute power output of the transducer. The dif- 

 ference S — L in decibels is called the transmission 

 loss and is denoted by the symbol H. 



Frequently, the transmission loss is represented in 

 terms of its deviation from the law of inverse square 

 spreading. If this law vere valid, the transmission 

 loss should amount to 20 log R, where R is the hori- 

 zontal range from the transmitter to the chosen 

 point. The expression H — 20 log R is called the 

 transmission anomaly and is denoted by A. Figure 1 

 shows the experimentally determined values of H and 

 A in a particular run with plots of the sound velocity 

 against depth and of the computed ray diagrams. 



H and A are quantities depending on the trans- 

 mission characteristics of the path under considera- 

 tion and on the directivity pattern of the transmitter. 

 They are independent of the power output of the 



