TRANSMISSION RUNS 



71 



-SHAPE OF PING ^ SHAPE OF SIGNAL 

 GOOD COHERENCE 



SHAPE OF SIGNAL 

 POOR COHERENCE 

 Figure 2. Examples of good and poor coherence. 



transmitter; moreover, A is an absolute quantity 

 wiiich is independent of tiie system of units chosen.* 



Transmission loss and transmission anomaly are 

 the principal quantities which characterize the propa- 

 gation of sound from the source to any point of in- 

 terest. For some purposes, it is also desired to obtain 

 information on the steadiness of the transmitted sig- 

 nal and on its "coherence." Slow changes in signal 

 strength that occur in the course of several minutes 

 are called variation. Changes that take place in the 

 course of seconds are called fluctuation. The coherence 

 of a signal may be loosely defined as the degree of 

 fidelity with which the envelope of the transmitted 

 signal is duplicated by the envelope of the received 

 signal. If transmission conditions in the ocean did not 

 change rapidly, one would be a perfect copy of the 

 other, except for a negligible transient. Actually, 

 conditions sometimes change so rapidly that the 

 shapes of the transmitted and received signal re- 

 semble each other only slightly. Figure 2 shows (A) a 

 case of good coherence, and (B) a case of poor coher- 

 ence. Both of these figures show oscillograms of re- 

 ceived supersonic signals recorded on the same equip- 

 ment. 



A detailed discussion of variation, fluctuation, and 

 coherence will be given in Chapter 7. 



4.2 DETERMINATION OF TRANSMISSION 

 LOSS 



Information on transmission loss has been ob- 

 tained by three distinct methods : first, transmission 

 runs; second, echo-ranging runs; and third, the sta- 



° A is ten times the logarithm of the latio between the 

 power flow per unit solid angle close to the source and the 

 power flow per unit solid angle at the specified location; both 

 of these quantities should be expressed in the same units. 

 The apex of the solid angle is in both cases formed by the 

 sound source. 



tistical analysis of observed echo and listening ranges. 

 Soimd transmission runs include all investigations in 

 which the source of sound is separate from the receiv- 

 ing instrument and in which the sound travels from 

 source to receiver without suffering reflection from a 

 target; slanting reflection from the surface or the 

 bottom of the sea is, however, not excluded. While 

 various setups have been used for transmission runs, 

 the most common one involves the use of two ships. 

 One ship carries the sound source, whereas the other 

 ship is equipped with hydrophones whose outputs are 

 recorded. In echo-ranging runs, the same transducer 

 is used as both source and receiver. The sound is 

 propagated to a target and then reflected back toward 

 the point of origin. The target may be a vessel, but is 

 more frequently an artificial target, that is, a device 

 used exclusively for research and training purposes. 

 The observed range information has been furnished 

 to the research groups in the form of log books and 

 patrol reports by naval craft on active duty. 



Of the three methods of investigation mentioned, 

 transmission runs have proved by far the most power- 

 ful and reliable tool. The other two methods, analysis 

 of observed ranges and echo runs, are now merely 

 subsidiary. 



4.3 



TRANSMISSION RUNS 



The characteristic feature of the transmission run 

 is the employment of separate devices for transmit- 

 ting and receiving the sound. It is, therefore, possible 

 to measure the transmission over any type of path by 

 varying the depth of the projector, the depth of the 

 hydrophone, and the horizontal distance between 

 them. Depending on the instrumentation, it is further 

 possible to vary other important acoustic parameters, 

 such as signal frequency and signal length, or to em- 

 ploy signals composed of several frequencies or a 

 continuous range of frequencies. The temperature 



