Chapter 1 



INTRODUCTION 



1.1 IMPORTANCE OF TRANSMISSION 

 STUDIES 



SINCE SOUND WAVES are transmitted through water 

 very much more readily than radio and light 

 waves, the use of underwater sound has become a 

 basic part of subsurface warfare. There are always 

 many different ways in which equipment can be de- 

 signed and used. An intelligent choice between the 

 different alternatives depends on accurate knowledge 

 of the different factors affecting final performance. 

 One of these factors is the extent to which sound is 

 weakened in passing from one point to another; this 

 weakening is called transmission loss. The present 

 volume summarizes the information available in 1945 

 on transmis.sion loss of imderwater sound." Much of 

 the detailed discussion refers to a sound frequency of 

 24 kc since this is the frequency most commonly used 

 in practical echo ranging, and most of the available 

 data are at that frequency. 



This information, although incomplete, is useful 

 in a variety of ways. In particular, it is helpful both 

 in the design of gear and in the development of opera- 

 tional doctrine. 



It is evident that the intelligent design of new 

 equipment requires reUable information on imder- 

 water sound transmission as well as on a variety of 

 other factors. For example, the choice of frequency 

 in any device usually involves a compromise between 

 high frequency for the sake of directivity and low 

 frequency for the sake of good transmission. It is 

 possible to arrive at a suitable compromise by trial- 

 and-error methods. However, the choice is made more 

 quickly if routine methods can be used to predict the 

 transmission loss at each frequency, the directivity. 



' This volume includes primarily those data applicable in 

 the frequency range above 200 cycles. Sound of lower fre- 

 quencies has not been used in sonar equipment and its trans- 

 mission has not been investigated by Division 6 of the NDRC, 

 except occasionally in connection with the transmission of 

 explosive pulses. 



and other factors, such as the noise level, which affect 

 performance. These different predictions can then 

 be combined to find which frequency gives the best 

 results. The optimum frequency will, of course, de- 

 pend on the purpose for which the equipment is de- 

 signed, and on the limitations of size, available 

 power, and other characteristics. Thus, in some types 

 of echo-ranging equipment, low-frequency gear with 

 a wide beam pattern and a long maximum range is 

 used in searching for submarines, but tilting high-fre- 

 quency gear is provided for tracking a submarine at 

 close range during an attack. 



The development of operational doctrine for the 

 gear already in use also depends on the results of 

 transmission studies largely because of the wide 

 variability of underwater sound transmission. If a 

 pulse of soimd is sent into the water and received near 

 the surface 3,000 yd away, the signal energy received 

 will sometimes be only a millionth of the signal energy 

 received at other times. This enormous variation is 

 due mainly to changes in the vertical temperature 

 gradients present in the water. These changes have a 

 direct effect on the maximum range at which sub- 

 marines can be detected by echo-ranging gear. When 

 the maximum range is known to be short, the gear 

 can be operated most effectively with a short keying 

 interval, since more rapid keying increases the chances 

 of finding a submarine which happens to be within the 

 maximum range. If a long keying interval were used 

 under these conditions, time would be wasted in 

 listening for echoes during periods when no echoes 

 would be possible. 



Information on the change of sound transmission 

 conditions with changing temperature conditions is 

 useful in the choice of antisubmarine tactics as well as 

 in the selection of rules for operating the sonar gear. 

 When the transmission loss of sound is high and the 

 maximum range of sonar gear is short, the spacing 

 between surface vessels conducting an antisubmarine 

 hunt must be reduced. Sharp temperature gradients 

 at considerable depths may weaken sound passing 



