FUTURE RESEARCH 



241 



10.3.3 High Supersonic Frequencies 



As far as is known, transmission in shallow water 

 at higii supersonic frequencies is similar to that at 

 24 kc, except for greatly increased absorption losses; 

 transmission anomalies in the presence of negative 

 gradients are linear and their slopes are somewhat 

 liigher than those in deep isothermal water. 



10.4 FLUCTUATION AND VARIATION 



The transmission loss measured at any instant in 

 the ocean will usually differ from the value found 

 several seconds earlier. This rapid change of sound 

 level is called fluctuation. Measured transmission 

 losses and transmission anomalies are averaged to 

 smooth out this fluctuation. 



Fluctuation is invariably observed in the trans- 

 mission of single-frequency supersonic signals trans- 

 mitted over a path at least 100 yd long. Fluctuation is 

 negligible over transmission paths of the order of 5 yd. 

 Little is known concerning fluctuation over inter- 

 mediate path lengths. For frequencies of 5 kc and 

 less, fluctuation appears to be less pronoimced than 

 at frequencies of 10 kc and higher. The summary 

 which follows is concerned only with the fluctuation 

 of supersonic signals transmitted over paths at 

 least 100 yd in length. 



10.4.1 Variance with Shallow Projector 



For a projector at a depth of 16 ft, the direct sound 

 from an echo-ranging projector cannot be distin- 

 guished from the surface-reflected sound. The fluc- 

 tuation is large and inexpUcably variable. Observed 

 values of variance average 40 per cent with an inter- 

 quartile spread of about 20 per cent. The variance at 

 24 kc is significantly correlated with the variance at 

 16 kc or 60 kc, the coefficient of correlation being 

 about 0.7. 



10.4.2 Variance with Deep Projector 



For a deep projector and a deep hydrophone, the 

 direct signal can be resolved from the surface-re- 

 flected signal. The observed fluctuation of the direct 

 signal is small; observed values of the variance at 

 24 kc lie between 5 and 10 per cent and may result 

 from the variability of the measuring equipment. 

 The surface-reflected pulse is highly variable with a 

 variance between 50 and 70 per cent. 



With explosive pulses, the direct sound can be re- 

 solved from the surface-reflected pulse even at shal- 



low depths. The observed variance for the direct 

 pulse is about 1 or 2 per cent if the transmission path 

 lies wholly in an isothermal layer, but up to 20 per 

 cent if part of the transmission path Kes in the 

 thermocline. 



10.4.3 Rapidity of Fluctuation 



The time during which the sound level is not likely 

 to change appreciably is also variable, but seems to 

 increase with increasing range. At a fixed range of 

 less than a few hundred yards, the transmission loss 

 for a shallow sound projector changes by about 

 20 per cent on the average during 0.5 sec. At a fixed 

 range of several thousand yards in the direct sound 

 field, the average time for a 20 per cent change might 

 be 2 sec; while in the shadow zone, this average time 

 is likely to be nearer 0.02 sec. 



10.4.4 



Variation 



Slow changes in the (averaged) transmission of 

 sound in the sea, which take place in several minutes 

 and which cannot be explained in terms of observable 

 changes in the vertical temperature pattern, are 

 called variations. It has been found that at 24 kc the 

 variation between two transmission runs about 20 

 minutes apart has an average value of about 4 db if 

 only pairs of transmission runs are considered in 

 which the bathythermograph pattern is significantly 

 the same. This average value for the variation does 

 not appear to depend significantly on range. 



10.5 



FUTURE RESEARCH 



During World War II research on the transmission 

 of underwater sound has been largely devoted to the 

 empirical investigation of certain practical problems. 

 A wealth of detailed information has been accimiu- 

 lated on the transmission loss of sound from a stand- 

 ard echo-ranging projector under conditions likely to 

 be observed in practice. Although this information 

 has been useful in subsurface warfare, it has not led 

 to any complete understanding of the physical proc- 

 esses involve4 in underwater sound transmission. 

 For example, the average attenuation in deep iso- 

 thermal water near San Diego has been extensively 

 measured, but the causes of this attenuation are 

 completely unknown. 



In the years to come, ressearch in this field will 

 probably change its character. The quest for empiri- 

 cal data on some particular situation has been carried 



