PROPAGATION ALONG WAKES 



509 



horizontally than vertically in case the wake has a 

 strong core and weaker fringes, because the vertical 

 measurements refer to the center of the wakes. 



32.4 PROPAGATION ALONG WAKES 



On the whole, the methods employed for the study 

 of sound propagation across wakes, described in 

 Section 32.3, have led to apparently consistent re- 

 sults. For sound propagation along wakes, however, 

 the observations do not fit easily into the general 

 picture; they are a few in number and provide in- 

 sufficient data to permit a complete analysis of all the 

 factors involved. 



A mechanical noisemaker ' was towed both in and 

 below the wake of a destroyer running at 10 and 14 

 knots, and sound levels were recorded simultaneously 

 by two hydrophones — one towed in the wake by the 

 destroyer and the other suspended at a depth of 10 ft 

 from a boat which was hove to. The destroyer fol- 

 lowed a straight course past this boat, while the 

 distance between the noisemaker and the towed 

 hydrophone was steadily increased from 50 ft to 

 1,200 ft by unreeling the hydrophone cable. 



As the cable lengthened the hydrophone gradually 

 descended, ultimately passing below the bottom of 

 the wake, which was assumed to be 20 ft below the 

 surface.^ The noisemaker was towed 50 ft behind the 

 destroyer, and the hydrophone reached a depth of 

 20 ft at distances of 400 ft (10 knots) and 1,000 ft (14 

 knots) behind the noisemaker. 



Finding the transmission loss along the wake would 

 require comparing the sound levels recorded by the 

 towed hydrophone with levels recorded by a hydro- 

 phone when no wake is present in the direction of the 

 noisemaker. Unfortunately, the levels recorded by 

 the stationary hydrophone, suspended from the boat 

 outside the wake, cannot be used, because the direc- 

 tivity pattern of the noisemaker is unknown. It 

 should be noted that the aspect of the noisemaker, as 

 viewed from the towed hydrophone, is practically 

 constant, while the aspect of the noisemaker relative 

 to the stationary hydrophone changes by about 90 

 degrees while the destroyer is moving toward, or re- 

 ceding from the point of closest approach. 



The sound levels obtained by the towed hydro- 

 phone with the noisemaker towed at a depth of 40 ft 

 may serve as an approximate reference level repre- 

 senting the wake-free state, because then most of the 

 path from the noisemaker to the hydrophone runs 

 below the wake. Subtracting these sound levels from 



the ones applying to the noisemaker towed in a wake, 

 an approximate value for the transmission loss along 

 the wake is found. The numerical values are about 

 6 db for 3-kc sound and about 13 db for 8-kc sound 

 at a speed of 10 knots; at 14 knots, the values are 

 about 13 db and 30 db for 3-kc and 8-kc sound re- 

 spectively. These transmission losses are of the same 

 order of magnitude as those found in propagation 

 across wakes. The increase of transmission loss with 

 frequency is also in agreement with what has been 

 learned about sound transmission across wakes. 



However, for the entire range covered (100 to 

 1,000 ft) the transmission loss along the wake does 

 not show the expected increase with distance from 

 hydrophone to noisemaker. The sound levels used as 

 reference values, with the noisemaker 40 ft below the 

 surface, vary inversely as the square of the distance 

 between hydrophone and noisemaker. But the sound 

 levels recorded with the noisemaker in the wake also 

 follow approximately the same inverse square law. In 

 other words, the measured transmission anomalies 

 fail to show any increase with distance behind the 

 noisemaker, which would readily be interpreted as 

 caused by attenuation inside the wake. There is even 

 a slight decrease, perhaps 3 or 4 db, over a range of 

 1,000 ft; however, this decrease may result from the 

 presence of bottom-reflected sound. Measurements of 

 the destroyer ship sound, with no noisemaker present, 

 gave results similar to those obtained with the noise- 

 maker. These observations are rather puzzling. 



The measurements of the sound output of a de- 

 stroyer, cruiser, and aircraft carrier - give additional 

 evidence of a very low transmission loss along wakes. 

 During the so-called Z runs, the vessel to be measured 

 passed the measuring vessel, which was hove to, and 

 then made a turn so that, during the receding run, the 

 axis of the wake coincided with the line connecting 

 the stationary vessel with the receding one. The sound 

 levels recorded were corrected for the transmission 

 loss resulting merely from geometrical divergence ac- 

 cording to the inverse square law, and from the cor- 

 rected levels a transmission anomaly was derived. 

 Attenuation coefficients along the wake were found 

 to be 10 to 80 db per kyd; these attenuation coef- 

 ficients were not judged sufficiently accurate to war- 

 rant a discussion of variation with speed (16 to 30 

 knots), frequency (5, 25, and 60 kc) and ship type. 

 In order to appreciate fully how small those attenua- 

 tion coefficients measured along wakes are, it should 

 be remembered that the measured transmission loss 

 across wakes (see Section 32.3) corresponds to attenu- 



