GENERAL CONSIDERATIONS 



Submarine radiated-noise measurements ' are presently being obtained 

 with a hydrophone array in which the cables are bundled, and the system is 

 allowed to drift with the listening ship in the manner shown in Figure 1. Be- 

 cause of ocean currents and winds, the system is set into motion and the 

 cables rruove relative to the water which results in the formation of a"Karman 

 Vortex Trail." 3 Above certain velocities, eddies break off alternately on 

 either side of the cable in a periodic fashion, as indicated in Figure 2. Thus 

 a staggered, stable arrangement or trail of vortices is formed behind the 

 cylinder. This alternate shedding produces periodic forces normal to the un- 

 disturbed flow which act first in one direction, and then in the opposite direc- 

 tion . The alternating forces cause the hydrophone cables to vibrate and the 

 cable vibrations are either received directly by the hydrophones as sound 

 waves or cause an actual acceleration in the sensitive hydrophone elements 

 which also results in noise. The resulting signals are of high amplitude and 

 tend to mask out lower-level noise components present in the low frequency 

 portion of the spectrum . Attempts to reduce the vibrations have been made 

 by sliding loose plastic tubing over the single cable ( see Figure 1) to break 

 up the flow around the cable. This technique has been partially successful, but 

 not to the degree necessary for accurate sound analysis. 



The motion of the system through the water also causes the hydrophone 

 array to tow in a catenary so that the hydrophones are neither at desired 

 depths nor in a true vertical plane with respect to the noise source. Since the 

 depth and configuration of the present type of array is difficult to predict, the 

 assumptions made with respect to the position of the hydrophones in the anal- 

 ysis of data are sometimes far from accurate. 



EXPERIMENTAL PROGRAM 



As mentioned in the Introduction, the experimental program was restricted 

 by limitations of test facilities at the Taylor Model Basin as to size, depth, 

 and background noise. Therefore, this investigation was conducted in three 

 phases: 



1 . Shallow-water towing tests to determine the magnitudes and 

 sources of vibrations or low-frequency noise components in the acoustic 

 system, 



2. Open-water tests to determine the effects of cable scope and 

 fairing in the reduction of vibrations and to obtain information relative to the 

 towing attitude of the system, and 



3. Evaluation tests at sea to determine the towing behavior and con- 

 figuration of a proposed system for submarine radiated noise measurements. 



References are listed on page 1; 



