levels among the conditions tested. However, observations made during the 

 tests indicated that the bare cable vibrated at these speeds and that these vi- 

 brations appeared to influence the motion of the hydrophone as well as its 

 signal as seen on an oscilloscope. The vibrations at these speeds are of 

 such low frequency that they do not appear in the analysis, since the lowest 

 l/3-octave band on the spectrometer is centered at 16 cycles per second. 



At speeds between 0.5 and 2.0 knots, the noise levels for the bare cable 

 condition are from 5 to 20 decibels higher in the low frequency bands (16 to 

 125 cycles per second) than for the faired cable condition. All test conditions 

 produced high-noise levels for speeds above 2.0 knots. 



The addition of plastic tubing (fairing) reduced cable vibration at all speeds 

 and seemed to completely eliminate the vibrations at the low speeds . The 

 shroud-ring tail on the hydrophone greatly reduced the very low-frequency os- 

 cillation of the hydrophone at all speeds. The effect of the reduction does not 

 appear in the analysis because the frequency of the oscillations is below the 

 frequency range of the instrumentation. The addition of the shroud-ring tail 

 to the hydrophone had no effect in reducing the higher frequency cable vibra- 

 tions . 



It should Be noted, however, that the results shown in Figure 10 may be 

 influenced by background noise in the basin, and noise and vibration of the tow . 

 carriage. Nevertheless, the data indicate that further investigation into the 

 effects of more refined fairing, greater cable scopes, and other towing con- 

 figurations is warranted. 



The experimental results obtained from the open-water tests conducted 

 in Chesapeake Bay are presented in Figure 1 1 . The results of theoretical 

 calculations, using the method outlined in Reference 5, are superimposed for 

 comparison. It may be seen that the computed position of the array does not 

 agree very well with the measured portion. Lack of agreement is attributed 

 mainly to the fact the system towed to one side. Using a constrained, flexible, 

 faired section that is not free to swivel, the tow member will cause the towline 

 to develop side forces which deflects the system to one side. This occurrence 

 is indicated in this system by lateral angular measurements which approached 

 45 degrees at 4 knots. The angular records and observations showed that al- 

 though the array towed to one side, it remained reasonably steady over the 

 speed range . 



A qualitative narrow-band frequency analysis was performed on the hydro- 

 phone signals recorded during the open-water tests. It was found that for the 

 condition with the hydrophone at 100-foot depth suspended on the faired cable, 

 the interference which might be attributed to cable or fairing vibration in the 

 0-to 50-cps frequency range was negligible. Thus, the broad-band signal could 

 be amplified so that other noise components such as the firing rate of the boat's 

 engine, signals from a passing tanker, random background noise, etc., could 

 be identified. However, the records from the two hydrophones secured to the 

 unfaired weighted rope were quite different. In particular, the record for the 

 100-foot depth hydrophone in the latter system showed many interfering high- 

 level noise components in the very low frequency range. The hydrophone was 

 taped directly to the rope which was the vibrating member in this case. 



12 



