E. J. Skudrzyk and G. P. Haddle 271 
Figure 14.10 shows the noise spectrum recorded at hydrophone 8, located 
at the rear end of the cylindrical section. The boundary layer there is much 
thicker and the noise level is, therefore, more intense at the flow frequencies, 
as predicted by the theory. Conversely, the high-frequency noise levels are 
correspondingly lower. The curves for the polished surface, for instance, slope 
down very steeply (20 db per frequency-doubling). The readings obtained with 
hydrophone 8 were practically the same as those obtained with hydrophone 10, 
located just beyond the rear of the cylindrical section; therefore, only a limited 
number of readings was taken with hydrophone 10. 
Not filling the joint between the nose and the cylindrical section increases 
the low-frequency noise level recorded by the nose-section hydrophones by as 
much as 20 db and also increases the vibration level of the shell. Uncovered 
joints apparently generate larger eddies and, consequently, considerable low- 
frequency noise (Strouhal noise, for instance). But these large eddies also seem 
to increase the effective thickness of the boundary layer and reduce the gradients 
near the wall, lowering the high-frequency noise level considerably. The large 
eddies produced by the unwaxed joints apparently absorb a considerable fraction 
of the energy that otherwise would be available for the excitation of the high- 
frequency noise spectrum. 
Figure 14.11 summarizes the effects of various surface finishes (high polish, 
spray paint, fine grit, and coarse grit) at low frequencies (frequency range of 
700 cps to 1.5 kc) and at high frequencies (frequency range of 20 to 40 kc). Grit 
can be seen to reduce the low-frequency noise level by as much as 10 to 16 db. 
Grit seems to have the effect of splitting up the larger eddies into smaller ones 
56 ee aay 4 
w 
oO 
Z 
I) 
fe) 
Ss NUMBERED CURVES CORRESPOND 
\ TO READINGS FROM BUOYANT -UNIT 
Ay HYDROPHONES LOCATED AS SHOWN 
IN ILLUSTRATION 
3 
DECIBEL SPECTRAL LEVEL (dbs) 
fe) 
\\ 
HIGH POLISH, UNWAXED yoInTs*X——\ 
-30 
CONDITIONS OF MEASUREMENT \ SAN 
—40 VELOCITY - 55 FPS 
DEPTH — 200 FT a 
—50 CORRECTED FOR SPEED 
0.1 ! 10 100 
FREQUENCY (kc) 
Fig. 14.10. Flow noise measured by hydrophone No. 8 on the buoyant unit for various finishes. 
