E. J. Skudrzyk and G. P. Haddle 275 
ee) THEORETICAL SLOPE FOR RECTANGULAR 
OR CYLINDRICAL HYDROPHONE 
40 
-20 
-40 
DECIBEL SPECTRAL LEVEL (dbs) 
THEORETICAL SLOPE FOR PERFECTLY 
-60I— SHADED HYDROPHONE 
-80 
0.01 0.1 | 10 
FREQUENCY (kc) 
Fig. 14.13, Hydrophone-shape effect, 
14.9. THE REDUCTION OF FLOW NOISE 
The flow-noise level depends greatly on the position of the hydrophone. The 
intensity of the low frequencies is relatively small at the front of the unit where 
the boundary layer is laminar, or turbulent and very thin. The high-frequency 
noise is very weak in the stagnation region, which is nonturbulent, and even 
weaker towards the rear of the unit where the boundary layer is very thick. The 
high-frequency level (above 15 kc) received by the head hydrophone can be re- 
duced considerably by the use of a flat head instead of a hemispherical head. 
Most of the noise that is received by this hydrophone is generated near the joint 
between the cylindrical section and the head. A flat head increases the shadow 
effect, and less noise is diffracted into the stagnation region. Turbulence- 
suppressing varnishes proved effective as noise reducers. A vaseline coating, 
for instance, reduced the noise level above 3 kc by almost 20 db. The noise level 
also depends greatly on the condition of the joint between the head and the 
cylindrical portion. If this joint is not filledin, the boundary layer becomes thick, 
as if it were tripped, or oscillates. Not filling in the joint reduces the high- 
frequency noise level but increases the low-frequency noise level. Methods are 
being examined that may lead to a further reduction in the noise level. 
