E. J. Skudrzyk and G. P. Haddle 273 
14.7. RADIATED PRESSURE AND SHELL VIBRATION LEVEL 
A large hydrophone has been shown to be very insensitive (by a factor of 
"h 000 or more) to the small-scale turbulence that generates the high-frequency 
noise. Such a hydrophone measures essentially only those pressure fluctuations 
that are correlated over greater distances, such as the true sound pressure 
produced by the generation and decay of the eddies in the turbulent boundary 
layer. The hydrophone gives an indication of the true radiation field only; the 
validity of this indication has already been verified by the rotating-cylinder ex- 
periment and, again, by all the buoyant-unit runs, where the pressure inside 
the boundary layer, and outside at distances of about 100 yards from the buoyant 
unit, has been recorded. At the higher frequencies, the external pressure, when 
corrected for the geometrical decrease of its amplitude with distance, is prac- 
tically the same as the pressure recorded inside the boundary layer. The radi- 
ated pressure is usually greater than that which would be deduced from the 
vibration amplitude of the shell even under the most favorable conditions (as - 
suming pe as the value of the radiation resistance for the shell vibrations; see 
Fig. 14.8, double-dot-dash curve). 
In the frequency range of the measurements, the bending wavelength of the 
shell is always smaller than the sound wavelength. The very-large-scale pres- 
sure fluctuations that are responsible forthetrue sound radiation are, therefore, 
alternately in phase and out of phase with the bending modes of the shell and do 
not excite these modes very much. (A bending mode can be excited only if the 
pressure pattern shows variations similar to the mode function.) 
14.8. EFFECT GF SIZE AND SHAPE OF HYDROPHONE ON THE RECEIVED NOISE LEVEL 
It has been shown above that the level of the nearfield flow noise that is 
picked up by a hydrophone greatly depends on the size of the hydrophone. How- 
ever, the experimental results show that this phenomenon is considerably more 
complex than was assumed initially. No area effect was found at the very low 
frequencies in the test section of the water tunnel; all the hydrophones were 
equally sensitive to flow noise in the frequency range 50 to 600 cps. This un- 
expected result leads to the conclusion that the correlation of the flow, in the 
test section of the water tunnel, in the transverse direction, is considerably 
greater than the boundary-layer thickness. That this conclusion is reasonable 
can be illustrated by smoke photographs of the turbulence around an airplane 
wing. Such photographs show that the turbulence is stratified in the direction 
transverse to the flow and that it is, therefore, correlated over great distances 
in this direction. The stratification seems to be particularly pronounced near 
the leading edge of the wing, where the turbulence is generated first, and is less 
pronounced toward the rear. At the frequencies above 1 kc, the area effect 
was very pronounced in the measurements in the water tunnel, whenever the 
hydrophone diameter was not very much largerthanthe boundary-layer thickness 
(see Fig. 14.5). A very pronounced area effect was also found for the rotating- 
cylinder measurements. At a frequency of 20 kc, a 5-in.-diameter hydrophone 
was 12 db less sensitive to flow noise than a 2,5-in.-diameter hydrophone. The 
noise level outside the boundary layer of the cylinder (3 ft distant from it) was 
