E. J. Skudrzyk and G. P. Haddle 277 
epee ee SHAPE EFFECT 17 IN. AFT 
“a Do I-IN.-DIA DISK 
5° a ~ FlOW Rs F go ——— I-IN.-SQUARE 
SSS pone SS page? eS ae — 
86 r~ Sa . \ DIRECTION GO I-IN.- SQUARE 
ES J © —-— FISH, |.4-IN. LENGTH, 
0.6-IN. WIDTH 
DBS (re! dyne) 
0.1 | 10 100 
FREQUENCY (kc) 
FIG. 14.14C 
Fig. 14.14. Hydrophone-shape effect. 
14.10. THE REPRODUCIBILITY OF THE MEASUREMENTS AND THE EFFECT OF SHELL VIBRA- 
TIONS ON THE RESULT 
The results were obtained on different days and normally did not vary by 
more than two or three decibels. There were times when greater discrepancies 
were observed, however—discrepancies that could usually be attributed to tem- 
perature discontinuities or to great changes in the temperature structure of 
the sea. Some of the more important buoyant-unit runs were repeated five times; 
two runs were reserved for the recording of the low and middle frequencies and 
the remainder for the middle and high frequencies. Thus, five different record- 
ings of the middle frequencies (1 kc to 20 kc) were available to check the 
accuracy of the recordings. 
The measurements were not affected by shell vibrations. The driving-point 
impedance of the shell was about the same as the impedance of a 5-g mass at 
1000 cps. The hydrophones had a mass of 800 g. Therefore, only "A 60 of the shell 
amplitude was transmitted to the hydrophones. This conclusion is in agreement 
with the experimental results obtained when the shell was driven in air with the 
same amplitude that normally would have been excited by the flow noise. The 
hydrophone readings were more than 40 db below the readings that would have 
been obtained if the shell had been excited by flow noise. In a second investi- 
gation, a heavy damping layer was applied to the shell, but the damping had no 
effect on the hydrophone responses. 
