274 Lecture 14 
r AREA EFFECT I7 IN. AFT 
soll A + ——— 1/8-IN.-DIA CIRCLE 
B x ———- I/4-IN.-DIA CIRCLE 
Aol ese C © ------- 1/2-IN.-DIA CIRCLE 
Bee D G —--— |-IN-DIA CIRCLE 
E 2-IN-DIA CIRCLE 
DBS (re! dyne) 
20 
L} 
a 
oO 
| 
S 
{e) 
' 
a 
oO 
0.1 | 10 100 
FREQUENCY (kc) 
Fig. 14.12, Hydrophone-area effect on buoyant unit. 
about the same as that recorded inside withthe larger hydrophone. It was there- 
fore concluded that the area effect occurred whenever the hydrophone diameter 
was smaller than 5 in. The buoyant units exhibit the theoretical area effect at 
the very low and middle frequencies up to about 20 ke for hydrophones of, at 
the most, 1/, in.-diameter (see Fig. 14.12). Atthe higher frequencies the recorded 
noise seems to be essentially radiation-field noise and the recorded noise is 
therefore practically independent of the diameter of the hydrophone. 
The area effect is greatly dependent on the ratio of the diameter of the hy- 
drophone to the boundary-layer thickness and vanishes whenever the hydrophone 
becomes large. In contrast, the shape effect seems to persist irrespective of 
the hydrophone size. 
A rectangular hydrophone turns out to be particularly flow-noise sensitive, 
whereas a circular hydrophone is much less sensitive. Figure 14.13 illustrates 
this for a cylindrical hydrophone, which is similar in its behavior to a rectan- 
gular hydrophone. A fish-shaped hydrophone indicates a small noise level if the 
head of the fish points in the direction of the flow, and the noise level is 15 db 
larger if the fish points transverse to the main flow (Fig. 14.14a). The square 
shape shows a similar result when the corner is pointed in the direction of the 
flow (Fig. 14.14b). At frequencies above 8 kc, the nearfield sensitivity of most 
of the hydrophones used in the experiments became so poor that they indicated 
almost nothing but radiation-field noise (Fig. 14.14c). As a consequence, area 
and shape effects vanish. 
