E, J. Richards, J. L. Willis, and D. J. M. Williams 291 
Fig. 15.14. Rig for experimental measurement of Sac. 
15.4.2. Noise from the Turbulence 
Noise from turbulence in the absence ofsolid surface is quadrupole in nature 
and is very inefficient at the low speeds occurring in water vehicles; hence, it is 
unlikely to be significant. This may not be so if the boundary layer separates 
from the surface, but even in this condition the noise radiated by the other me- 
chanisms above are likely to predominate. 
15.4.3. Turbulent Boundary-Layer Wall Pressure Noise 
The radiated noise from a modern submarine has been calculated using our 
own measured levels of wall pressure fluctuations and correlation areas taken 
in conditions of a small pressure gradient. The results are shown in Fig. 15.16 
for a series of distances and submarine speeds. Comparison with data on ra- 
diated submarine noise available in the literature suggests that these levels are 
some 30 to 40 db below the noise actually radiated, and it must be presumed that 
the noise is certainly not simply that due to the fluctuating force field arising 
from the pressure fluctuations on a nominally smooth rigid wall. We must, 
therefore, investigate other effects, including the influence of roughness on the 
force field, the possibility of structural vibrations whichin turn reradiate noise, 
and, presumably, other effects with which we ourselves have had little experience 
in the underwater noise field, such as cavitation and separated and vortex flows 
of the kind studied in [5]. 
