290 Lecture 15 
Boc= 2.4.1 | (Litter) ard)? 
Mt?(xy) dxdy 
Where P,c are density and velocity of sound in fluid medium 
f = frequency 
= surface density of panel 
f(xy) = mode shape 
10:0 
2 
) 
ratio 6ac x 0? 
an 
oO 
damping 
Measurement on 
cylindrical baffle 
Acoustic 
) 2 4 6 8 10 
N—Length breadth ratio 
Fig. 15.13. Comparison of variation of acoustic damping ratio with length-to- 
breadth ratio calculated from uniform and nonuniform pressure theories. 
The radiation due to over-all modes ofacylinder, in which the whole surface 
vibrates in a sinusoidal mode, has been calculated by Junger [11]. He shows that 
if the lengthwise distance between nodal points is less than the wavelength of the 
radiated sound, then a cutoff in radiation occurs, and the acoustic damping is 
then zero (Fig. 15.15). Thus the amount of reradiation in the underwater case can 
be assessed in qualitative terms, once the modes of oscillation and excitation 
are determined. 
15.4. APPLICATIONS TO UNDERWATER NOISE PROBLEMS 
15.4.1. Radiated Noise 
The radiated noise from a ship's hull or submarine has several components 
some of which are clearly not of hydrodynamic origin. Thus, propeller noise and 
noise originating from machinery inside the hull are excluded from the discus - 
sions of this paper, even though the mechanism of machine noise radiation is 
similar to that discussed here. Thus, we are interested in four separate me- 
chanisms of underwater noise excitation: 
1. Noise from the turbulence in the boundary layer itself, in the absence of 
surfaces; 
Noise from the fluctuations of force exerted by the structure on the fluid 
and arising from the constraint imposed on the turbulent boundary layer 
by the presence of the structure which is assumed rigid; 
3. Asin No. 2, but due to the roughnesses of the surface; 
4, The reradiation of sound from the structural deflections arising from the 
above forces. 
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