E. J. Richards, J. L. Willis, and D. J. M. Williams 287 
25 
A400 - ae 
300; aN 
/ 
2 aca cane. 
ie Ss 
6B PRESSURE GRADIENT (Lo/exy, =) 
A (Peonsr - Peranc) Lejs 
4 (ins) 
DISTANCE BEHINO STEP 
Fig. 15.9. Measurements taken behind 0.10-in. step. 
the noise radiated from one part of a body to another is to relate it to the 
theoretical total acoustic power output, modified to allow empirically for the 
directional pattern as, for example, obtained in Fig. 15.10. Thus, some idea of 
the noise environment of a laminar region of a wing produced by the radiated 
noise from the turbulent region downstream can be obtained from the theoretically 
computed total acoustic output reduced by some 6 db (see Fig. 15.10) to allow 
for the directionality of the sound field. This distribution of the sound field is of 
course a function of the boundary-layer thickness, but this may not introduce a 
great error since the frequency at which the wall pressure fluctuations fall off 
(and therefore the frequency of greatest noise emission) is also related to the 
boundary-layer thickness. 
15.3, STRUCTURAL RESPONSE AND RERADIATION 
A structure as complicated as that of an airplane or submarine has a large 
number of possible modes of vibration, all of which can conceivably be excited 
in any particular case. Clarkson, however, has indicated by careful cross- 
correlation techniques that at least in the Caravelle [7] and the Comet [8] the 
number of modes excited by jet noise is quite small and manageable. For 
example, Fig. 15.11 shows the root-mean-square stress level in one plane of 
the Caravelle while Fig. 15.12 shows the cross correlation at each frequency 
