288 
—— —-— Theoretical dipole 
Measured 
g = 150 Ibs/fr? Measured 
at aerofoil position 
Flat plate 
\" chord 
Fig. 15.10. Polar plot of noise from small flat-plate aerofoil at zero incidence measured 
at three-foot radius. 
between the stresses in two adjacent panels aroundthe periphery of the fuselage. 
In this case, at least, the actual motion is independent between frames but takes 
on an approximate "hourglass" or "“eggtimer" mode around the periphery, 
adjacent panels being antiphase. 
The reradiation from such a motion can be calculated and the acoustic 
damping of such an array estimated. Calculations have been carried out by Mead 
[9] of the acoustic damping ratio of flat, rectangular, simply supported panels 
of various length-to-breadth ratios, mounted in a rigid structure and vibrating 
in their fundamental modes (see Fig. 15.13). The reradiation is simply calcu- 
lated in such cases and can be used to indicate the noise radiated once the 
panel movements are known. 
Preliminary tests on the acoustic damping of arrays of panels have com- 
menced at Southampton [10] by testing a single panel as shown in Fig. 15.14. As 
shown in Fig. 15.13, the agreement with theory in this particular case is good. 
