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666
ON  RESONANT REFLEXION OF  SOUND
[446
It is natural to expect an evanescence of reflexion when the damping is small and the tuning such as to give good resonance. In this case we may suppose kt and TT - 2^* to be small, and then (27) gives approximately
"¥).......(28)
By (25)
kS = &! tanh 7^ = k-i tanh
Jc2 tan
so that
6
.(29)
Since S is large and positive, the condition for no reflexion can be satisfied by making the perforated area a- small enough.
For a more general discussion we may trace the curves (B, A, Fig. 1) 4
Fig. 1. * So that wave-length is 4 times I.                „                  — ik' sin k'                                        ( .>.>\