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Introduction to Acoustics

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Radiated Power. The mean acoustic power in the fluids<br />

are given by<br />

⎧<br />

〈P1〉 =<br />

⎪⎨<br />

�<br />

(S) 1 2 Re � p1 [iωξ] ∗ � dS<br />

=− 1 2 ω2 �<br />

ρ1c1 (S) Re<br />

�<br />

k1 �<br />

k2 1−k2x −k2 �<br />

y<br />

× � �ξ(x, y) � �2 dS ,<br />

〈P2〉 = �<br />

(S) 1 2 Re � p2 [iωξ] ∗ � dS<br />

=+ 1 2 ω2 �<br />

ρ2c2 (S) Re<br />

� �<br />

k1<br />

⎪⎩<br />

× � �ξ(x, y) � � 2 dS .<br />

�<br />

k 2 2 −k2 x −k2 y<br />

(22.306)<br />

The minus sign in 〈P1〉 follows from the fact that the<br />

acoustic intensity vec<strong>to</strong>r in fluid 1 is oriented <strong>to</strong>wards<br />

the negative direction of the z-axis. The radiation efficiencies<br />

σi of the plate in both fluids (with i ∈[1, 2])are<br />

defined as<br />

�<br />

�<br />

σi = �<br />

〈Pi〉<br />

�ρici〈V<br />

2 �<br />

�<br />

�<br />

〉 �<br />

with<br />

〈V 2 〉= 1<br />

�<br />

ω<br />

2<br />

2 |ξ(x, y)| 2 dS (22.307)<br />

which gives<br />

σi = Re<br />

(S)<br />

� ki<br />

k i z<br />

�<br />

. (22.308)<br />

These quantities represent the ratio between the amount<br />

of acoustic energy radiated in fluid and the <strong>to</strong>tal kinetic<br />

energy of the plate. Equation (22.308) showsthat,if<br />

k z i is real, we have a propagating acoustic wave and<br />

the mean radiated power is different from zero. In the<br />

opposite case, the acoustic wave is evanescent and the<br />

mean radiated power is equal <strong>to</strong> zero.<br />

Particular Case of Two Identical Fluids. One particular<br />

case of interest corresponds <strong>to</strong> the practical situation<br />

where the plate is coupled <strong>to</strong> the same fluid on both<br />

sides. In this case, the dispersion equation becomes:<br />

2iω 2<br />

ρ<br />

�<br />

k2 − k2 − ω<br />

F<br />

2 ρsh + Dk 4 F = 0 , (22.309)<br />

where kx = kF cos θ and ky = kF sin θ. kF is the flexural<br />

wavenumber coupled <strong>to</strong> the fluid, θ is the angle of<br />

propagation and k = ω/c is the acoustic wavenumber.<br />

Structural <strong>Acoustics</strong> and Vibrations 22.5 Structural–Acoustic Coupling 937<br />

For a plate vibrating in vacuo, the flexural wavenumber<br />

is written<br />

�<br />

ω2 �1/4 ρsh<br />

kF0 =<br />

(22.310)<br />

D<br />

so that the dispersion equation can be rewritten<br />

k 4 F<br />

k 4 F0<br />

= 1 − 2iρ<br />

ρshkz<br />

(22.311)<br />

Light-Fluid Approximation. The light-fluid approximation<br />

corresponds <strong>to</strong><br />

� �<br />

�<br />

�<br />

2iρ �<br />

�<br />

�ρshkz<br />

� ≪ 1 . (22.312)<br />

In this case, the wavenumbers are given by:<br />

kF = kF0 or kF = ikF0 . (22.313)<br />

The case kF = kF0 corresponds <strong>to</strong> propagating waves. In<br />

this case, the radiation efficiency becomes<br />

⎧ � �<br />

k 1<br />

σ = Re = �<br />

15<br />

10<br />

5<br />

⎪⎨<br />

⎪⎩<br />

σ<br />

0<br />

0<br />

kz<br />

1 − k2 F0<br />

k 2<br />

1<br />

= �<br />

1 − ωc<br />

ω<br />

if ω>ωc<br />

with ωc = c2<br />

�<br />

h<br />

12(1 − ν2 )µ<br />

E<br />

= 0ifω

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