A energy equation for a thin plate which includes internal loss and conveys the relation between sound intensity and structural intensity was derived. It is applied to measure near field sound intensity and loss factor.
The measurement method of near field sound intensity uses a strain gauge bridge and a velocity sensor. It does not require to measure sound pressure on the surface of plate which it is difficult to measure for evanescent wave, deflection, and mean flow of surrounding fluid. The associated Green's function which depends on geometrical shape, boundary condition, and flow condition of sound field is not also required. The measurement method of loss factor uses a strain gauge bridge, a velocity sensor, and two pressure sensor. It works in any surrounding fluid with plate which is arbitrary supported; the conventional methods need vacuum condition and very controlled boundary conditions.
To prove the potentials of two measurement methods and consider the errors caused by the strain gauge bridge, numerical simulations for a vibro-acoustic coupled system was performed.
Experiments show that sound intensity for a thin steel plate using the suggested method is the same as that using surface intensity method at the resonance frequencies.
