QUASI-FAR FIELD CRITERION OF UNDERWATER ELASTIC STRUCTURE FOR RADIATED SOUND POWER MEASUREMENT

This paper derives a method for determining the quasi-far field criterion for measuring the radiated sound power from underwater elastic structures by method of mean squared sound pressure. A numerical experiment is presented for a finite elastic cylindrical shell excited by a point force in the water using finite element method (FEM). The radiation noise field at points on the acoustic axis of the shell is simulated. The phase difference between the pressure and velocity on those field points, as well as the attenuation law of sound pressure amplitude with the testing distance, are analyzed to determine the quasi-far field criterion. The simulation results show that the radiated sound power from the cylindrical shell can be precisely measured by method of mean squared sound pressure according to this quasi-far field criterion, which is much less than the classic criterion of far field L/λ for sound sources in general. This means that sound pressure can be measured at points nearer to the elastic structure to determine its radiated sound power by method of mean squared sound pressure, which is very valuable for sound source level measurements when the signal-to-noise ratio is low.