A low-frequency sound power measurement technique based on the separation of near-field acoustic energy flux in a non-anechoic tank

Abstract

To determine the radiated sound power of an underwater sound source below the first resonance frequency in a non-anechoic tank, a sound intensity measurement technique based on separating near-field acoustic energy flux is proposed. A theoretical analysis of vibration and acoustic radiation of underwater structures shows that the variation in the radiation impedance of the sound source in the given frequency band is much smaller than the mechanical impedance, and the vibration state of the sound source changes negligibly compared with that of the sound source in the free field. This theoretical analysis is verified experimentally. Based on this analysis, the acoustic energy flux is determined at various locations on the enveloping surface by first measuring the near-field sound pressure and the normal component of the particle velocity on the surface. The efflux and influx sound energies at an enveloping surface are separated to obtain the free-field radiated sound power of the source to be tested. This approach is tested by measuring the low-frequency radiated sound power of an underwater transducer source in a glass tank. The test results show that the maximum difference between the sound power measured in the glass tank and that measured in the anechoic tank is less than 0.8 dB.