Investigation On Low-frequency Radiated Sound Power Measurements Of Underwater Sound Sources In Enclosed Space

The acoustic stealth of underwater vehicles is related to their own vitality,so it is important to reduce the radiation noise of underwater vehicles,especially its low-frequency spectrum.However,in order to evaluate the effectiveness of vibration and noise reduction measures,it is necessary to measure the radiation noise of underwater vehicles accurately,and the measurement method is the core link.Non-anechoic water tank is an important acoustic measurement device which has several advantages such as low background noise,easy test environment,high accuracy of measurement results and good repeatability when used to measure the radiated power of underwater sound sources.The reverberation method provides an effective technical means for the acoustic measurement of underwater vehicles.The low-frequency sound field characteristics of non-anechoic water tank are the hot research on the sound field characteristics of enclosed space.The low-frequency radiation impedance of the underwater sound source has a significant change compared with the free field due to the less modal amount in the non-anechoic water tank,which leading to dramatic fluctuations in the low-frequency sound field,so the traditional reverberation method is no longer applicable.In order to expand the low-frequency applications of the non-anechoic water tank,the measurement methods of low-frequency acoustic characteristics of underwater sound sources in the non-anechoic water tank are discussed in this work.And the comparison method used in “the low-frequency resonance frequency band” as well as the sound intensity method used in “the acoustic cut-off frequency band” are elaborated subsequently.“The low-frequency resonance frequency band” refers to the frequency between the first-order normal frequency and the lower limit frequency of the reverberant sound field,in this frequency band,a comparison method is proposed by measuring the difference between the mean square sound pressure in the non-anechoic water tank and that in the free-field,and the radiated sound power of a complex structure is accurately measured.Using this comparison method,the reverberation method is expanded to be valid in non-anechoic water tank below the lower limit frequency range,and the measurement of reverberation time in the non-anechoic water tank is avoided.On the basis of the typical sound source,the low-frequency radiated sound power measurement test based on comparison method from a cylindrical shell model is carried out,compared to the radiated sound power tested in the anechoic water tank,the narrowband spectrum deviation tested in non-anechoic water tank is less than 1.5dB.The test results show that,the comparison method is reasonable and effective for the radiated sound power measurement in “the low-frequency resonance frequency band” of the reverberant sound field.“The acoustic cut-off frequency band” refers to the frequency below the first-order normal frequency of the reverberant sound field,in this frequency band,a sound intensity method based on the separation of near-field sound energy flow is proposed by measuring the radiation energy of the sound source in near field,and the radiated sound power of a complex structure is accurately measured by separating the inflow and outflow sound energy on the enveloping surface.Using this sound intensity method,the reverberant measurement is expanded to be valid in non-anechoic water tank below the first-order normal frequency,and the measurement of mean square sound pressure in the non-anechoic water tank is avoided.The sound radiation characteristics of the cylindrical shell model in enclosed space are analyzed by numerical computation,and the validity of the sound intensity method on reverberant measurement is verified.On the basis of the typical sound source,the low-frequency radiated sound power measurement test based on sound intensity method from a cylindrical shell model is carried out,compared to the radiated sound power tested in the anechoic water tank,the narrowband spectrum deviation tested in non-anechoic water tank is less than 2dB.The test results show that,the sound intensity method is reasonable and effective for the radiated sound power measurement in“the acoustic cut-off frequency band” of the reverberant sound field.