Investigation On Low-frequency Radiated Sound Field Characteristics Of Cylindrical Shells In Tanks

In the ocean,the low-frequency sound waves can travel a long distance.The underwater vehicles are an important part of China’s national defense and their low-frequency line spectrums of radiated noise have a significant impact on concealment and vitality of the vehicles in the ocean.The radiated sound power is an important acoustic parameter of underwater vehicles.Accurate measurement of the radiated sound power of underwater vehicles can be used as an acoustic index to evaluate the effectiveness of vibration and noise reduction measures.Compared with external field experiments,measuring the radiated sound power of underwater sound sources in non-anechoic tanks has advantages such as lower measurement cost,better measurement repeatability,less environmental interference and so on.The typical structure of underwater vehicles is the cylindrical shell model.In this paper,the low-frequency sound radiation characteristics and the measurement methods of radiated sound power of cylindrical shells in non-anechoic tanks were studied.In the low frequency band below the lower limit frequency of the reverberant field in a non-anechoic tank,the sound field is unevenly distributed.Compared with that in the free field,the radiation impedance of the sound sources changes significantly at modes of sound field in the non-anechoic tank,but changes less at non-modes.These sound field characteristics are the physical basis for applying the low-frequency measurement methods in non-anechoic tanks.The applicability of the comparison method for measuring the radiated sound power of cylindrical shells was analyzed in this paper.The finite element software was used to establish the radiated sound field models of the cylindrical shell and point sound source in the non-anechoic tank,and the effects of whether moving the sound source position or changing the sound source posture in the non-anechoic tank on the measurement results of the comparison method were studied.Numerical simulation and experimental research results showed that,the comparison method was based on the fact that difference between the sound field in non-anechoic tanks and that in the free field is only related to the sound field characteristics of the non-anechoic tanks,and has nothing to do with the characteristics of the sound sources.The applicability of the comparison method was analyzed from the two frequency bands with low acoustic modal density and high acoustic modal density in anon-anechoic tank : in the low frequency band with high acoustic modal density,when the position of cylindrical shell was fixed but the posture was changed,the deviation of the wideband radiated sound power tested in the non-anechoic tank was less than 3 d B;in the low frequency band with low acoustic modal density,increasing the position and changing the posture of cylindrical shell in the non-anechoic tank could decrease the measurement uncertainty of the comparison method.In the low frequency band of the non-resonant frequency,especially the very low frequency band below the cut-off frequency of the sound field in a non-anechoic tank,the energy in the sound field is mainly concentrated near the sound source and rapidly attenuates as it moves away from the sound source.When measuring the radiated sound power of cylindrical shells in this low frequency range,the low SNR makes the comparison method no longer applicable.Therefore,the applicability of the near-field acoustic energy flux method to measure the low-frequency radiated sound power of cylindrical shells was analyzed in this paper.By measuring the sound energy flux information on the envelope surface near the sound source,the near-field sound energy flux method can separate the acoustic energies flowing into and out of the envelope surface,thereby the radiated sound power of the sound source can be obtained.In this paper,numerical model was used to study the effect of the distance between the measurement envelope surface and the radiation surface of cylindrical shell on the measurement results of near-field acoustic energy flux method in non-anechoic tanks.The experiments had verified that the distance between the measurement envelope surface and the radiation surface of the cylindrical shell can affect the accuracy of the measurement results.The near-field acoustic energy flux method has provided an effective method for the low-frequency measurement of sound sources at fixed positions in non-anechoic tanks.