Research On The Reverberation Measuring Technique Of Underwater Complex Sound Sources

The acoustic characteristics of underwater moving targets is the important researchaspect of underwater acoustics and most of the underwater moving targets including differentkinds of sound sources, such as mechanical, hydrodynamic and propeller sources havecomplicated structure. The acoustic characteristic of underwater complicated sound sourcesincludes sound power level, directivity and frequency spectrum characteristics. It is not easyto correct the reflection of ocean bottom and ocean surface in case of the corresponding oceanenvironmental condition, so it is difficult to get the radiated sound power and the frequencyspectrum of the underwater complicated sound sources, and the noise sources can not beenseparated also. Reverberation method is a general method used in Architectural acoustics formeasuring the radiated sound power of sound sources and has been standardizedinternationally. It is seldom used in the radiated sound power measurement of underwatersound sources due to the low reflection coefficient of the pool wall and non-ideal reverberantfield. The main research of the paper focuses on the reverberation measuring techniques usedfor measuring the radiated sound power of underwater complicated sound sources in thenon-ideal reverberant field (non-anechoic pool). It will be proved that the radiated soundpower of underwater complicated sound sources can be measured accurately in the non-idealreverberant field by theoretical analysis and experimental verification.The sound field characteristic in the rectangular non-anechoic pool whose wall is notrigid is first analyzed in this paper. The relation between the spatial averaging squared soundpressure where the hydrophone is located far from the directional source and the radiatedsound power of the source is derived by taking the Green function method and thesuperposition law of sound sources is verified. The theoretical formula of reverberationmethod for underwater complicated sound sources is established. To the wall effect for themeasurement of low frequency sound sources, the Waterhouse revision is expanded and thestatistical average correction is proposed to correct the wall effect. It is great significant forthe measurement of underwater low frequency sound sources. Secondly, the spatial averagingmeasurement technique is studied. It is investigated whether the spatial average, spatialaveraging in different ways, whether the average of the sound source have effect on theradiated sound power measurement of sources. Thirdly, the underwater complicated soundsources are measured and investigated in the non-anechoic pool. The radiated sound power ofa spherical standard sound source is measured. The radiated sound power of directional sound sources, such as the coherent sound sources and the piston sound source is measured. Thecharacteristics of the scale effect for different size pools are experimentally studied. Finally,the uncertainty for the radiated sound power of the underwater sound sources is analyzed.Measurement and research results show that the bigger the spatial average range, the bettereffect and much better effect if the source is also spatially averaged. The difference betweenthe radiated sound power of a standard spherical sound source measured by reverberationmethod and that measured in free field is not more than1dB. The difference between theradiated power of low frequency sound sources in the glass tank revised by statistical averageand that gotten in freedom field is not more than1dB. The difference between themeasurement results of the radiated sound power for the coherent spherical sound sources andthe theoretical results is not more than1dB. The radiated sound power of the piston soundsource measure in the non-anechoic pool is almost equal to that measured in free fields. Theradiated sound power of two spherical sound sources working at same time is just equals tothe sum of each one working independently. The greater the non-anechoic pool, the lower thespatial average sound pressure levels, the lower the signal-to-noise and the Schroeder cutofffrequency. The class A measurement uncertainty of radiated sound power for underwatersound sources is less than1.5dB.A reverberation chamber method is proposed to measure the flow-induced noise of theunderwater hydrofoil structure. A experimental system was built in the gravity water tunnel,the radiated sound power of the underwater hydrofoil model was measured using this method.It is investigated also that flow speed has impact on the radiated sound power. The resultsshow that this method can avoid the hydrophone by the impact of sound field distortion in thewater tunnel. the radiated sound power will increase withU6∞dependence (U∞is the flowspeed) and show the dependence of radiated sound power on speed for dipole whenU∞isless than5m/s; the radiated sound power will increase withU10±1∞dependence and notindicate the dependence for dipole whenU∞is more than5m/s. The absorption of the watertank’s wall to the sound wave is small, so the characteristics of diffuse sound field in the tankis much better than that in the pool whose wall is made of cement or tile. In order to improvethe characteristics of diffuse sound field, it is suggested to design the underwaterreverberation chamber according to the water tank.