The Study On Measurement Of Radiated Noise From Submarine Due To Inhomogeneous Distribution Of Acoustic Energy In Shallow Sea

With the constantly development of acoustic stealth technology, the radiated noise level of low noise submarine has already approached the ocean ambient noise, which brought great difficulty for radiated noise measurement. It is a difficult and urgent task for our country to research radiated noise measurement technology for quiet submarines which is consistent with our national conditions.As continental shelf of China is comparatively gentle and coastal areas are mostly shallow, it is difficult to find appropriate ocean area to build deep-water testing site. To satisfy the requirement of quiet submarine radiated noise measurement, it is more appropriate for China to test within short range shallow water. However, distribution of acoustic fields is quite inhomogeneous because of the short range shallow water environment, submarine's complicated acoustic source structure and its radiated characteristics. Since great improvements have been made on the factors influencing accuracy of submarine radiated noise measurement, such as measurement equipments, ranging, approaches and so on, inhomogeneous distribution of acoustic energy in measuring sea has become an important factor influencing accuracy of submarine radiated noise measurement in China. Based on this, taken the practical engineering program of low noise measurement system as background, this paper does comprehensive theoretical and experimental investigation on underwater acoustic focusing characteristics within radiated noise measuring distance of target.Based on Ray Theory, mathematical expressions of sound pressure field and the particle velocity field were deduced for typical acoustic source and complicated acoustic source under the condition of boundary constraint. This paper came up with a calculating model of the vector acoustic field in shallow water using the image source method. The model was established and applied to the vector acoustic field computation when sound sources were broadband signals. Method of Small Perturbation (MSP) was applied to describe the acoustic fields of uneven surface with smooth slop and small fluctuation variance, which provided theoretical calculating methods for acoustic field's characteristics research within testing sea.Characteristics of short-range vector propagation of target radiated sound in shallow water have been studied from the aspect of practical engineering in submarine radiated noise measurement, which shows that the inhomogeneous distribution of acoustic fields in testing sea is related to many factors like target sound source structure, radiation directivity, the band width and frequency of the signals, boundary constraint, seabed acoustic properties, the spatial location of sound sources, navigation condition of target acoustic sources and so on. In order to reduce the influence on measurement caused by inhomogeneous distribution of acoustic energy in specific measurement environment, acoustic field's spatial distribution analysis has been conducted according to testing environment and testing targets. Measurement accuracy could be improved by choosing appropriate spatial location of acoustic source and receiver in areas with less acoustic energy convergence.The degree of measurement error caused by inhomogeneous distribution of acoustic energy has been quantitatively analyzed by comparing measurement results in measurement environment with spherical wave propagation law. It is feasible to correct radiated noise measurement results by using spherical wave propagation law in the far fields and free fields.It is also feasible for non-directional acoustic source within the short measurement distance. But when it comes to directional acoustic source or when boundary constraint exists, there is deviation in distance correction of measurement results by using spherical wave propagation. Measurement results and spherical wave propagation law are much larger than the true results especially for the acoustic energy offset areas.There needs to be specific correction according to radiated characteristics and spatial location of acoustic source.Toward the main influencing factors of acoustic energy inhomogeneous distribution in testing environment, this paper probed into methods and related technology for reducing inhomogeneous distribution of acoustic fields to further improve measurement accuracy. It is put forward and theoretically proved that the measurement model based on vector hydrophone array can effectively smooth the channel space transfer function, which provided a basis for how to reduce acoustic energy inhomogeneous distribution's influence on target radiated noise measurement caused by boundary constraint of the testing sea. By estimating target equivalent acoustic center in different frequency bands and distance correcting, it is feasible to reduce the influence on results of target radiated noise measurement caused by volume distribution source through rationally selecting equivalent acoustic center of the practically complicated acoustic source.By systematical experiments on acoustic propagation measurement in short range, we have finished sensitivity calibration of vector hydrophone, out field testing system design, and validated the characteristics of acoustic energy inhomogeneous distribution within underwater target radiated noise measurement distance, related laws and methods on reducing acoustic energy inhomogeneous distribution's influence on radiated noise measurement, which provided experimental basis for data analysis of practical engineering and engineering application of vector hydrophone array in underwater radiated noise measurement.