【摘要】 在侧壁无消声的波浪槽(70×2×0.6m~3)内,利用9根有水平指向性的长圆管换能器作阵元,实现了前四号简正波的空间过滤,定性地研究了简正波过滤阵抑制近场点源干扰的理论概念,并对起伏波浪表面对各号简正波振幅起伏的影响作了初步观察。更多还原

【Abstract】 The spatial filtering of modes is very important both for theoretical purpose and practical purpose. Spatial filtering of modes is a main approach to verify the normal-mode theory and it is also the foundation to realize the field matching in the wave guide. It will have an important application in the investigation of the interaction between the sound wave and some environment factors such as the acoustic characteristics of ocean bottom, internal waves… etc, because the process of separated mode is simpler than the point source.In this paper, the model experiment includ following three parts:(1). to realize the spatial filtering of mode in a long water tank without absorbers on the side wall.(2). to verify a new conception[7] on suppresing the near interference point source by means of selected mode.(3). to observe the effect of surface waves on each individual mode as well as a point receiving signal.The spatial filtering of mode is based on the orthogonality of modes. For a Pekeris’s wave guide, we have : Un(z) = Na · sin [k0sinα · z] an is determined by the dispersion equation :2 Hk0sinαn+argVH - π = 2 nπ n = 0, 1, 2,… For small grazing angle we have[9]:argVH= - π + P·a wherethus, we obtain the approximation solutionand then:In this experiment the weight functions for the mode filtering were calculated according to the above formula. The experiment was completed in two water tanks: one is small (24× 1×0.92 m3) the other is large (70×2×0.6m3) with strengthened glass walls of 1cm thick and concrete bottom covered by 10 cm sand in the large tank. Surface waves can be produced by blower or wave-making machine. In order to circumvent the side wall’s effects, the long cylindrical transducers (56.4cm long with 3cm diameter) were used both for receiving array and for transmitting source. The receiving array consisting of 9 long cylindrical tran-sdusers. The directivity of the receiving array weighted according to a certain mode function has been measured in free space, the experimental result agree well with the theoretical. It is shown that these transducers are satisfied to the requirement of setting up array for mode filtering.The main results obtained in this experiment are :(1) The spatial filtering of lth-4th modes has been realized in a long water tank (70× 2×0.6m3) without absorbers on the side walls by using long cylindrical transducers as source and receivers.(2) The conception of suppressing the near interference by using a mode selective array has been verified. When the target source r, and the interference source rN are both in the "3/2" region, the gain of the experimental result agree with the theoretical result given by[7] :(3) The amplitude fluctuation of each individual mode induced by the surface waves is apparently smaller than the "point" receiving signal, and the higher the order of mode the larger the fluctuations.(4) The spectrum of the amplitude fluctuation of the "point" receiving signal conforms with the spectrum of the surface waves near the reveiver approximately.更多还原