Effect of the fluctuant acoustic channel on the gain of a linear array in the ocean waveguide

The inhomogenous ocean waveguide, which leads the amplitude and phase of the signal arriving at a hydrophone array to fluctuate, is one of the causes that make the array gain deviate from its ideal value. The relationship between the array gain and the fluctuant acoustic channel is studied theoretically. The analytical expression of the array gain is derived via an acoustic channel transfer function on the assumption that the ambient noise field is isotropic. The expression is expanded via the Euler formula to give an insight into the effect of the fluctuant acoustic channel on the array gain. The result demonstrates that the amplitude fluctuation of the acoustic channel transfer functions has a slight effect on the array gain; however, the uniformity of the phase difference between the weighting coefficient and the channel transfer function on all the hydrophones in the array is a major factor that leads the array gain to further deviate from its ideal value. The numerical verification is conducted in the downslope waveguide, in which the gain of a horizontal uniform linear array（HLA） with a wide-aperture operating in the continental slope area is considered. Numerical result is consistent with the theoretical analysis.

The fluctuation of acoustic field due to internal waveson the thermocline in typical shallow water

In this paper, the results of experiments concerning the fluctuations of internal waves on the thermocline and the fluctuations of acoustic amplitudes at different ranges in typical shallow water are presented. Recorded time is 48.96 h. Thickness of the thermocline is about 2 m-4 m. The deviation of temperature recorded at a fixed depth is 10℃-15℃. The vertical displacement of isotherm curves is about 5 m-6 m, which is mainly dominated by the lowest model. The measured spectrums of vertical displacement have obvious spectral packet within the range of 0.05 cpm (20 min) and 0. 143 cpm (7 min) besides the diurnal and semidiurnal tide frequency bands. Spectrum attenuation coefficients are between -1.5 and -1.7. Numerical calculation shows that phase velocity and group velocity of internal waves are equal to 0.329 m/s at lower frequencies. And with the increasing of frequency, the group velocity differs from phase velocity at about 0.03 cpm (33.33 min) and drops more quickly. The measured fluctuation of acoustic amplitude is about 8 dB-10 dB. With the increasing of range, the fluctuation trends to be faster and acoustic signals are declining. Spectrum attenuation coefficients are between -1.45 and -2.0. At last, PE method is used to simulate the acoustic amplitude fluctuation, in which internal waves are the important element.