## 3 Results  
One of the most frequently occurring and usually most harmful components of underwater noise in shallow coastal water areas is the nonstationary noise caused by moving and anchoring vessels and motor boats. An oscillogram of a 16-min fragment of non-standardized signal at the hydrophone preamplifier output is shown in Figure 1, top; the vertical axis scale is conventional. The oscillogram looks like a noise track with sharp and smooth maximums and peaks. Its images may be considered typical for the given region; the mean level of noise track depends generally on the state of the sea. There is a smooth increase, maximum noise level around 710 s, and a smooth decrease afterwards--all caused by the passage of a cargo vessel with water displacement of approximately 800 metric tons and over 2 m draft (approach-departure) at a distance of about 0.3-0.5 nautical miles from the buoy that the hydrophone was mounted to. The sharp increase in noise level in the area of 790 s was caused by a passing motor boat at the distance of about 0.1 nautical miles from the buoy. Barely noticeable in this scale maximums in the areas before 500 s and past 900 s comply with the passages of vessels at distances of over 3 miles. The short peaks on the oscillogram are caused by the abrupt movements of the flexible mount of the hydrophone which are inevitable in rough sea surfaces; the cable connects the buoy and the boat. Peaks at the time intervals of 650-820 s are caused by hitches of the elastic suspension and the cable, which were observed when there were surface waves, caused by movement of the vessel and motor boat. Figure 1 (bottom) is a spectrogram (fast Fourier transform (FFT) size 1024, Hann window) of the signal and scale that reflect the dependency of color on the levels of spectral components of the signal in dB; the scale is conventional. Spreading in a fan-like shape, slope lines on the spectrogram physically represent an interference pattern changing over time, typical for the moving source of radiation in a hydroacoustic waveguide [7,8]. When listening to the signal, one can hear clicks at the time interval that corresponds to the minimum distance of the passing vessel; these clicks are caused by the propeller cavitation. The character of the spectrograms is typical for the waveguide propagation of sound with damping of modes at frequencies below the critical value (cutoff frequency). The cutoff frequency \(f_{0}\) can be calculated using the following formula [8]:  
Figure 1: Oscillogram (top) and spectrogram (bottom) of the signal at the hydrophone preamplifier output during the passage of a vessel and a boat. Over the spectrogram there is a scale that illustrates the dependence of color on the levels of spectral components of the signal in dB. The scale is conventional. \[f_{0}=\frac{c_{w}}{4D\sqrt{1-\left(c_{w}/c_{b}\right)^{2}}}\.\]  
This formula is exact only for a homogeneous water layer of depth \(D\) and sound speed \(c_{w}\) overlying a homogeneous bottom of sound speed \(c_{b}\). For the depth \(D\) = 30 m, assuming \(c_{w}\) = 1500 m/s and \(c_{b}\) = 1600 m/s (sand and silt), the cutoff frequency is \(f_{0}\) = 33 Hz. Changes not only in levels, but also in spectral content of the noise over time bring additional complications to the problem of the hydroacoustic signal detection. On a number of occasions, methods of automatic gain control (AGC) and normalization of the signal according to the required level are used in receiving devices of radio- and hydroacoustic communication the for the purposes of decreasing the dynamic range and stabilization of the terminal signal and noise levels. There are many methods of AGC implementation in analog and digital signal processing. If the stationary random process can be used as a model of noise, then AGC is usually necessary to maintain the received signal at a constant signal strength. The statistic theory of AGC for this particular case is developed; its conclusions are widely used in practice.