# Study of Low-Frequency Hydroacoustic Waves' Behavior at the Shelf of Decreasing Depth
## 2 Description of the Experiment  
The experiment was conducted in Vityaz' Bay of the Sea of Japan. The map of the experiment is shown in Figure 1. The research vessel was anchored at the \"radiation point\" (coordinates N 42\({}^{\circ}\)35.6827, E 131\({}^{\circ}\)09.8707) at a depth of 32 m. From its board, a low-frequency hydroacoustic transmitter of the electromagnetic type was immersed to a depth of 18 m [4], generating in the water harmonic signals at a frequency of 22 Hz. The transmitter was a part of a transmitting hydroacoustic system, consisting of a transmitter with an electromagnetic transducer, a suspension frame for the transmitter, a cable-hose with a control pressure gauge, an electric power supply, an electric pump, a control hydrophone, and two calibration accelerometers. The transmitting hydroacoustic system was designed to generate harmonic and phase-manipulated hydroacoustic signals in the frequency band of about 1 Hz in the range of 19-26 Hz. Figure 1: Map scheme of the experiment. The labelled “radiation point” is the location of the vessel with the transmitter. Points P1–P7 show the positions of the receiving stations. The angle 27.4\({}^{\circ}\) is the angle between the reception route and the “North–South” direction. The amplitude of the volumetric oscillatory displacements of the transmitter reached 0.0123 m\({}^{3}\). At the frequency of 20 Hz in unlimited water, this corresponded to a transmitted acoustic power of 1000 W. The configuration of the transmitting hydroacoustic system is shown in Figure 2.  
The transmitter had a mass of 260 kg in air and 40 kg in water. It contains a cylindrical case (1) and a pair of transmitting pistons (2), oscillating in mutually antithetic directions and creating in-phase flows of volumetric oscillatory velocity. The oscillations were induced by the electromagnetic type transducer with U-shaped composite halves of the core (3) and four coils (4). Between the edges of the pistons, a set of 312 cylindrical springs (5) was clamped. Their preliminary compression was achieved due to the decrease in air pressure by 0.5 atm in the transmitter cavity relative to the hydrostatic pressure at the depth of its immersion. To compensate for the hydrostatic pressure during immersion or lifting, a 60 m-long hose with a control pressure gauge and two nipples was used. The gaps between the flanges of the case and the edges of the pistons were sealed with rubber-fabric collars (6). The coils in each pair at half of the core (3) were connected in series. The current was supplied to them by means of elastic conductors put out through seals on the pistons (2). The conductors were commutated directly to the transmitter with terminal-bolt connections with polyisobutylene waterproofing and corresponded to the series connection of the pairs. A battery of series-connected (from 3 to 22 pieces, depending on the required power) acid batteries with a voltage of 12 V and capacity of 90 A*h was used as the primary source of direct current. The power source was a bridge key amplifier made of two half-bridge IGBT modules and equipped with a compensating capacitor battery of 420 uF, a circuit breaker, and a DC ammeter. When operating at stations P3-P7, the low-frequency hydroacoustic transmitter created a pressure of about 7 kPa, and when operating at stations P1-P2, about 5.8 kPa. The transmitted hydroacoustic signal was recorded with a Bruel & Kjaer 8104 hydrophone at different depths at points P1-P7 of the study ground (see Figure 1); it was immersed from aboard the vessel. The hydrophone was placed on a mobile capsule containing a hydrophone preamplifier, an analog-to-digital converter, and an autonomous recording device. At each reception point, the recording was carried out every 1 m from the surface to the bottom. The hydrophone was used for the vertical sounding of the hydroacoustic field near the transmitter, on which, subsequently, the energy density created in the water by the transmitter was determined. Figure 2: Hydroacoustic transmitter of 19–26 Hz.