Hydroacoustic Installation

an aggregate of acoustic, electronic, and electrical instruments and devices that are interconnected by design and construction and are used for the reception or emission or the combined reception and emission of acoustic vibrations in water.

Two types of hydroacoustic installations are distinguished: those that only receive acoustic energy (passive operation) and those that both receive and emit acoustic signals (active operation). Passive hydroacoustic stations (noise hydrophone [Figure l,a], surveillance hydroacoustic installation, sound ranging station, and so forth) are used to detect and determine the direction of a noise-emitting object (a moving ship, an active hydroacoustic installation, and the like) from an acoustic signal (noise) generated by the object. They are also used to listen to, analyze, and classify the received signals. The operation of passive hydroacoustic stations is concealed and cannot be detected. Active hydroacoustic stations (sonar [Figure l,b], fish-finders, sonic depth finders, and the

Figure 1. Simplified block diagram of a hydroacoustic installation. (a) Noise direction finder: (1) stationary acoustic system, (2) phase shifter, (3) amplifier, and (4) indicator system, (b) Sonar: (1) moving acoustic system, (2) radome, (3) rotor, (4) “receive-transmit” switch, (5) generator, (6) amplifier, and (7) indicator system.

like) are used in detecting and determining the direction of and distance to an object that is completely or partially submerged in water (submarine, ship, iceberg, school offish, sea bottom, and the like). This objective is achieved by sending short acoustic pulse signals either in a single direction or in all directions and receiving the pulse reflected from the object in the intervals between these emitted pulses. Active hydroacoustic installations can detect not only noise-generating objects but also objects generating no noise, as well as moving and stationary objects. However, the signals emitted by the active hydroacoustic installations can be detected and hence their location can be found, which is somewhat of a shortcoming. Active hydroacoustic installations include installations for underwater sound communication, hydroacoustic beacons, hydroacoustic logs, and ice echo sounders. For a more detailed discussion of the methods used in direction finding and ranging see HYDROACOUSTICS and SONAR.

The principal components of a passive hydroacoustic installation are the acoustic system (antenna), compensator, amplifier, and indicator system. An active hydroacoustic station also has a generator and a transfer device, or a “receive-transmit” switch.

The acoustic system of a hydroacoustic installation consists of many electroacoustic transducers (hydrophones for receiving hydroacoustic installations and oscillators for receiving-emitting hydroacoustic installations), which are used to obtain the required directional response pattern for reception and emission. Depending on the type and purpose of the hydroacoustic installation, these transducers can be located beneath the hull of the surface ship (mounted on a rotating-sliding device); installed in a stationary, acoustically permeable radome; or built into the outer planking (shell) of the vessel. They can also be mounted in a container that is either towed by the ship or lowered by a helicopter or on a stanchion on the bottom of the sea. The phase shifter introduces a phase shift into the alternating currents flowing in the electric circuits of the spatially separated hydrophones. This phase shift is equivalent to the difference in the arrival times of the acoustic vibrations at the individual hydrophones. The numerical values of these shifts indicate the angle between the axis of the directional response pattern of a stationary acoustic system and the direction of the object. After amplification, the electrical signals are transmitted to the indicating equipment (telephone or a cathode-ray tube), in order to fix the direction of the noise-generating object. The generator of an active hydroacoustic. system generates short electrical signal pulses, which subsequently are transmitted by the oscillators as acoustic vibrations. During the intervals between these pulses, the signals reflected from the object are received by the same oscillators (which during this time are connected by the “receive-transmit” switch to the electrical oscillation amplifier). The range, or distance, to the object is determined on the indicator from the delay time of the reflected signal relative to the directly transmitted signal.

Hydroacoustic installations, depending on their type and purpose, operate in the infrasonic, sonic, or (more frequently) ultrasonic frequency range (from tens of Hz to hundreds of kHz) and emit power ranging from tens of watts (for continuous generation) to hundreds of kW (per pulse). The precision of direction finding is from one degree to a fraction of a degree, depending on the method used (maximizing method, phase method, amplitude-phase method); on the sharpness of the directional response pattern, which is influenced by the frequency and dimensions of the acoustic system; and on the method of display. The hydroacoustic installations operate at distances ranging from hundreds of meters to more than tens of km and primarily depend on the installation parameters, on the reflecting properties of the object (the strength of the target), or on its noise-emission level, as well as on the physical phenomena of the propagation of sound vibrations in water (refraction and reverberation) and on the noise level of the hydroacoustic installation owing to the motion of its own vessel.

Figure 2. Operational schematic diagram of hydroacoustic installation on a surface vessel: (1) sonic depth finder transducer, (2) hydroacoustic post, (3) sonar transducer, (4) detected mine, and (5) detected submarine

A hydroacoustic installation is installed on submarines, naval surface vessels (Figure 2), or in helicopters. It is also installed on land for antisubmarine defense, enemy reconais-sance, or communications between submarines and surface vessels. It can provide data for firing rocket-torpedoes or torpedoes or ensuring navigational safety. Transport, fishing, and exploration vessels use hydroacoustic stations for navigation, in searching for clusters of fish, for oceanographie and hydrological work, for communication with divers, and other purposes.