Hydophone: Designs And Principle

Introduction

Any systems (except for communication) which employ underwater acoustics energy are called sonar. It is a branch of applied acoustics that utilizes water as propagating medium [1]. Receiver (hydrophone) is compulsory for every sonar system to detect underwater sound. Only active sonar system utilizes transmitter [2]. Sonar system manipulates longitudinal characteristics of acoustical waves underwater. Longitudinal (irrotational) parameters include pressure, particle displacement, particle velocity, medium density, specific impedance, energy transfer speed, wavelength and temperature [3]. Physics behind sonar receiver is as simple as converting acoustics signal into electrical signals. The design should comply correct resolution and enough sensitivity to detect not only transmitted signal by transmitter (in active sonar system), but whole underwater noises within desired range. Hydrophones age nearly reach 100 years. From the first design patented in 1919 until the latest one; there were just too great advancements made in both performance and size, only a little will be discuss here.

Materials and Designs

The heart of hydrophone is piezoelectric (pressure-electric) device/material. Piezoelectric effect occurs naturally in a material when it physical dimensions changes upon application of pressure and produces electrical field. Piezoelectric effect is also reversible theoretically and in many occasions practically. This effect could be achieved using mechanically assemble components. However, in natural piezoelectric materials, effect occurs when a material consist of innumerable electric dipoles. A stress could cause a deformation of a material and reorientation of the dipoles induces net charges between electrodes. Several piezoelectric materials (not natural) are lead zirconate titanate ceramic (Pb(Zr,Ti)O3) or PZT, lead metaniobate (PbNb2O6), barium titanite (BaTiO3) and lithium niobate (LiNbO3). Certain PZT can be doped for better piezoelectric properties.

One of the earliest hydrophones [4] was sealed in the container for complete submersible operation as shown in Fig. 1. It contains circular metal diaphragm placed in the circular rim and tightly attached to the metal plate, with button type microphone placed in the center chamber.

Conducting leads was attached as output terminals and sent from container through water-proof clamp. Operating principles is very simple. Acoustics pressure will vibrate the diaphragm. Vibrating diaphragm then generates sound inside the center chamber, and the secondary generated sounds will be detected by microphone.

This design suffered from several weaknesses. It confused by the noise generated by ship engine. Proper calibration could not be done properly since it contains moving mechanical parts as transducer which worsens the resolution and sensitivity. During 1950s, capacitive-type hydrophone was introduced [5]. Details cross-section design of hydrophone shows in Fig. 2.

Fig. 1: First patented hydrophone design.

When immersed in the water, any pressure changes detected by bag, as a diaphragm. Capacitance between water and electrode will change as the function of the pressure, since the size of air cells also changed. Capacitance changes are then converted in electrical signal by any method. Compare to the previous design, it was simpler and cheaper, but lack of versatility when only capable of detecting low frequency signal. During 1960s, hydrophone design is totally depends on piezoelectric materials [6]. Almost all design follows the same pattern as shown in Fig. 3. This design employs a stack of piezoelectric plates as active material. By using stacked configuration, higher capacitance or lower impedance is obtainable. Cover and housing usually is made from flexible material such as rubber to sustain hydrostatic pressure. Fluid medium fills the entire volume between stack and cover. Produced electrical field by plates was sent through electrodes to the pre-amplifier, filter and conditioning circuits.

Arrays

Arrays are transducer assemblies with more than one element. These elements may be in line, rectangular and square in shapes, ring shaped and circular array. Hydrophone array also may be a combination of one of those for example linear array in three-dimensional arrangements. By using array, signals can be added from the desired direction while subtracted signals from other directions. Usually, array of hydrophones (and transmitter) is controlled by beamformer. Studying array requires deeper knowledge in signal processing. It deals with complicated mathematical model, complex analyses and parametric estimation, depending to the type of array [7-9].