SONAR is an acronym for SOund NAvigation and Ranging – a classic example of this acoustic application in air is the flying bat, and in water, dolphins, whales, ships, submarines and their weapons, such as torpedoes.
SONAR is the underwater equivalent of radar, and like radar its functions are many and varied. These include the detection, identification, location and speed indication of ‘targets of interest‘.
There are basically two sonar modes of operation, ‘Passive‘ (receiving or listening) and ‘Active‘ (transmitting and receiving).
Radar operates on the principle of transmitting and/or receiving electromagnetic waves, travelling at approximately the speed of light
(3×108 m/sec) through space, or the atmosphere.
SONAR on the other hand, relies on sound or acoustic energy in water travelling at approximately 1,500m/sec, depending upon conditions such as temperature, depth, and salinity.
In the dull, dense and conductive environment of seawater, electromagnetic waves and light rays are severely attenuated. Acoustic energy however has proven to be more tolerant and controllable in this medium, but the sound is still influenced in many ways during its passage through the sea, consequently substantial acoustic energy is essential for many sonar applications.
The operational frequency of a specific sonar system is chosen to accommodate the required mode of application; this is critical to the system’s performance, and its receiving/transmitting transducers.
For example low frequency (e.g. 1 kHz) systems may be used for long range ‘listening’ and detection, whereas a higher frequency (>10 kHz) systems could be employed to resolve ‘target signatures‘, and to pinpoint the target location.
The accepted formula for calculating wavelength (λ) is –
λ= c/f meters
Where c = 1,500m/sec (approx. speed of sound in seawater),
and f = the operational frequency, e.g. 1kHz (1,000Hz).