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A "Hydrophone" is a device which will listen to, or pick up, the acoustic energy underwater. A hydrophone converts acoustic energy into electrical energy and is used in passive underwater systems to listen only. Hydrophones are usually used below their resonance frequency over a much wider frequency band where they provide uniform output levels. Underwater echo detection systems were developed for the purpose of underwater navigation by submarines in World war I and in particular after the Titanic sank in ***2. Alexander Belm in Vienna described an underwater echo-sounding device in the same year. The first patent for an underwater echo ranging sonar was filed at the British Patent Office by English metereologist Lewis Richardson, one month after the sinking of the Titanic. The first working sonar system was designed and built in the United States by Canadian Reginald Fessenden in ***4. The Fessenden sonar was an electromagnetic moving-coil oscillator that emitted a low-frequency noise and then switched to a receiver to listen for echoes. It was able to detect an iceberg underwater from 2 miles away, although it could not precisely determine its direction. The turn of the century also saw the invention of the Diode and the Triode, allowing powerful electronic amplifications necessary for developments in ultrasonic instruments. Powerful high frequency ultrasonic echo-sounding device was developed by emminent French physicist Paul Lang�in and Russian scientist Constantin Chilowsky. They called their device the 'hydrophone'. The transducer that Langevin used in the earlier days was a mosaic of thin quartz crystals glued between two steel plates (the composite having a resonant frequency of about **0 KHz), mounted in a housing suitable for submersion. In the underwater community the word transducer means a device that has the capability of both transmitting and receiving sound. A projector is a device that transmits sound underwater. Projectors are used in active systems. In active systems, after the sound has been generated, the sound waves travel to a target and return as echoes to be detected. Projectors are usually used near their resonance frequencies where they provide the highest acoustic output.