04-02-2012, 03:49 PM
Hydrophone
INTRODUCTION
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 1912. 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 1914. 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 Langévin 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 150 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.
Hydrophones 2 Seminar Report 2009-2010
Dept. of AE MTPC Mattakkara
HYDROPHONE/RECEIVER
A hydrophone is an underwater listening device. Just as a microphone is used to detect sound in the air, a hydrophone detects sounds in the water and converts the acoustic energy into electrical energy. Hydrophones listen to sounds in the sea, but do not transmit any sound, making them passive listening devices.
Humans were listening to underwater sounds with air tubes as early as 1490, when Leonardo da Vinci wrote about it. It was not until the mid to late 1800's that the science developed to convert acoustic energy into electrical energy. This technology advanced enough that by World War II, British, French, and American scientists were using echo-ranging to locate icebergs and submarines. The ability to locate submarines was invaluable against the German U-Boats.
Most hydrophones are made from a piezoelectric material. This material produces small electrical charges when exposed to pressure changes. The pressure changes associated with a sound wave can be detected by a piezoelectric element. Under the pressure of a sound wave, the piezoelectric element flexes and in return gives off electrical signals. These electrical signals can be recorded and later analyzed with computer programs.
Photograph of a standard hydrophone. The upper picture is the preamplifier. The center picture is the ceramic element/preamplifier assembly - the preamplifier is housed inside the ceramic element. The lower picture is the final product that is encapsulated in urethane. Photo courtesy of Adrienne Spychalski, High Tech, Inc. (http://home.att~hightechinc/)
Hydrophones 3 Seminar Report 2009-2010
Dept. of AE MTPC Mattakkara
Some hydrophones, called omnidirectional hydrophones, record sounds from all directions with equal sensitivity. Other hydrophones, called directional hydrophones, have a higher sensitivity to signals from a particular direction. Directional hydrophones are typically used in systems for locating and tracking objects.
Hydrophones are specially designed for underwater use. They are normally encased in a rubber boot to provide protection from seawater. Hydrophones can be mounted in several different ways. They can be attached to a boat, towed, or placed in a fixed position underwater.
