Active Acoustics is a means of measuring the range to an object and its relative size by producing a pulse of
soundand measuring the time it takes for an echoto return from the object and the amplitudeof the returned echo. The range is calculated as a function of the speed of sound and the time it takes for the echo to return.
Active acoustics is the principle behind
SONARor hydroacousticsfor underwater measurements.
Active acoustic split-beam systems are capable of detecting organisms as small as krill, with no limit on upper sizes. Fish position within the ensonified beam can be accurately located in split-beam systems, allowing calculation of target strength, swim speed, and direction of movement. New directions for active acoustics in science include multibeam, multi-frequency, and high frequency imaging systems. While traditional systems have a beam width of 7-12 degrees sampling a slice of the water column, Multibeam systems such as the EM3002 can be used to examine school shape and behavior with a 130 degree beam width. In addition, multibeam systems are used for details bathymetric mapping and bottom typing based on the returned echo from the bottom. By using multiple frequency systems, fish of varying sizes can be differentiated due to resonance frequencies. High frequency systems such as the DIDSON provide camera-like accuracy of targets in the water column or on the bottom.
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