Hydroacoustic Technology, Inc.
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> What are hydroacoustics? What are echo sounders?

Basic hydroacoustic concept.  
 

Hydroacoustics is defined as active sound in water (sonar) that is used to study fish. Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish. While the first serious attempts to quantify fish biomass were conducted in the 1960's, major advances in equipment and techniques took place at hydropower dams in the 1980’s. Some evaluations monitored fish passage 24 hours a day for over a year, producing estimates of fish entrainment rates, fish sizes, and spatial and temporal distributions.

In the 1970’s, the dual-beam technique was invented, permitting direct estimation of fish size in-situ via its target strength. The first portable split-beam hydroacoustic system was developed by HTI in 1991, and provided more accurate and less variable estimates of fish target strength than the dual-beam method. It also permitted tracking of fish in 3D, giving each fish’s swimming path and absolute direction of movement. This feature proved important for evaluations of entrained fish in water diversions as well as for studies of migratory fish in rivers. In the last 35 years, tens of thousands of mobile and fixed-location hydroacoustic evaluations have been conducted worldwide.

 
Device that records digital processor's electronic sampling.
       Typical hydroacoustic system.
 Actual hydroacoustic fish survey.
    > How do hydroacoustics work?
 
 

Hydroacoustics may be one of the most misunderstood techniques for fisheries research. In essence, hydroacoustics monitoring is based on a few relatively simple principles. An acoustic echo sounder transmits a pulse of acoustic energy into the water. The pulse of energy travels through the water at a speed of approximately 1500 m/sec. When the acoustic pulse encounters an object, such as a fish or the bottom,
some of the energy (i.e. an echo) is reflected back to the transducer. The echo sounder amplifies the received signal and then sends it to an output device (such as a chart recorder or video display) and digital echo processor.

If the signal level exceeds a mark threshold level, a mark appears on the output display device. The distance from the top of the display to the mark is proportional to the travel time for the pulse to travel from the transducer to the target and back. Since the velocity in water is known, range (distance from the transducer) can be calculated from this travel time.
By displaying the output resulting from many consecutive transmissions, the time “change of range” and direction of objects in the acoustic beam can be monitored.

The basic system includes a high frequency echo sounder, one or more transducers with cables, a chart recorder, an oscilloscope, and a computer-based echo processing system. If more than one transducer is used, a multiplexer is also required. To learn about the individual components, mouse over the “Typical hydroacoustic system” (left).

Only research-quality equipment that does not drift in its parameters and can be accurately calibrated will allow month-to-month, or even hour-to-hour, comparisons of results.

   Click to view the hydroacoustic graphics and screen saves below.
Split-Beam, River Application   Split-beam, Lake Application   Split-beam Screen Save  
     
Multi-Frequency Screen Save Concept of Mobile Survey Mapping Concept of Pole-Mounted Sampling
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How do hydroacoustics work?

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Electro-mechanical device which translates electrical energy to sound energy to produce the hydroacoustic signal, and converts returning echoes back into electrical signals. The transmission of sound waves and measuring the time it takes for their echo to return after reaching an object. Acoustic waves reflected from a target of density differing from the medium in which the sound is traveling. Handles the precise transmitting, receiving, filtering, amplification, and timing functions of the signals. Used to display the echo signals on recordings called echograms. Echograms provide a permanent hardcopy record of all targets which have an amplitude greater than the predetermined minimum size threshold. Monitors the system operation, and is particularly useful if acoustic interference is prevalent. It gives a visual indication of the amplitude of the returning signals, and aids in aiming fixed-location transducers near boundaries (i.e., solid objects, the bottom, or the surface). Device that records digital processor's electronic sampling. Device that records digital processor's electronic sampling. System comprising acoustic transmitter, echo receiver and display. Central processing system (aka PC). email HTI home site map