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# Fisheries Acoustics 101 - Principles

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Introduction to Fisheries Hydroacoustics 101 - BioSonics, Inc

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### Fisheries Acoustics 101 - Principles

5. 5. Step 1: Timing and Transmission • User Selects Pulse Rate • User Selects Pulse Duration • User Selects Transmission Power • Echosounder synchronizes with high-precision clock 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
6. 6. Characteristics of Transmission • A – Frequency • B – Amplitude • C – Pulse Duration 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
7. 7. Units • Frequency is in Hertz (cycles / second) or typically kiloHertz (kHz, or 1000 Hertz) • Amplitude is in Volts • Pulse Duration is in Time (milliseconds) 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
9. 9. Features of Transducer • Piezoelectric • Designed to resonate component at proper frequency • Active array or • Shapes the sound field element dimensions in the water • Main lobe and side • Pattern of sound lobes production • Directivity or • Amount of focus Directivity Index along acoustic axis 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
10. 10. Characteristics of Sound Field • Maximum sound intensity is along the acoustic (Z) axis • Main and side lobes are formed 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
11. 11. Units • Sound Pressure • Newton/square meter (force per unit area) (Pascal) • Power • Watts • Intensity • Watts/square meter (power per unit area) 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
12. 12. The Decibel (dB) • A logarithmically compressed unit • Defined as 10 times the ratio between an intensity and a reference intensity 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
13. 13. Nearfield of a Transducer • In the nearfield, a transducer does not act like a point source projector. • NF < d2/ where d is diameter of the transmitter and is acoustic wavelength. 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
14. 14. Directivity Index of a Transducer • Directivity Index is the sound level difference between a directional and non-directional sound projector. 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
15. 15. Beam Pattern Factor for Point-Source Targets • Directivity is a map of intensity with angle • The sound pressure transmitted and received at an angle off axis is reduced 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
16. 16. Mean Squared Beam Pattern Factor for Volumetric Targets • An equivalent non- directional hemispherical pattern is calculated from the directional pattern, used as a weighting factor in the echo integration scaling. 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
17. 17. Sound Traveling in the Water • Sound pressure travels as a pressure wave spreading in a spherical shape • Sound intensity is focused in the direction of the acoustic axis to form the directivity 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
18. 18. Sound Velocity • Sound velocity varies in a complex fashion with temperature and salinity • Sound velocity is critical in converting time-based data to range-based data • “c” is used for velocity 2003-2009 Copyright BioSonics, Inc. All Rights Reserved