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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

  1. 1. Principles of Hydroacoustics By BioSonics, Inc. 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  2. 2. Sound Generation and Reflection (echo) Dr. Schorsch Georg Wiora SOund Navigation And Ranging 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  3. 3. Scientific Echosounder 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  4. 4. Breaking Down into Steps 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  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
  8. 8. Production of Sound 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
  19. 19. Spherical Spreading • Radiated power is constant • Power per unit area decreases with range due to spreading • Mathematical decrease is given as: 20 log R 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  20. 20. Absorption • Absorption of energy is like a frictional loss, causing localized „heating‟ of the water • Absorption is significantly higher in salt water for frequencies up to 1 Megahertz 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  21. 21. Sound Reflection • Sound traveling through water reflects off of objects that have different density than the surrounding water 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  22. 22. Transmission of Reflected Sound • Sound reflects from the object back to the transducer • The same directivity losses observed during transmit are also repeated during return of the echo 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  23. 23. Energy Conditioning by the Echo Sounder Receiver • After conversion from mechanical to electrical energy by the transducer, the signal is filtered and amplified by the echo sounder‟s receiver • A quantitative output value is provided 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  24. 24. A Review of Transmission Loss • Spherical spreading creates a reduction in intensity at any specified angle • Sound absorption reduces the signal intensity due to frictional losses • is the term for absorption, units are dB/m 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  25. 25. Effects of Transmission Loss • A string of identical targets is suspended below the transducer • Two-way transmission loss is 40 log R + 2 R 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  26. 26. Time-Varied Gain • With proper TVG applied, transmission loss is compensated for • Targets of the same size produce echoes of the same size independent of range 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  27. 27. Acoustic Energy Budget • 1+2 Sound level transmitted is called the Source Level • 3+5 Transmission loss is 40 log R + 2 R • 4 Target Reflectivity is Target Strength (TS) • 6 + 7 Signal conversion efficiency is Receive Sensitivity 2003-2009 Copyright BioSonics, Inc. All Rights Reserved
  28. 28. The Sonar Equation: input-output 2003-2009 Copyright BioSonics, Inc. All Rights Reserved

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