Fisheries Acoustics 101 - Principles

Principles of Hydroacoustics

By
BioSonics, Inc.

2003-2009 Copyright BioSonics, Inc. All Rights Reserved

Sound Generation and Reflection
(echo)

Dr. Schorsch Georg Wiora

SOund Navigation And Ranging


Scientific Echosounder


Breaking Down into Steps


Step 1: Timing and Transmission
• User Selects Pulse Rate
• User Selects Pulse Duration
• User Selects Transmission
Power
• Echosounder synchronizes
with high-precision clock


Characteristics
of Transmission
• A – Frequency
• B – Amplitude
• C – Pulse Duration


Units
• Frequency is in Hertz (cycles / second) or
typically kiloHertz (kHz, or 1000 Hertz)

• Amplitude is in Volts

• Pulse Duration is in Time (milliseconds)


Production
of Sound


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


Characteristics of Sound Field
• Maximum
sound intensity
is along the
acoustic (Z)
axis
• Main and side
lobes are
formed


Units
• Sound Pressure • Newton/square meter
(force per unit area) (Pascal)

• Power • Watts

• Intensity • Watts/square meter
(power per unit area)


The Decibel (dB)
• A logarithmically
compressed unit
• Defined as 10 times
the ratio between an
intensity and a
reference intensity


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.


Directivity Index of a Transducer
• Directivity Index
is the sound level
difference between
a directional and
non-directional
sound projector.


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


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.


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


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

Spherical Spreading
• Radiated power
is constant
• Power per unit
area decreases
with range due to
spreading
• Mathematical
decrease is given
as: 20 log R


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


Sound Reflection
• Sound traveling
through water reflects
off of objects that have
different density than
the surrounding water


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


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


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


Effects of Transmission Loss

• A string of identical targets is suspended below
the transducer
• Two-way transmission loss is 40 log R + 2 R


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


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


The Sonar Equation: input-output


Fisheries Acoustics 101 - Principles

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