Glossary of Terms

# | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A/D conversion

Analog to Digital (A/D) Conversion.  A device or process used to convert a continuous time (analog) signal into digital form.  Specified by the number of bits per sample and the number of samples per second (sampling rate).


Loss of acoustic signal strength due to conversion of acoustic energy into heat.  The dominant cause of absorption below 100 kHz is due to ionic relaxation of the magnesium sulfate (MgSO4) molecules in seawater.

absorption coefficient

The rate of Absorption per unit distance. Represented by the letter α (alpha), the absorption coefficient gives the attenuation of the sound level in dB/m during the transmission of the signal through water.  This attenuation can vary according to the seawater conditions (particularly salinity).  For example, the absorption in sea water at frequencies between 5 and 50 kHz has been found to be up to 30 times that in distilled water.  It also increases with the square of the frequency.

absorption loss

One-way linear loss of acoustic energy along the travel path of range R, Expressed as αR. Dependent on the interaction of acoustic frequency with temperature and salinity.


The transmission and reception of sound waves, and measurement of transmission travel times and arrival times of echoes.

acoustic axis

A line projected perpendicular to the transducer face and at center of the acoustic sound field, characterized by the highest intensity and maximum response.  The direction of maximum in-phase condition or maximum correlation.

acoustic beam

The cone-shaped volume of water which is filled with pulses of sound when a transducer is transmitting.

acoustic calibration

A method of measuring the absolute acoustic transmission and receiving characteristics of the echosounder and the directivity function of the transducer.

acoustic equipment

Devices for the generation or reception of acoustic waves.

acoustic equation

See SONAR Equation

acoustic intensity

Amount of acoustic power traveling through a unit area. Reference is a plane wave intensity having an rms pressure equal to 1 μPa (one micro pascal) (symbol I: unit dB/1μPa).

acoustic power

Acoustic energy per unit time.  Usually given in dB||1 watt.

acoustic signature

Particular reverberation of sound and reflections from a target which may provide target classification clues.

acoustic speed

Speed at which acoustic waves travel (symbol c; unit m/s).


The branch of the science of physics that deals with sound.


The theory of acoustic waves and their propagation.


A group of organisms of the same or different species distributed closely together.

ambient noise

The minimum level of noise present from biological and electro-mechanical sources.


Amount by which a signal is increased (unit dB).


The device which increases signal amplitude.


Size of an echo, in Volts or Digital Counts.

angular resolution

The amount of discrimination between targets separated in angle (unit degrees). This expresses, in degrees, the echosounder's precision in determining the angular position of targets.  Transducers with narrow beams have good angular resolution.


The cultivation of aquatic animals and plants, esp. fish, shellfish, and seaweed, in natural or controlled marine or freshwater environments; underwater agriculture.


Multi-element transducer.


The decrease in signal strength due to several factors, including geometric spreading, absorption, shading, and multiple scattering.  Total attenuation is typically a function of range, and must be accurately compensated for in quantitative processes.

b values

Obsolete values used to correct Time Varied Gain errors in older echosounders with analog circuitry.

back scattering

Amount of acoustic power scattered by a target in the direction of the transmitting transducer.

back scattering cross-section

A measure of the reflectivity of a target.  Target strength (TS) is equal to 10 times the base 10 logarithm of the backscattering cross section σbs.  σbs is related to the scattering cross section σ as σbs = σ/4π.  The scattering cross section is defined as: σ = 4πR2Ib/Ii where R = range to target; Ii = intensity at the midpoint of the incident sound pulse at the target; Ib = intensity at the midpoint of the backscattering sound pulse.

back scattering layer

Biomass layer which back-scatters acoustic power.

background noise

Sound or electrical energy in the acoustic system from sources other than the desired targets.


The bandwidth of an amplifier is given as a difference between the two frequencies (in Hz) where a drop of 3 dB occurs in the amplification on each side of the center frequency.  Bandwidth of an echosounder should be set to approximately 2/pulse length.  For example, a transmitted pulse of 1 msec. should be received through a 2 kHz bandwidth.

beam angle

Full included angle between the half-power  (-3 dB) points (symbol θ; unit degrees).

beam deflection

Angular amount by which a beam is refracted from its normal acoustic axis, typically by a thermocline.

beam half angle

Angle where the acoustic power is half that of the axis (symbol q /2 unit degrees).

beam overlap

Amount by which successive pings cover the same area.

beam pattern

See Directivity Pattern

beam pattern factor

The amount of signal intensity lost due to the angular distance of the target from the acoustic axis.

beam width (½ power)

See Beam Angle

beam width (effective)

The angular width of the underwater sound beam in degrees (dependent on many factors, including transducer electronics, echosounder transmit power, echosounder receiving sensitivity, and target strength of the organism being sampled.

behavioral studies

Research that focuses on the behvioral aspects (position, velocity, direction of travel, nearest neighbor distances), in contrast to traditional surveys that might focus on abundance estimation.  Fixed-location acoustic techniques are very good at providing behavioral data.


The study of the relationship between living organisms and sound.

biological background noise

Noise geneated by biological sources, such as cetatian vocalizations, drumming by groupers, and clicking by shrimp.


The amount of living matter in a given habitat, expressed either as the numbers or weight of organisms per unit area or per unit volume of habitat.

biomass density

Example units includes g/m3, kg/10,000 m3, # fish/10,000 m3, or # fish/10,000 m2.

BioSonics EcoSAVTM

BioSonics EcoSAVTMis the only commercially available hydroacoustic processing software specifically developed for submerged aquatic vegetation assessment.  Data analysis using EcoSAVTM software and provides a GIS-ready table of information presenting the location of SAV beds, the canopy height over the seabed, the density of the SAV bed and the bathymetry at each location.  BioSonics developed EcoSAVTM under a cooperative research and development agreement with the US Army Corps of Engineers.

BioSonics Visual AcquisitionTM

BioSonics Visual AcquisitionTM is a data collection software application which enables the user to interface with BioSonics DT-X Digital Scientific Echosounder Systems.  Visual AcquisitionTM allows complete control of the operational settings and functions of the surface unit (echosounder) and the transducer(s) attached to the system.  Primary access to the interface is provided through clearly marked function buttons.  Through the user’s computer, Visual AcquisitionTM is used to upload data logging parameters to the embedded processor (inside the surface unit), visualize the data in real-time as it is being collected, and store the data for post processing and playback.

BioSonics Visual AnalyzerTM

BioSonics Visual AnalyzerTM is a post-processing software application used primarily to estimate fish densities through echo integration.  Secondary functions include visually examining acoustic data files produced by BioSonics DT-X Series Echosounder Systems, and performing bathymetry and target strength analysis for single targets.  BioSonics Visual AnalyzerTM is organized around a high-resolution color echogram and oscilloscope in which BioSonics acoustic data files can be viewed. 

BioSonics VBTTM

BioSonics VBT™ is a post-processing software application used to determine depth, and to extract features for seabed classification.  It is designed for use with BioSonics DT-X Series Echosounder Systems.  The output files of the BioSonics VBT™ Seabed Classifier are produced in ASCII format in geographical context (Latitude and Longitude).  BioSonics VBT™ can also be run concurrently with navigation software or with GIS applications to produce thematic charts of bathymetry and seabed classification.

bottom discrimination

Determining the nature of the bottom.

bottom echo

Acoustic pulse reflected from the bottom substrate back toward the transducer.

bottom lock

A device which "locks" the recording or processing range of the display or processor relative to the bottom, instead of the surface.  The bottom signal forms a reference for echoes just above it (also seabed lock).

bottom noise

Noise generated by tidal flow.

bottom tracking

A special algorithm that predicts the location of the bottom based on previous bottom detections.  Bottom tracking is used to terminate processing of the acoustic return just prior to the bottom pulse.  Older analog designs simulated this function with an electronic circuit.

bottom window

User-selectable range window centered around the range of the leading edge of the bottom echo.  The range window is used in the bottom tracking algorithm.

cal tone

In obsolete analog designs, a cal tone represents a signal injected into the input stage of the receiver to facilitate documenting TVG performance and analog recordings.


Scientific measurement of the transmitting and receiving performance of an acoustic system, including transducer directivity measurements.  These measurements allow verification that acoustic performance of the echosounder is unchanging over use and time.

calibration equipment

Equipment used to determine the acoustic performance of an echosounder, including signal generators, hydrophones, standard targets,  oscilloscopes, voltmeters, plotters, etc.


Production of voids in the water due to negative pressure.  Produced by over-driving a transducer with excessive power, also produced by boat propellers.


Restriction of acoustic waves by boundaries.


A sonar beam that is cone-shaped.

dead zone

Volume of the transducer beam, usually close to the seabed, where echoes from the sea floor overlap echoes from near-bottom targets.  Targets cannot generally be detected in this zone.


A logarithmic system for expressing the wide range of values in the sonar equation.  Intensity level in decibels (dB) is defined as 10 log (I1/Ist), where I1 is the intensity of sound at a given point in an acoustic field, and Ist is a standard or reference intensity. For power, a 3 dB change corresponds to a factor of 2. For voltage gain (voltage squared is proportional to power), a 6 dB change corresponds to a factor of 2, since voltage level in dB is 20 log (V1/Vst).

deep scattering layer

Layer of small fish and invertebrates in the deep ocean which undergoes diel vertical migrations and shows up as a 'fake bottom' echo on echograms.  See back scattering layer.

default value

A value that an algorithm or microprocessor will unless the user enters a superseding value.


Process of extracting information from a signal.

depth finder

Simple hydroacoustic device for determining water depth; often not suitable for fisheries research.

depth interval

Selected interval between two depths, also known as a range window or depth slice (unit meters).

depth range

The total depth indicated on the display (unit meters).

depth recorder

Device which indicates and records the depth of acoustic targets and the seabed.

detected signal

The positive going envelope of a signal with carrier frequency filtered out.

detection threshold

For filters, the signal power in the receiver bandwidth relative to the noise power in a 1 Hz band which permits the detection of a target against specified criteria.  For signal processing, the user-selected level above which signals are processed (unit: dB).

digital signal cable

A digital signal cable is used to connect the echosounder to the transducer.  Digital signal cables eliminate cable loss of received signal, as well as coupled and induced noise.  Digital signal cables are waterproof, rugged and shielded to reduce noise.

digital transducer

Acoustic transducers operate in the following way:  An electric signal is delivered to a piezoelectric ceramic where the energy is transformed into a pressure pulse and transmitted out into the underwater environment. Part the energy that hits any target present in the acoustic beam is reflected back to the ceramic and transformed back into electricity. As this electric signal travels up to the surface unit (often over hundreds of meters) there is signal loss, and signal distortion introduced by environmental sources. In the mid 1990’s BioSonics again revolutionized the scientific hydroacoustic industry by introducing the Digital Transducer. All receive and transmit signal are digitized within the transducer itself, reducing the analog portion of the system from hundreds of meters to only a few centimeters. The resulting echosounder system introduced as the DT Series in 1995 featured and extremely low (industry leading) noise floor, drastically increasing the signal to noise capability of the system. Targets that were once invisible to the system could now be detected. An additional feature of a digital transducer system is that each transducer is treated as a node on a digital network, much like a computer network. Multiple nodes can be individually addressed and be networked from a single digital signal cable.

digital sampling

See A/D Conversion

directed net fishing

Use of hull, towed, and especially net mounted transducers to direct trawls to proper location and depth to maximize catch.

directivity pattern

A linear or polar plot of relative sensitivity or efficiency of a transducer in transmitting or receiving acoustic signals as a function of angle.  Highest sensitivity of 0 dB is on axis, and sensitivity falls off (directivity function becomes negative) as the angle from the axis increases.


Acoustic echoes received by a transducer from the direct transmission signal of that transducer. Direct-path echoes always arrive first and usually have a stronger signal strength than multi-path echoes.

display unit

For the display of signals and other information relating to the echosounder.


A spatial or temporal array of living organisms, rooted vegetation, and other items measured by the echosounder.

Doppler effect

The alteration of apparent frequency when the sound source is moving relative to the observer, or when the target is moving relative to a transducer.  The frequency shift (Δf) in Hz is given by:  Δf = f + (fv/c) where f = the frequency of the transmitted pulse in Hz, v = the relative speed between the acoustic source and a receiving hydrophone in m/sec, c = the velocity of sound in sea water in m/sec, and f = the frequency shift in Hz.

down-scan sonar

A downward-looking transducer.


Multi-element transducer from which two concentric beams of the same frequency but different beam widths are formed. By comparing target amplitudes from the two beams, the angular distance from the acoustic axis can be estimated.

dynamic range

The range of signals that can be simultaneously and instantaneously processed without clipping large signals and losing small targets in the noise. Higher values are beneficial.


Sound energy transmitted by the echosounder and reflected by a target of density differing from the medium in which the sound is traveling.

echo integration

The processing technique that determines the average squared echosounder output voltage for selected range intervals and average times.  The integrator output is proportional to fish density or biomass (see Echo Integrator Scaling Constant).

echo integrator

Algorithm or device to process and add the acoustic intensities from selected depth intervals.

echo integrator scaling constant

A value that contains echosounder calibration constants, transducer and sound velocity parameters, duration of transmitted pulse, and mean backscattering cross section estimates.  The output of the echo integrator is density if the back scattering cross section is used, and is biomass if the back scattering cross section per unit weight of fish is used.

echo level

Acoustic intensity at the receiving transducer (symbol: EL; unit dB).

echo ranging

Finding the distance to a target by measuring the time from transmission to echo.

echo sounding

Finding the depth of a target by measuring the time from transmission to echo.


A (paper or electronic) display of a time series of received echo pulses.  The Y axis is range or depth, while the X axis represents a time series (minutes).


Electronic instrument comprised of an acoustic transmitter and echo receiver which, when connected to an underwater transducer, detects underwater targets.


Electrically Erasable Programmable Read-Only Memory. An integrated circuit or “chip” that can have programming instructions dynamically loaded.  Systems with EEPROMS can easily be upgraded when software changes or new algorithms are introduced.

electro-acoustical efficiency

The ability of a transducer to convert electrical energy into acoustic energy and acoustic energy into electrical energy.  It can be expressed in dB [efficiency in dB = 10 log (% efficiency)].


Material which changes its dimensions under the influence of an electric field.


A sonar beam whose cross section takes the form of an ellipse.

ensonified volume

Volume of water into which acoustic signals are directed to obtain biomass information.  The volume is confined within a pulse duration by the angular edges of the beam.

equivalent beam angle

The Equivalent Beam Angle, or eba, represents a solid angle that would produce the same echo integral as a real transducer when the targets are randomly distributed throughout the beam.  Directivities inside the eba are 1, and are 0 outside.  The equivalent beam angle is represented by the symbol Ψ, and is expressed in Steradians.  It may also be expressed as EBA in logarithmic units, where EBA = 10 (log10 Ψ).

event mark

Creation of a label, symbol, or line initiated by the operator for the purpose of marking the chart when significant events are observed.

false bottom

An apparent bottom echo that in reality represents the arrival time of the bottom echo from the previous ping.  It often appears in mid-water and appears as a biological layer or target.


Distance beyond where the initial fluctuations of intensity are damped out to form a spherically advancing wave front when transmitted by a transducer.

figure of merit

Comparative performance of acoustic systems based on maximum allowable two-way transmission loss related to a target strength of 0 dB (symbol: FM; unit: dB).

fish abundance

The quantity of fish in a population.

fish detection

Location of fish by acoustic means.

fish traces

A target shape formed over time by spatial correlation of echoes from a target passing through the echosounder beam.

fisheries acoustics

Acoustics applied to fisheries research and management.

fixed-location hydroacoustics

A hydroacoustic survey technique where the transducer is attached to a solid object, with its aiming angle set and stable.  In contrast to a mobile survey, the fixed-location survey samples fish as they pass through the acoustic beam.

fm slide/chirp

A technique that varies the frequency within the transmitted pulse, often improving the range resolution and signal-to-noise performance in hydroacoustic assessment systems.


Volume of water clear of boundaries.


The number of pressure maxima (or minima) passing a fixed point in one second.  Units are Hertz (Hz) or kilohertz (kHz).  Deep-water marine echosounders typically operate at frequencies of 20-70 kHz.  Riverine sonar systems usually operate in the 100-500 kHz range. Systems for studying plankton operate in the 400-2000 kHz range.

frequency counter

A device to count the number of complete cycles to pass a given point in a given time.

frequency response

The extent to which a system is sensitive to a range of frequencies (unit Hz).


Amount by which the amplitude (size) of a signal is increased (unit dB).

geometric cross-section

Projected area of a target in the direction of esonification.

geometric spreading

The increase in the ensonified cross-sectional area with distance traveled by the sound wave.  This process can be visualized by watching the rings spread when a rock is thrown into still water.

geometric zone

Where the relationship of wavelength λ to the dimensions of a fish enables TS to be deduced from the laws of geometric optics.

geometric loss

Dispersal of energy of an acoustic wave due to the spreading effect within the geometry of the beam.

geometrical spreading

The increase in the ensonified cross-sectional area with distance traveled by the sound waves.  Usually called Spherical Spreading.


Global Positioning System. GPS receivers track satellites and provide current data on time, Latitude and Longitude.

gps position

A location defined by a Global Positioning System.

ground truth

Use of trawls, gill-nets, etc., to independently estimate biomass and provide species identification to hydroacoustic data.


A unit of frequency equivalent to one cycle per second.  It is also used to characterize the A/D sampling rate (samples per second). Kilohertz (kHz) is 1000 Hertz or 1000 cycles per second.

horizontal distribution

The frequency of occurrence or the natural range of aquatic life viewed along a distance traveled or within a survey area.


Hydroacoustics is the application of controlled sound energy in water to remotely obtain information about the physical characteristics of the water body, its bathymetry, or biotic populations.


An instrument used for listening to sound transmitted through water.


Ratio of generally complex quantities of pressure and particle velocity or of voltage and current at the same time and place.

incident intensity

Acoustic intensity impacting on a target.

incident sound

Sound which impinges on a target.


“In the original location”.  Used to describe fish targets in their free-swimming, undisturbed state.  Also refers to calibrations using standard spheres suspended in the water column.


To 'illuminate' by means of acoustic waves.

integrated layer

Layer of water, defined by upper and lower depths, in which density and/or biomass estimates of fish are provided.


The power (amount of energy per unit of time) crossing a unit area.


Having non-directional properties.


One thousand bytes, used to measure storage space for computer files.

kilohertz (kHz)

1,000 Hz

log book

Document in which all significant observations are recorded during data collection.  These include environmental, biological, and meteorological observations, as well as data collection and processing parameters.

low side lobe transducer

The theoretical maximum side lobe suppression for a transducer’s acoustic beam is approximately -17 dB.  A typical BioSonics transducer beam has side lobe suppression in the range of -25 to –30 dB.  This low side lobe capability allows the transducer to operate much more effectively in shallow complex environments, particularly when bottom and surface boundaries are a factor. Side lobe energy can reflect off of the boundaries and substantially increases noise, which minimizes the ability to detect targets at range.


Substance in which sound is traveling.


Standard unit of pressure often used in scientific sonar systems, equal to 1 millionth of a pascal. A pascal is a unit of pressure equal to the pressure resulting from a force of 1 newton, acting uniformly over an area of 1 square meter.

minimum recordable signal

Smallest amplitude (size) of signal which can be detected above the background noise.

mobile survey

A hydroacoustic survey conducted from a moving boat.


The process of impressing information on a signal (e.g. pulse).


Multiple acoustic echoes received from the same target due to reflections from the water surface or surrounding structures.   Multipath echoes always arrive at the transducer after the direct-path transmission, and are usually (but not always) weaker in signal strength compared to direct-path echoes.

multiple targets

More than one target within the pulse resolution volume.


The use of two or more transducers with a single X-Series Echosounder surface unit.  The BioSonics X-Series Echosounder supports multiplexing up to sixteen transducers.

near field

The distance from the transducer within which the sound pressure wave is not coherently formed, I.e, the inverse square law does not apply.  Quantitative measurements should not be made inside this range due to fluctuating intensity.


Unwanted signals that interfere with the signals to be quantified.  Sources include self (internally generated) noise, received noise radiated into the system through the transducer cable, flow noise from water passing across the transducer at high velocity, volume reverberation noise from unwanted particles, bubbles, or animals distributed throughout the sound field or from the sound field grazing a boundary, and false targets (such as rocks, debris, resident fish, etc.).

noise floor

The self-noise expressed as an on-axis target strength, i.e. the smallest target strength of an organism that can be detected at the closest range.

noise level

Number of dB by which noise is above or below a given reference.

noise limited

Distance at which detection is no longer possible because the signal is obscured by noise.

noise reduction

Number of dB by which noise is reduced from a reference.

noise spectrum level

Noise power for one cycle of energy, (symbol: SPL; unit: dB/1 m Pa/Hz).


Electronic circuit for generating controlled oscillations.


An instrument for displaying and measuring signals (voltages).


Pinging from a single transducer while alternating between two or more configurations.  For example:  Parameter-plexing three configurations (A,B,C) on a single transducer would yield the following ping sequence:  A | B | C | A | B | C | ...

pc card

A standardized input/output format for plug in memory cards designed for laptop computers as defined by the Personal Computer Memory Card International Association (PCMCIA).  The cards themselves are also referred to as PC Cards.


Personal Computer Memory Card International Association.  Commonly called PC cards, these credit card sized units plug into slots in computers to provide connectivity to a wide variety of other devices (modems, Ethernet, SCSI, etc).  Some modern acoustic systems connect to the computer via the PCMCIA slot.

performance test

Measurements to establish the standard to which a system is working.


Time required for a single oscillation of a sine wave.  The period equals 1/f where f is the frequency.  Presented in msecs or seconds.


The time relationship of one sine wave to another (symbol f, units in degrees).

piezoelectric effect

Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential in response to applied mechanical stress.  The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied).


An informal name for the transmitted acoustic pulse and its associated reception.


Amount of energy per second.  A typical unit of acoustic power is the watt.


Boosts signals before the main amplifier.


The effect which occurs when a Force is applied on a surface; its measure is the force exerted per unit area. Common unit of pressure is micropascal (µPa) equal to micro newton per square meter.


Transmitter of acoustic power.


Ability of acoustic signals to progress outward in a medium.


Sound in water. A momentary, sudden fluctuation in an electrical quantity, as in voltage or current.

pulse duration

Length of time a pulse of a given frequency is emitted by the transducer.

pulse length

The distance a pulse extends (unit meters).

pulse rate

Number of pulses transmitted per unit time.  Since traditional echosounders must receive echoes before transmitting the next pulse, pulse rate is limited by range of ensonification.

pulse rate

See Pulse Repetition Rate

pulse repetition rate

The rate of repetitive acoustic pulses, of a given duration and frequency, emitted by a transducer.  Typically referred to as the "ping rate."

pulse volume

The volume contained within the included angle of the beam for the extent of one pulse length at a given range (unit m3).

pulse width

The duration in time (usually milliseconds) of the transmitted pulse.  Also called pulse duration or pulse length.


Distance from the transducer face to the target.  Often used synonymously with depth in vertical sounding.  Typically expressed in meters, but often expressed in terms of the time interval between transmission and reception of the echo (in msec).

range resolution

The minimum range separation between distinguishable targets. For a monotone pulse acoustic system, the range resolution is equal to cτ/2, where c is the velocity of sound through the medium and τ is the pulse length transmitted by the echosounder.

rayleigh scattering zone

The region inside which a target scattering cross-section varies inversely with the fourth power of wavelength, fish length much greater than l.


Of or relating to computer systems that update information at the same rate as they receive data, enabling them to direct or control a process such as an automatic pilot.

real-time tracking

Any auto-tracking (2D and/or 3D tracking) procedure used during the data acquisition phase of an acoustic system.

receive sensitivity

Number of dB volts or dB counts (relative to 1 volt or 1 count) coming out of the receiver for a given acoustic pressure at the transducer face.


The component of an echosounder that receives and conditions the signals from the transducer.


Exhibited by mutually interchangeable transducers.


The "bouncing" of sound off a target, due to the differences in density between medium and target and target orientation.


Deflection of sound from a straight path, e.g., when passing through a thermo cline at an angle.


When a circuit, or a target, is excited to different modes of vibration by a particular frequency.

resonant frequency

The natural frequency of operation for a transducer or circuit (symbol f: unit Hz).


Acoustic interference caused by scattering of objects other than those of interest.  The main sources of reverberation in fisheries acoustics are the bottom, surface, other boundaries, air bubbles, plankton, and particles in the water.

rf (radio frequency) output

Signal out of the sounder that has not been shifted to a lower frequency.

root mean square (rms)

The square root of the averaged sum of all squared values of a waveform (symbol RMS).

sample period

A sequence is divided up into a number of intervals referred to individually as a sample period.  Sample periods are linear by definition and are always associated with the sequence in which they are contained. For example, the sample periods defined for sequence one of a sampling plan would be labeled as "S1P1" for period one, "S1P2" for period two and so on. Whenever more than one sampling period is defined for a sequence, the system is in fast multiplexing mode.  The system will execute the sample periods within a sequence in the order in which they have been defined, starting with the first and ending with the last defined sample period for that particular sequence. After the last sample period has been completed, the system will then begin with the first sample period for that sequence once again.

sampling cross section

The cross-sectional area sampled by the acoustic beam.

sampling plan 

Data collection is performed by defining a sampling plan within the system based on start times and duration, as well as ping rates. The sampling plan can be set to sample within hourly intervals, or by a set of defined intervals which are repeated. Defining a sampling plan must be the first step taken before any data are acquired by the system.

sampling volume

The volume of water ensonified by the acoustic beam.

scientific echosounder

An echosounder designed for quantitave measurement, requiring accurate measures of Source Level, Receive Sensitivity, and Directivity, as well as having circuits designed for high dynamic range, stability, and linearity.

second echo

When the echo from a target or seabed is reflected back from the surface and causes a second echo from the target or seabed to be received.

sector scanning

The use of a multiple transducer array, with each transducer ensonifying only a portion of the total area of interest, to increase overall transducer coverage.


Degree of response to an acoustic or electrical signal.


A specific time event within the application when data is processed. A sampling plan is comprised of defined time intervals referred to individually as a sequence. These sequences are linear by definition and are labeled as "S1" for sequence one, "S2" for sequence two, and so on.  The system will execute a particular sequence for its defined duration. When the duration for a sequence has been completed, the system switches to the next defined sequence in the sampling plan. After the last defined sequence has been completed, the system will then execute the initial sequence it started and will continuously repeat the procedure.


The effect caused by one target lying in the 'shadow' of another.

side lobe

All transmit\receive beams of a transducer except the main beam.

side-scan sonar

Side-looking transducer, usually used by commercial fisherman to spot distant fish schools.

signal encoding

Encoding an identifying code within the transmitted pulse.

signal generator

Instrument which produces electrical signals at controlled frequencies and amplitudes.

signal strength

Intensity of an acoustic wave or amplitude of an electrical wave.

signal-to-noise ratio (snr)

The amount by which a signal exceeds noise, or is below noise (symbol SNR, unit dB).

single beam sonar

An echosounder designed to measure range to a target, using acoustic energy to generate an echo.  Sound is transmitted (through a medium, such as water) and the SONAR measures the time energy takes to travel to and return from the target.


The simplest use of an X-Series Echosounder: continuous sampling on one transducer attached to one surface unit.

solid angle

The solid angle, Ω, is the angle that an object subtends at a point. It is a measure of how big that object appears to an observer at that point. For instance, a small object nearby could subtend the same solid angle as a large object far away. The solid angle is proportional to the surface area, S, of a projection of that object onto a sphere centered at that point, divided by the square of the sphere's radius, R. (Symbolically, Ω = k S/R2, where k is the proportionality constant.) A solid angle is related to the surface area of a sphere in the same way an ordinary angle is related to the circumference of a circle.


SOund NAvigation and Ranging systems, or SONAR, is a technique that uses sound propagation under water to navigate, communicate or to detect.

sonar equation

The equalities from which the performance of an acoustic system can be calculated, (units in dB) Now called acoustic equation.

sound intensity

Power of sound waves, measured in ergs/cm3/s.

sound radiation

Spreading of sound equally in all directions.

sound velocity

Velocity of sound through a medium; in water, about 1500 m/s, and dependent upon temperature, salinity and depth.  Usually expressed by the symbol 'c'.

sound wave

Pressure maxima and minima moving within a compressible medium.

source level

The transmitted sound pressure in dB relative to a 1 μPa plane wave measured 1 meter from the face of the transducer.

spherical spreading loss

Describes the decrease in sound intensity as the beam spreads (i.e., decreases with range).

split beam sonar

An echosounder designed to directly measure target strength.  The position of a target in the sound field is calculated by accurately measuring the differences in an echo’s arrival time to individual elements in the transducer.

standard target

A target with a known acoustic size used for the calibration of acoustic systems.  Standard targets are designed to be omni-directional, and have stable reflective properties with depth and temperature.  Examples include tungsten carbide and copper spheres.

stationary transducer

Transducer fixed to a buoy or to the bottom looking upward, sideways or downward.


The steradian (symbol: sr) is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane.  The steradian is dimensionless.

swimbladder resonance

Characteristic "ringing" of air-filled swim bladders when ensonified by a hydroacoustic system.

swimming path

Track or travel of an organism in water.


The keeping together in time of recorders: units comprising an echo-sounder, a sonar, or electrical waveforms.


An object to be detected, located or measured.

target strength

A measure of the reflecting power of a target expressed in decibels. The ratio of the acoustic intensity IR reflected from a fish and measured 1 m away, to the incident acoustic intensity Ii, 10 log (IR/Ii). The technical definition of target strength is based on a perfectly reflecting sphere of 2 meters radius having a target strength of 0 dB.

thermal noise

The ultimate limit to detection due to molecular activity, mainly evident above 100 kHz.


Temperature discontinuity where organisms often collect, thus making it acoustically 'visible'.


The least value of a voltage, pressure, current, or other quantity that produces the minimum detectable response in an instrument or system.  A chosen signal level below which no signals are processed.

time varied gain (tvg)

An instantaneous amplification with range (time) applied to a received ping to compensate for transmission losses, including spreading loss and absorption.  The TVG may be applied in hardware in analog systems and in software in digital systems.


The time reference to which signals on a paper recorder, or an oscilloscope, are displayed.

time-varied-gain (tvg)

A successive increase in the amplification of the receiver with range (time) during the reception period of each sounding.  For single targets, 40 log(R) compensates for geometric spreading loss and absorption.  For multiple targets, such as produced by fish schools, a 20 log(R) TVG will provide an output that is a function of the density of the scattering and not a function of range.  Analysis of surfaces (sea bottom classification) often uses a 30 log(R) TVG.


Also referred to as tow body, towing vehicle or BioFin. Hydrodynamically shaped body into which a transducer may be fitted for towing.

track fish

To observe or follow the course of a fish in 2D or 3D.


A transducer converts electrical energy into acoustic energy in transmission mode and converts acoustic energy to electrical energy during receiving.  The efficiency in this conversion process is a function of angle is called directivity.  Transducers for most scientific sounders are very efficient through a narrow frequency range.

transducer beamwidth

Angular width of the beam measured at the half-power point (i.e., at -3 dB points down).

transducer impedance

An electrical characteristic of the transducer that must be matched to the cable and echosounder to have maximum efficiency of operation.

transducer, electrostrictive

Transducers with elements made of ceramic materials such as barium titanate or lead zincronate.  They expand and contract according to the electric field.

transducer, magnetostrictive

Transducer which consists of nickel plates which expand and contract according to the magnetic field induced in it.

transmission locked

Display of signals related to time of transmission.

transmission loss

Sum of absorption loss and geometric loss (symbol TL, unit dB).


The component of the echosounder that produces the high-energy electrical pulse at the required frequency to be converted to acoustic energy by the transducer.

travel times

Referring to time calculations that measure a trajectory or a distance (points of reference along a waterway) for a particular fish track.

trigger interval

Amount of time between sound transmissions.  A pulse repetition rate of 2 pulses/sec corresponds to a trigger interval of 0.5 sec/pulse.

trigger pulse

A pulse generated either by an external device or internally by hardware or software, occurring at the time of transmission, and used to synchronize processes in all system components.

tungsten carbide

A chemical compound that is durable, and produces stable reflective characteristics (form factor) for standard calibration spheres.

volt meter

Device for the measurement of voltages, either arising from direct or alternating current.


The distance traveled by a sinusoidal wave of the acoustic frequency in a time equal to the period of the sine wave.  The wavelength, represented by λ, is important in determining the directivity of the transducers and the scattering characteristics of various sized targets.


Spatial frequency of a propagating sine wave acoustic signal.

white line

Effect of a circuit which cuts off the seabed echo recording shortly after it appears, then allows it to resume after a fixed period.

zero line

Base line of a chart recorder (echosounder, depth sounder) representing zero time (zero depth or depth of the transducer).


source level


target strength


back scattering cross section


receive sensitivity


transmission loss


time varied gain










foot per second




hertz or cycle per second


















nautical mile








Digital Count


watt second per square centimeter































Back to Top
GSA Advantage Program

BioSonics is a GSA Contractor. GSA Advantage is the most reliable and proven shopping source for Federal purchasers worldwide. Contact us for details.


Our Document Library is a great source of information on a wide range of hydroacoustic assessment topics.
read more


Our Hydroacoustic Assessment Workshops provide training in the latest techniques for hydroacoustic data collection and processing.
read more