Hydroacoustic Technology, Inc.
  Hydroacoustic Technology, Inc.    
    This glossary provides the most common acoustic and telemetry terminology used throughout this website and includes brief definitions. You can scroll down to read through the definitions alphabetically, or click on the first letter of the word below to go to that alpha grouping.    
# | 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
2D-echo A general term for an acoustic tag echo which has been assigned a Tag ID, either through a 2D-tracking procedure or from manual marking.  
2D-tracking An automated procedure for extracting tag data from the ambient noise on an individual hydrophone basis.  
3D-echo See description for position echoes.  
3D-positioning A process by which tracked acoustic tag data is used to create a 3D position echo. In order to calculate a 3D position, data must be obtained from at least four hydrophones which have their exact locations (x,y, and z) known.  
3D-tracking See 3D-positioning.  
8 kHz output The acoustic output from an echo sounder's receiver shifted from the operating frequency to 8000 Hz (cycles/sec).  
A byte An eight bit (0 and 1) digital number.  
A calibration Method of defining and setting characteristics of the electronic/mechanical equipment which allows repeatability of results.  Very important in quantitative hydroacoustic work.  
A constant A value that contains echo sounder calibration constants, transducer and sound velocity parameters, duration of transmitted pulse, and mean backscattering cross section estimates.  It is used to scale the output of the integrator to obtain biomass or density estimates.  
A device bottom tracking A special circuit or 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.  
A/D Analog to digital converter.  A device used to convert a continuous time (analog) signal into a digital form.  Specified by the number of bits per sample and the number of samples per second (sampling rate).  
absorption coefficient The coefficient a, denoting the power loss due to absorption (symbol a). This gives the attenuation of the sound level in dB/m during the transmission of the signal through sea water.  This attenuation can vary according to the sea water 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 A temperature and frequency dependent power loss to acoustic waves, linear with distance (symbol a R: unit dB).  
acoustic The transmission of sound waves and measuring the time it takes for their echo to return after reaching an object. Having to do with the science of sound (see also sonar).  
acoustic axis The center axis of the acoustic beam.  The direction of highest acoustic intensity.  Region of maximum response, normally perpendicular to the face of a transducer.  
acoustic calibration Measuring the performance of an acoustic system to a specified standard (unit dB).  
acoustic equation See sonar equation.  
acoustic equipment Devices for the generation or reception of acoustic waves.  
acoustic intensity Amount of acoustic power through unit area. Reference is a plane wave intensity having an rms pressure equal to 1 m Pa (one micro pascal) (symbol I: unit dB/1 m Pa).  
acoustic power Acoustic energy per unit time.  Usually given in dB||1 watt.  
acoustic signature Particular reverberation of sound and reflections from a target (usually with swim bladder), which typifies that target and may someday be used for species identification. up
acoustic speed Speed at which acoustic waves travel (symbol c; unit m/s).  
acoustic tag echo Repetition of sound produced by the reflection of sound waves returning from an obstruction.  
acoustics The theory of acoustic waves and their propagation.  
AcousticTag Acoustic tag software that can collect data and post-process data.  During data collection, it communicates with the Acoustic Tag Receiver (ATR), which can receive up to 16 separate channels of acoustic data. One channel is assigned to each hydrophone. The received signals are synchronized in order to determine time of arrival for each detected pulse (see conceptual ping illustration). The arrival times of the transmitted signal pulses are used to determine the 3D location of the acoustic tag passing through the study area.   
aggregation A group of organisms of the same or different species living closely together.  
airbladder resonance See swimbladder resonance.  
amplification Amount by which a signal is increased, see gain (unit dB).  
amplifier The device which increases signal size.  
amplitude Size of a signal.  
angular resolution The amount of discrimination between targets separated in angle (unit degrees). This expresses, in degrees, the echo sounder's ability to distinguish between targets at different bearings but at the same distance from the transducer.  Transducers with narrow beams have good angular resolution.  
aquaculture The cultivation of aquatic animals and plants, esp. fish, shellfish, and seaweed, in natural or controlled marine or freshwater environments; underwater agriculture. up
array Multi-element transducer.  
ASCI-based text files General formatting for database files.  
attenuation Loss or reduction of acoustic signal strength due to spherical spreading and absorption of the waves (unit dB/km) or  internal friction within a water body.  Attenuation is greater for salt water systems.  
automatic gain control Amplification varied in proportion to a received signal to reduce output voltage variation.  
auto-tracking An automated procedure for extracting tag data from the ambient noise. Several methods and algorithms are employed within the auto-tracking phase and can be adjusted via tracking settings.  
B values A user-selected input for the echo integrator.  A different B value may be calculated for each discrete depth interval selected by the operator to compensate for errors in the TVG.  
back scattering Amount of acoustic power scattered by a target into the direction of the transmitting transducer.  
back scattering layer Biomass layer which back-scatters acoustic power.  
back scattering cross-section A measure of the reflectivity of a target.  Target strength (TS) is equal to 10 log10(bs/4) of the backscattering cross section bs, which is defined by the relationship: bs = 4R2Ib/Ii where R = range to target; Ii = intensity at the midpoint of the incident wave at the target; Ib = intensity at the midpoint of the backscattering pulse.  
bandwidth The bandwidth of an amplifier is given as a difference between the two frequencies (in Hz) where a drop of dB occurs in the amplification of each side of the center frequency.  Bandwidth of a sounder should be set to approximately 2/pulse length.  For example, 1 msec pulse should have a 2 kHz bandwidth.  The amount frequencies extend on either side of the nominal acoustic frequency (symbol BW; unit Hz). up
beam angle Full included angle between the half-power points (symbol q ; unit degrees).  
beam deflection Amount by which a beam is moved in angle from its normal acoustic axis.  
beam dual See dual beam.  
beam half angle Angle where the acoustic power is half that of the axis (symbol q /2 unit degrees).  
beam narrow When full angle is less than 10°.  
beam overlap Amount by which successive pings cover the same area.  
beam pattern Two-dimensional pattern showing the relative response of a beam.  The beam pattern is shown as a polar plot of the sensitivity of the transducer against direction.  
beam pattern factor In active hydroacoustics, the loss in signal intensity due to the target’s position in the beam.  If the target is located on the central axis of the transducer (in both X and Y planes), then the beam pattern factor will be zero.  As the target moves further away from the transducer axis, the beam pattern factor becomes more negative, at a rate that is dependent on the transducer beam width.  
beam width A nominal value in degrees describing the full angular width of the acoustic sound cone, usually determined by the angle at which the transducer directivity pattern is 3 dB down for one way (transmitting or receiving).  Transducers are usually classified as wide (perhaps 15) or narrow (perhaps 4) beam units.  
behavioral studies Scientific surveys, population modeling, and fishery management strategies are all dependent upon a fundamental understanding of fish behavior.  Behavioral studies provide critical information needed to improve predictions on population abundance, distribution and survival, and to conserve populations of economically significant resource species and their habitats.  
biological background noise Noise due to biological sources, usually much lower in frequency than hydroacoustic signals and not a problem in signal interpretation.  
biomass The amount of living matter in a given habitat, expressed either as the weight of organisms per unit area or as the volume of organisms per unit volume of habitat. up
biomass density Measured as g/m3, or kg/10,000 m3, or # fish/10,000 m3, for example.  
blocking When receiving function is stopped by a very large signal, as in 'white line'.  
body towed See towed-body.  
bookmarking To mark or track a particular location or track within a data set in an acoustic tag tracking software application.  
bottom detect A circuit that generates a square pulse to inform a processor of the bottom position.  A user adjustable threshold usually determines the signal level at which the bottom detect circuit creates it pulse.  
bottom discrimination Determining the nature of the bottom.  
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 pulse Electrical pulse produced from bottom echo.  
bottom tracking A special circuit or 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.  
bottom window User-selectable range window centered around the range of the leading edge of the bottom detect pulse.  The range window is used in the bottom tracking algorithm.  
cal tone A known level signal that is injected into the echo sounder receiving electronics.  For recorded data, the cal tone signal can be recorded and later used to adjust playback level. up
calibration Method of defining and setting characteristics of the electronic/mechanical equipment which allows repeatability of results.  Measuring or adjusting the performance of a system to a specified standard is very important in quantitative hydroacoustic work.  
calibration equipment Signal generators, hydrophones, standard targets, projectors, oscilloscopes, voltmeters, etc.  
cavitation Production of voids in the water due to negative pressure.  
channeling Restriction of acoustic waves by boundaries.  
chart recorder Equipment used for data acquisition.  
conical-beam A sonar beam that is cone-shaped.  
dead zone Volume of the transducer beam, usually close to the seabed, where targets cannot be detected.  
decibel 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.  
default value A value that a microprocessor like a fish tracker will accept unless the user enters a replacement value. up
demodulation Process of extracting information from a signal.  
DEP Data collection software for hydroacoustic studies.  Provides the interface for properly configuring the system and visually monitoring real-time operation of a hydroacoustic system during data collection.  
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 gate (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.  
depth sounder See depth finder.  
detected signal The positive going envelope of a signal with carrier frequency filtered out.  
detection threshold 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 (unit: dB).  
digital sampling Electronic sampling or storing X and Y points of volts over time at a rate of 48,000 bits per second.  Allows hydroacoustic data to be internally processed in fine-scale detail.  
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 diagram showing the angular response of a transducer.  Pattern of sensitivity or efficiency of a transducer in transmitting and receiving hydroacoustic signals.  Best efficiency is on-axis, usually falling off rapidly off-axis. up
direct-path Acoustic echoes received by a hydrophone from the direct transmission signal of a tag. 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 echo sounder.  
distributions A spatial or temporal array of aquatic life.  
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 in Hz is given by: f = (2v/c)f where f = the frequency of the transmitted pulse in Hz, v = the relative speed between the acoustic source and the target in m/sec, c = the velocity of sound in sea water in m/sec, and f = the frequency shift in Hz.  
double-pulse An HTI Model 795 Acoustic Tag which produces a primary transmit signal followed by a secondary transmit signal.  The primary transmit signal is based on the tag's defined Period (i.e. ping rate) while the secondary transmit signal is based on the period and the defined subcode setting.  
down-scan sonar A downward-looking transducer.  
dual-beam Multi-element transducer from which two concentric beams of the same frequency but different beam widths are formed.  
dual-beam sonar Simultaneous use of wide and narrow beam transducers, allowing in-situ estimation of target strength.  
dynamic range The extent to which signals can be processed without distortion (unit dB).  
echo An acoustic wave reflected from a target of density differing from the medium in which the sound is traveling. up
echo integration The processing technique that determines the average squared echo sounder output voltage for selected range intervals and average times.  The integrator output is proportional to fish biomass.  
echo integrator Unit to process and add the acoustic intensities from selected depth intervals.  
echo level Acoustic intensity at the receiving transducer (symbol: EL; unit dB).  
echo paper dry Recording paper conductive with high voltage.  
echo paper moist Recording paper conductive with low voltage.  
echo ranging Finding the distance to a target by measuring the time from transmission to echo.  
echo sounder System comprising acoustic transmitter, echo receiver and display.  
echo sounding Finding the depth of a target by measuring the time from transmission to echo.  
echo trace Mark on a record caused by an echo.  
echogram Record of a sequence of echoes.  
EchoScape Hydroacoustic post-processing software that provides instant and automatic tracking of fish data on-screen.  Used to perform data analysis and display results (works with HTI’s split-beam
electro-acoustical efficiency The ability of a transducer to convert electrical energy into acoustic energy.  It can be expressed in dB [efficiency in dB = 10 log (% efficiency)]. up
electro-strictive Material which changes its dimensions under the influence of an electric field.  
elliptical-beam A sonar beam having the form of an ellipse.  
entrainment Trapping.  
equipment log sheet A table of equipment readings.  
equivalent angle The included angle of an 'ideal' beam, calculated from actual transducer characteristics.  
expanded dynamic range A technique to improve the range of tones on a paper record.  
expanded scale Display of a portion of range or depth at a size exceeding its basic scale.  
far-field Distance beyond where the initial fluctuations of intensity occur 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 target strength 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 dB (symbol: TS; unit: dB).  
fish traces Acoustic tracks or ‘traces’ of fish travel.  
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 move toward and pass through the acoustic beam. up
FM slide/chirp A technique for significantly improving the signal-to-noise performance in hydroacoustic assessment systems.  
free-field Volume of water clear of boundaries.  
frequency The number of oscillations a sinusoidal signal source makes each second.  Usually expressed in Hertz (Hz, cycles/sec) or kiloHertz (kHz, 1000 cycles/sec).  Hydroacoustic systems usually have frequencies in the range of 20-500 kHz.  
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).  
gain 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 isonification.  
geometric zone Where the relationship of wavelength l to the dimensions of a fish enables TS to be deduced from the laws of geometric optics.  
geometrical 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.  
ghost echo An echo falsely related to the depth scale.  
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. up
Hertz Frequency, defined as one-per-second, abbreviated as Hz.  
history (within tag programming sessions) A systematic account of any set of software steps in a sequential order in time.  
horizontal distribution The frequency of occurrence or the natural range of aquatic life viewed in a parallel or horizontal perspective within a water column.  
hydroacoustics The study or use of sound in water to remotely obtain information about the physical characteristics of the water body, its bathymetry, or biotic populations.  
hydrophone Device to receive acoustic waves and convert them to electrical signals.  
impedance 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 falling on a target.  
incident sound Sound which impinges on a target.  
insonified volume Volume of water into which acoustic signals are directed to obtain biomass information.  
insonify To 'illuminate' by means of acoustic waves.  
integrated layer Layer of water, defined by upper and lower depths, on which integrated biomass estimates or fish counts are based.  
integrator The computerized integration of fish echoes to estimate biomass.  


The acoustic power per unit area of a propagating acoustic wave. 

interval mark

A voltage pulse created by the integrator for the purpose of marking the chart at each printed output.

isotropic Having non-directional properties.  
kilohertz (kHz) 1,000 Hz  
layer See scattering layer.  
live fish calibration Overall calibration of an echo-sounder/echo-integrator system by insonifying captive fish and measuring the received intensity.  
magneto-strictive Material which changes its dimensions under the influence of a magnetic field.  
marked A raw acoustic tag echo which has been assigned a specific tag identification (i.e. period and subcode).   


The process of assigning a raw acoustic tag echo a specific Tag Identification (i.e. period and subcode).  Marking raw echoes can be either a manual or an automated (i.e. auto-tracked) process. up
MarkTags A post-processing program, the primary acquisition and 3D analysis software for Model 290/291/295 Acoustic Tag acquisition systems.  MarkTags selects and separates all the data contained in the raw data files and makes marked data files. These files now contain Tracked Acoustic Tag Echoes (TAT files).  The file can be used to produce summary information (i.e. survival studies, travel times, horizontal distribution) or can be further processed in AcousticTag to produce 3D analysis results.    
medium Substance in which sound is traveling.  
minimum recordable signal Smallest amplitude (size) of signal which can be seen on the display.  
mobile survey A hydroacoustic survey conducted from a moving boat.  
modulation The process of impressing information on a signal (e.g. pulse).  
multi-path Acoustic echoes received from the same tag but from different sources due to reflections from the water surface or surrounding structures.   Multipath echoes always arrive at the hydrophone 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 beam of the transducer.  

The term multiplexing refers to switching from one set of data acquisition parameters or sample period to another sample period, within a sampling plan. Part of defining a sample period is the transducer calibration and DES settings. In defining the sample period's calibration for a particular transducer, a multiplex channel, or port must be defined as to where the transducer is actually connected to the back panel of the DES. Although each sample period must have the multiplex channel specified in order for the system to know what type of transducer is connected (and where), sample periods can use the same multiplex channel number and thus the same transducer within their definitions. Thus, multiplexing between sample periods can be accomplished by using just one transducer, or a maximum of 16, the total number of transducers possible that can be connected to the DES.

near field This is the region in front of the transducer where the wave-fronts produced by the transducer are not parallel and the beam is not properly formed (inverse square law does not apply).   
noise Unwanted electrical signals originating within the equipment or from hull or water sounds picked-up by the transducer. up
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).  
oscillation An uncontrollable state of an amplifier, or the result of an oscillatory state.  
oscillator Electronic circuit for generating controlled oscillations.  
oscilloscope An instrument for viewing and measuring oscillations or signals.  
performance test Measurements to establish the standard to which a system is working.  
period Time required for a single oscillation of a sine wave.  The period equals 1/f where f is the frequency.  Always presented in msecs.  
phase The time relationship of one wave to another.  
ping A name for the transmitted acoustic pulse.  
Position Acoustic Tag Echo Also known as a position or 3D Echo, is an echo position which has been calculated using a 3D-positioning process using tracked acoustic tag echo data.  
Position Acoustic Tag Files (*.PAT) These files contain the individual position acoustic tag echoes generated within the 3D tracking process. This file type is created during real-time tracking, or when exporting position echoes.  
position echoes Echoes which have been generated by the 3D-positioning process either during real-time data acquisition or post processing procedures.  To generate real-time position echoes, the ATS must be properly configured in terms of having valid hydrophone positions, correct released tag information, and proper settings for the auto-tracking and 3D-positioning processes.  up
position set A group or set of defined hydrophones with known positions (X,Y,Z coordinates) used either in data acquisition and/or in post-analysis.   
pre-amplifier Boosts signals before the main amplifier.  
projector Transmitter of acoustic power.  
propagation Ability of acoustic signals to progress outward in a medium.  
pulse 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 in a given time.  
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 width or duration in time of the transmitted acoustic pulse, usually expressed in msec.  
radio tags Radio tags technology identifies objects remotely through the use of radio frequencies that requires transmitters to emit the signals, receivers to detect and record them, and additional supporting equipment. They depend on steering the fish in a particular path (requiring the fish to be routed through a restricted sensing area).  
range Distance from the transducer face to the target (symbol: R; unit: m).  Often used synonymously with depth in vertical sounding.  
range resolution The minimum range separation between distinguishable targets.  For a CW pulse acoustic system, the range resolution is equal to c/2 where c is the velocity of sound and is the pulse length.  
Raw Acoustic Tag Echoes Echoes received by the individual hydrophones of an ATS. These echoes can be comprised of direct path as well as multipath sources.  Raw echoes are not associated with Tag ID. up

Raw Acoustic Tag Files (*.RAT)

The AcousticTag program generates raw acoustic tag echoes based on the echo selection criteria using the data collected from the hydrophones of an ATS.  These raw echoes are written to a *.RAT (RAT  = Raw Acoustic Tag) files during data collection. Located at the beginning of each *.RAT file is header information which contains the data acquisition settings used when the file was created. Following this header information are the individual entries for each raw acoustic echo.  It is important to note that these raw echoes are not associated with any specific Tag ID, nor is there any spatial (i.e. x, y or z) positioning assigned to a raw echo. 

Rayleigh scattering zone Where fish scattering cross-section varies inversely with the fourth power of wavelength, fish length much greater than l.  
real-time 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 the AcousticTag program.  
receiver Instrument to amplify, filter and otherwise process electronic signals (echoes) produced by the transducer.  
receiving voltage response Number of dB relative to 1 Volt for a given acoustic pressure at the transducer face (symbol: VRT; unit: dB/V).  
reciprocity Exhibited by mutually interchangeable transducers.  
reflection The "bouncing" of sound off a target, due to the differences in density between medium and target and target orientation. up
refraction Deflection of sound from a straight path, e.g., when passing through a thermo cline at an angle.  
resonance 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).  
reverberation Acoustic interference caused by scattering off objects other than those of interest.  The main source of reverberation in fisheries assessment are the bottom, surface, other boundaries, air bubbles, plankton, and particles in water.  
Rf (radio frequency) output Signal out of the sounder that has not been shifted to a lower frequency.  
root mean square 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.  
scattering layer Extensive horizontal distribution of acoustic targets.  
secondary 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. up
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.  
sensitivity 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.

shadowing The effect caused by one target lying in the 'shadow' of another.  
side lobe All 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 Proprietary encoded code-phase modulated pulse/signal.  
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) Ratio of signal strength to background noise level (symbol: SNR; unit: dB).  
single-pulse An HTI Model 795 Acoustic Tag which produces a single transmit signal.  The transmit signal is based on the tag's defined Period (i.e. ping rate). up
sonar Commonly referred to as the transmission of sound waves and measuring the time it takes for their echo to return after reaching an object.  
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/sec, and dependent upon temperature, salinity and depth.  
sound wave Pressure maxima and minima moving within a compressible medium.  
source level Ratio of acoustic intensity on the axis of a source at 1 m, to a plane wave of rms pressure 1 m Pa (symbol: SL; unit: dB/1 m Pa/m).  
speed of acoustic waves See acoustic speed.  
spherical spreading loss Describes the decrease in sound intensity as the beam spreads (i.e., decreases with range).  
standard target A target possessing a known target strength, used for the calibration of acoustic systems (unit dB).  
stationary transducer Transducer fixed to a buoy or to the bottom looking upward, sideways or downward.  
subcode A setting used for double-pulse tags. Subcode settings range from 1 to 15 which defines a specific time interval at which the secondary signal is transmitted after the primary signal has been transmitted.  
sub-meter Less than one meter in diameter.  Often referred to with HTI’s Model 795 acoustic tags fine-scale tracks.  

Summary Files (*.SUM)

Files containing summary information for all raw acoustic tag echoes which have been marked. *.SUM files contain a list of Tags in which data was marked for each hydrophone of the system.  Within MarkTags, summary data is only available after the data has been marked either through manual or Auto-Tracking methods. up
survival studies Research or investigations to assess the effects and influences on the long-term viability of fish stocks, commonly used for salmonids listed as threatened or endangered under the U.S. Endangered Species Act (ESA).  
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.  
synchronization The keeping together in time of recorders: units comprising an echo-sounder, a sonar, or electrical waveforms.  

tag double-pulse mode

In addition to the period, there is another setting that can also be used to increase the number of possible unique tag identifiers.  Within the tag programming software, TagProgrammer, there is an option for enabling the double pulse or subcode mode.  If this option is turned on, the tag will produce a primary transmit signal followed by a secondary transmit (subcode) signal.  There are 15 different subcodes possible for each period.  This gives up to 100,000 unique codes. This option should be used when a project involves the release of thousands of tags. Using this feature does affect the life of the tag since the tag's signal is being transmitted twice.  
tag ID The tag's identification which consists of the period and subcode setting used when the tag was programmed. A tag ID is always represented by the period value followed by a "." then the subcode. For example a Tag ID of 4200.05 designates a tag was programmed with a period of 4200 msecs with a subcode setting of 05. If a tag is not programmed with a subcode setting, it would be designated as 4200.00. All single-pulse tag IDs will have a subcode of "00" ( ex. 1000.00,  2010.00, etc). up

tag period

The period (i.e. rep rate, tag code) is the rate at which the tag emits a pulse into the water.  Period is measured from the leading edge of one pulse to the leading edge of the next pulse in sequence.  By using slightly different periods, single pulse tags can be individually identified (up to 16,000 unique periods). The timing of the start of each transmission is precisely controlled by a microprocessor within the tag.  When conducting a study where multiple tags are to be released, each tag must be programmed to have its own unique period in order to identify one tag from one another.  

tag programming

Model 795 Acoustic Tags are NOT pre-programmed and need to be programmed by the user prior to use.  Tags have a finite shelf life and a finite operational life. Do not turn tags on until they are intended to be used.  To program a Model 795 Acoustic Tag requires the Model 490 Acoustic Tag Programmer,  a computer installed with the TagProgrammer software, and a supplied serial cable connecting the computer to the Model 490. It is also advisable to have an oscilloscope connected to the Model 490 to verify and measure the programmed state of the tag. There are two main settings that need to be addressed prior to programming tags, these being the pulse width and period.  The tag's battery life is limited and is affected by the combination of these two settings.  Refer to the tag life section on how these two settings affect the longevity of the battery.


tag pulse width

The pulse width (or pulse length) for each Model 795 Acoustic Tag is configurable where the duration of the tag transmission is precisely controlled by a microprocessor within the tag. When conducting a study where multiple tags are to be released, all tags must be programmed with the same pulse width.  This is so the Model 290/291/295 AcousticTag Receivers can "filter" out the specific pulses coming from the acoustic tags from the ambient background noise.   



Software that programs acoustic tags and can turn the tags “on”.  Model 795 Acoustic Tags are NOT pre-programmed, allowing the user to configure tags prior to the study and keeps them in a sleep mode until they are ready for use. up
target strength Acoustic size of target in dB (see backscattering cross section).  The ability of a given target to reflect acoustic signals; usually given in terms of negative dB's.  
thermal noise The ultimate limit to detection due to molecular activity, mainly evident above 100 kHz.  
thermocline Temperature discontinuity where organisms often collect, thus making it acoustically 'visible'.  
threshold An amplitude value below which all echoes are rejected.  A threshold is applied to reject noise and signals from very small targets which are not of interest.  
timebase 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.  
towed-body Also refered to as towed-body. 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.  
tracked Raw acoustic tag echoes which have been selected and assigned a tag ID through an auto-tracking method. Also referred as auto-tracked.  
tracked acoustic tag echo Tracked echo is a raw echo which has been assigned a tag ID. Assigning a tag ID to an echo can be done using a manual or auto-tracked method. All echoes within a *.TAT file are assigned tracked echoes. up

Tracked Acoustic Tag Files (*.TAT)

These files contain the individual tracked acoustic tag echoes which have been assigned a tag ID.   Within the 2D tracking phase of the real-time tracking process, raw echoes are assigned tag IDs based on the specific tag defined  for real-time tracking within the tag information dialog box.  The *.TAT (TAT = Tracked Acoustic Tag) files are always associated with a specific *.RAT file since the data within a *.TAT file will always be a subset of the data found in a *.RAT file.

Located at the beginning of each *.TAT file is header information which contains the data acquisition settings, including the auto-tracking settings used when the file was created. Following this header information are the individual entries for each tracked acoustic tag echo. As with *.RAT files, no spatial positioning is assigned to a Tracked Acoustic Echo.
transducer Electro-mechanical device which translates electrical energy to sound energy to produce the hydroacoustic signal, and converts returning echoes back into electrical signals.  
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 echo sounder 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).  
transmitter Unit which produces electrical power at the required frequency.  
travel times Referring to time calculations that measure a journey 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 the chart recorder or by the echo sounder that occurs at the time of transmission.  Its purpose is to keep all equipment synchronized for data collection and display.  
tungsten carbide A chemical compound that is unbelievably durable, and is a cheaper and more heat-resistant alternative to diamond.  
voltmeter Device for the measurement of voltages, either arising from direct or alternating current.  
wavelength The distance traveled by a sinusoidal acoustic wave in a time equal to the period of the sine wave.  The wavelength is important in determining the directivity of transducers and the characteristics of scattering.   
wavenumber 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 (echo sounder, depth sounder) representing zero time (zero depth or depth of the transducer). up
Helpful Links
View DemosAdditional ResourcesRecent StudiesRequest More Info
© Hydroacoustic Technology, Inc. ll Rights Reserved.
email HTI home site map