Hydroacoustic Technology, Inc.

  Hydroacoustic Technology, Inc.    
  Acoustic Tag and Hydroacoustic Glossary  
     
    The following 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.
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.
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).
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 dual-beam sonar applications, this is the ratio of the received signal intensities from the narrow and wide channels(In/Iw).  Assuming that the received signals come from one target, the ratio depends on the direction of the target and can be used to estimate the amount of signal intensity lost due to the angle of the target from the acoustic axis.  
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.
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.
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.
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.
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.
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
systems).		  
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)].
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.
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.
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.  

intensity

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).   

marking

 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.
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.  
multiplexing 

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.
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. 
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.

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.
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 avera