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The International System of Units has received worldwide acceptance, so it is especially important that projects associated with the United Nations Organization should make use of this system whenever possible to ensure uniformity in its training programmes and in reporting the results of its work.

Systeme International d'Unités (International System of Units) abbreviated SI, was adopted by the 11th General Conference of Weights and Measures in 1960. It includes three classes of units:

1. Base units
2. Supplementary units
3. Derived units which together form the coherent system of SI units.

1. Base Units: SI is founded on the seven base units below.


Name of Base Unit











electrical current



thermodynamic temperature



luminous intensity



amount of substance



2. Supplementary Units

These units have not yet been classified as either base units or derived units.




plane angle



solid angle



3. Derived Units

Only those thought to be appropriate to this manual are listed below.




Expressed as




1 Hz = 1s-1


newt on


1 N = 1 kg m/s2

pressure and stress



1 Pa = 1 N/m2

work, energy, quantity of heat



1J = 1 Nm




1 W = 1 J/s

electrical potential



1 V = 1 W/A

electrical capacitance



1 F = 1 As/V

electrical resistance



1 W = 1 V/A

electrical conductance



1 S = 1 W -1

4. Other Units




Expressed as





















wave speed



acoustic resistance


r c


acoustic cross-section




duration of time




angular velocity




angle (ideal beam)





electrical power



acoustic power



source level


dB/1m Pa/1m

directivity index


10 log(4p area)/l 2

transmission loss



target strength



receiving transducer sensitivity


dB/1V/1m Pa

radiation resistance



voltage at transducer terminals



echo level


dB/1m Pa

band level



calibrated voltage output



5. Multiples of SI Units

Again, only those multiples likely to occur in this manual are included:

Multiplication Factor































1 cm3 = (10-2m)3 = 10-6m3
1 m s-1 = (10-6s)-1 = 106s-1
1 mm2/s = (10-3m)2/s = 10-6m2/s

compound prefixes should not be used, write nm (nanometre) NOT mm m.

The choice of multiples (decimal multiple or submultiple) of an SI unit is taken for convenience, i.e. the particular multiple chosen will be the one leading to numerical values within a practical range. It is usually possible to keep these values between 0.1 and 1000.


0.00394m can be written as 3.94 mm
1401 Pa can be written as 1.401 kPa
3.1 x 10-8s can be written as 31 ns

In calculations, errors may be avoided more easily if quantities are expressed in SI with the prefixes being replaced by powers of 10.

Symbols for units are printed in lower case up-right type and remain unaltered in the plural, they should be written without a final full stop and be placed after the complete numerical value. Upper case letters are used where the name of the unit is derived from a proper name, e.g. Newton, N or Pascal, P.

Compound units, formed by dividing one unit by another, e.g. are indicated as m/s, or ms-1 the latter form is preferable where multiple units occur on the same line.

6. Notes

1) The unit of pressure in older textbooks was the m bar, this has been replaced by the m Pa. To convert m bar to m Pa, add 100 dB,

e.g. 110 dB/1m bar/1m = 210 dB/1m Pa/1m
or -110 dB/1V/1m bar = -210 dB/1V/1m bar.

2) Use of the nautical mile is standard for seagoing purposes, where the terms mile or mile2 are used, a nautical mile is implied. (Never use nm as an abbreviation for nautical mile).

3) The prefix hydro is regarded as superfluous when fisheries acoustics are being considered. It is not used in this manual.

4) Some duplication in the use of symbols inevitably occurs but an explanation is given in the section concerned.



1. absorption coefficient: the coefficient a , denoting the power loss due to absorption (symbol a )

2. absorption loss: a temperature and frequency dependent power loss to acoustic waves, linear with distance (symbol a R: unit dB)

3. acoustic axis: region of maximum response, normally perpendicular to the face of a transducer.

4. acoustics: the theory of acoustic waves and their propagation

5. acoustic calibration: measuring the performance of an acoustic system to a specified standard (unit dB)

6. acoustic equation: see sonar equation

7. acoustic equipment: devices for the generation or reception of acoustic waves.

8. 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)

9. acoustic power: acoustic energy per unit time

10. acoustic speed: speed at which acoustic waves travel (symbol c; unit m/s)

11. amplification: amount by which a signal is increased, see gain (unit dB)

12. amplifier: the device which increases signal size

13. amplitude: size of a signal.

14. analogue: functional similarity: (a circuit or device to generate voltages proportional to quantities)

15. angular resolution: the amount of discrimination between targets separated in angle (unit degrees)

16. array: multi-element transducer

17. attenuation: reduction of acoustic power due to spherical spreading and absorption of the waves (unit dB/km)

18. automatic gain control: amplification varied in proportion to a received signal to reduce output voltage variation.


1. back scattering: amount of acoustic power scattered by a target into the direction of the transmitting transducer.

2. back scattering layer: biomass layer which back-scatters acoustic power.

3. band width: the amount frequencies extend on either side of the nominal acoustic frequency (symbol BW; unit Hz)

4. beam angle: full included angle between the half-power points (symbol q ; unit degrees)

5. beam half angle: angle where the acoustic power is half that of the axis (symbol q /2 unit degrees)

6. beam dual: see dual beam system

7. beam equivalent: see equivalent beam

8. beam deflection: amount by which a beam is moved in angle from its normal acoustic axis.

9. beam narrow: when full angle is less than 10°.

10. beam pattern: two-dimensional pattern showing the relative response of a beam

11. beam wide: when full angle is more than 10°.

12. beam overlap: amount by which successive pings cover the same area.

13. beam width: distance across the acoustic beam at a given range. (unit: metres)

14. blocking: when receiving function is stopped by a very large signal, as in 'white line'.

15. body towed: see towed body

16. bottom lock: when the bottom signal forms a reference for echoes just above it. (also seabed lock).

17. bottom discrimination: determining the nature of the bottom.

18. bottom noise: noise generated by tidal flow.

19. bottom pulse: electrical pulse produced from bottom echo.


1. calibration: measuring or adjusting the performance of a system to a specified standard.

2. calibration equipment: signal generators, hydrophones, standard targets, projectors, oscilloscopes, voltmeters, etc.

3. cavitation: production of voids in the water due to negative pressure.

4. channelling: restriction of acoustic waves by boundaries.

5. contour line: similar function to 'white line'

6. cross section: see scattering-cross section.


1. dead zone: volume of the transducer beam, usually close to the seabed, where targets cannot be detected.

2. decibel: logarithmic ratio used to express relative levels of acoustic or electrical signals (unit: dB)

3. demodulation: process of extracting information from a signal.

4. depth interval: selected interval between two depths, also known as a gate (unit metres)

5. depth range: the total depth indicated on the display (unit metres)

6. depth recorder: device which indicates and records the depth of acoustic targets and the seabed.

7. 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)

8. digital: having the circuit state of 'OFF' or 'ON'.

9. directivity index: concentrating power of a transducer related to dimensions and acoustic wavelength, expressed in logarithmic form (symbol DI: unit dB)

10. directivity pattern: diagram of the concentrating power of a transducer in terms of beam angle and relative amplitude of the lobes.

11. display unit: for the display of signals and other information relating to the echo-sounder

12. doppler effect: frequency difference (f) between the transmitted pulse (f0) and the received echo, due to the relative speed (V in m/s) between the ship and the target in water where the acoustic velocity is c, m/s and f = 2f0V/c

13. dual beam: multi-element transducer from which two concentric beams of same frequency but different beamwidths are formed.

14. dynamic range: the extent to which signals can be processed without distortion (unit dB)


1. echo: an acoustic wave reflected from a target.

2. echo level: acoustic intensity at the receiving transducer (symbol: EL; unit dB)

3. echo paper dry: recording paper conductive with high voltage.

4. echo paper moist: recording paper conductive with low voltage.

5. echogram: record of a sequence of echoes.

6. echo integrator: unit to process and add the acoustic intensities from selected depth intervals.

7. echo sounder: system comprising acoustic transmitter, echo receiver and display.

8. echo sounding: finding the depth of a target by measuring the time from transmission to echo.

9. echo ranging: finding the distance to a target by measuring the time from transmission to echo.

10. echo trace: mark on a record caused by an echo.

11. electro-acoustic: process to convert electrical energy into acoustic energy.

12. electro-strictive: material which changes its dimensions under the influence of an electric field.

13. electro-acoustical efficiency: efficiency of electrical to acoustic energy conversion (symbol h ; unit: dB, or %)

14. equipment log sheet: a table of equipment readings.

15. equivalent angle: the included angle of an 'ideal' beam, calculated from actual transducer characteristics.

16. expanded dynamic range: a technique to improve the range of tones on a paper record.

17. expanded scale: display of a portion of range or depth at a size exceeding its basic scale.


1. far-field: distance beyond where the initial fluctuations of intensity occur when transmitted by a transducer.

2. 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)

3. fish abundance: how many fish are in a population.

4. fish detection: location of fish by acoustic means.

5. 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)

6. free-field: volume of water clear of boundaries.

7. frequency: number of complete cycles of an electrical or acoustic wave to pass a given point in one second (symbol: f; unit: Hz)

8. frequency counter: a device to count the number of complete cycles to pass a given point in a given time.

9. frequency response: the extent to which a system is sensitive to a range of frequencies (unit Hz)


1. gain: amount by which the amplitude (size) of a signal is increased (unit dB)

2. geometrical loss: dispersal of energy of an acoustic wave due to the spreading effect within the geometry of the beam.

3. geometrical spreading: increase in the cross section of an acoustic beam, proportional to distance squared.

4. geometric cross-section: projected area of a target in the direction of isonification.

5. 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. l very much less than fish length.

6. ghost echo: an echo falsely related to the depth scale.


1. hydrophone: device to receive acoustic waves and convert them to electrical signals.


1. impedance: ratio of generally complex quantities of pressure and particle velocity or of voltage and current at the same time and place.

2. incident intensity: acoustic intensity falling on a target.

3. insonify: to 'illuminate' by means of acoustic waves.

4. isotropic: having non-directional properties.

5. integrator: see echo integrator.

6. intensity: see acoustic intensity (symbol I; unit dB/m Pa)

7. interface: the matching unit between one instrument and another.

8. interference: an unwanted signal or a malfunction.


1. layer: see scattering

2. live fish calibration: overall calibration of an echo-sounder/echo-integrator system by insonifying captive fish and measuring the received intensity

3. lobe: see side lobe

4. log sheet: see survey equipment


1. manual: hand control

2. magneto-strictive: material which changes its dimensions under the influence of a magnetic field.

3. marker button: a press button to make a line on a paper record.

4. minimum recordable signal: smallest amplitude (size) of signal which can be seen on the display.

5. modulation: the process of impressing information on a signal (e.g. pulse)


1. nautical mile marker: signal from ships log to mark a record, or to zero an integrator.

2. near field: the distance within which transducer measurements should not be made.

3. noise level: number of dB by which noise is above or below a given reference.

4. noise limited: distance at which detection is no longer possible because the signal is obscured by noise.

5. noise reduction: number of dB by which noise is reduced from a reference.

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


1. oscillation: an uncontrollable state of an amplifier, or the result of an oscillatory state.
2. oscillator: electronic circuit for generating controlled oscillations.
3. oscilloscope: an instrument for viewing and measuring oscillations or signals


1. performance test: measurements to establish the standard to which a system is working.

2. ping: a name for the transmitted acoustic pulse.

3. power consumption: electrical power needed to operate acoustic equipment (unit Watts or kilo Watts)

4. phase: the time relationship of one wave to another (symbol f : unit degrees)

5. pre-amplifier: boosts signals before the main amplifier.

6. projector: transmitter of acoustic power.

7. propagation: transmission and travel of acoustic waves through a medium.

8. pulse duration: the time for which a pulse continues (symbol: t ; unit: ms).

9. pulse length: the distance a pulse extends (unit metres)

10. pulse rate: number of pulses in a given time.

11. 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)


1. range: distance to a target (symbol: R; unit: m).

2. range resolution: the minimum distance between targets which allows them to be separated from each other (symbol: ct /2; unit: m)

3. Rayleigh scattering zone: where fish scattering cross-section varies inversely with the fourth power of wavelength, fish length much greater than l .

4. receiving voltage response: number of dB relative to 1 Volt for a given acoustic pressure at the transducer face (symbol: VRT; unit: dB/V)

5. reciprocity: exhibited by mutually interchangeable transducers.

6. resonance: when a circuit, or a target, is excited to different modes of vibration by a particular frequency.

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

8. reverberation: the sum of all scattered acoustic energy.

9. root mean square: the square root of the averaged sum of all squared values of a waveform (symbol rms)


1. sampling cross-section: area across the beam at a given range (unit m2)

2. sampling volume: selected volume within the acoustic beam.

3. 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).

4. scattering layer: extensive horizontal distribution of acoustic targets.

5. 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.

6. sensitivity: degree of response to an acoustic or electrical signal.

7. shadowing: the effect caused by one target lying in the 'shadow' of another.

8. signal generator: instrument which produces electrical signals at controlled frequencies and amplitudes.

9. signal strength: intensity of an acoustic wave or amplitude of an electrical wave.

10. signal-to-noise ratio: amount by which a signal exceeds noise, or is below noise (symbol: SNR; unit: dB).

11. side lobes: all beams of a transducer except the main beam.

12. sonar: all SOund NAvigation and Ranging systems but usually reserved for near horizontally directed beams.

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

14. speed of acoustic waves: see acoustic speed

15. standard target: a target possessing a known target strength, used for the calibration of acoustic systems (unit dB)

16. stylus: the writing tip, or tips in a paper recorder.

17. survey log sheet: table of data relating to survey readings.

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


1. target strength: ratio of the echo intensity at 1 m from a target to the incident intensity (symbol: TS; unit: dB).

2. thermal noise: the ultimate limit to detection due to molecular activity, mainly evident above 100 kHz.

3. thermocline: temperature discontinuity where organisms often collect, thus making it acoustically 'visible'.

4. threshold: a chosen signal level below which no signals are processed.

5. timebase: the time reference to which signals on a paper recorder, or an oscilloscope, are displayed.

6. time-varied-gain: accurately controlled amplification (gain) relative to time after the transmission, used to correct for transmission loss (symbol TVG)

7. towed-body: hydrodynamically shaped body into which a transducer may be fitted.

8. transducer: device for the conversion of acoustic energy into electrical energy and vice-versa: see hydrophones, projectors, electrostrictive and magnetostrictive.

9. transceiver: combined transmitter and receiver of an echo-sounder.

10. transmission locked: display of signals related to time of transmission.

11. transistor: electronic device which amplifies, switches and oscillates in electronic circuits as required.

12. transmission loss: sum of absorption loss and geometric loss (symbol TL, unit dB)

13. transmitter: unit which produces electrical power at the required frequency.

14. trigger contact: switch to initiate transmission, or to start some function.

15. trigger pulse: electrical signal which initiates a circuit action.


1. voltmeter: device for the measurement of voltages, either arising from direct or alternating current.


1. wavelength: distance between the crests or the troughs of a sinewave (symbol: l ; unit: m).

2. 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.


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