Underwater acoustics — Terminology

ISO 18405:2017 defines terms and expressions used in the field of underwater acoustics, including natural, biological and anthropogenic (i.e. man-made) sound. It includes the generation, propagation and reception of underwater sound and its scattering, including reflection, in the underwater environment including the seabed (or sea bottom), sea surface and biological organisms. It also includes all aspects of the effects of underwater sound on the underwater environment, humans and aquatic life. The properties of underwater acoustical systems are excluded.

Acoustique sous-marine — Terminologie

General Information

Status
Published
Publication Date
24-Apr-2017
Current Stage
9093 - International Standard confirmed
Completion Date
07-Nov-2022
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INTERNATIONAL ISO
STANDARD 18405
First edition
2017-04
Underwater acoustics — Terminology
Acoustique sous-marine — Terminologie
Reference number
ISO 18405:2017(E)
ISO 2017
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ISO 18405:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form

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ISO 18405:2017(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

3.1 General terms ........................................................................................................................................................................................... 1

3.1.1 General...................................................................................................................................................................................... 1

3.1.2 Acoustical field quantities........................................................................................................................................ 2

3.1.3 Acoustical power quantities .................................................................................................................................. 5

3.1.4 Logarithmic frequency intervals .....................................................................................................................10

3.1.5 Other acoustical quantities ..................................................................................................................................11

3.2 Levels used in underwater acoustics ................................................................................................................................13

3.2.1 Levels of acoustical power quantities .........................................................................................................13

3.2.2 Levels of acoustical field quantities ..............................................................................................................16

3.3 Terms for properties of underwater sound sources ............................................................................................17

3.3.1 Source waveforms and factors ..........................................................................................................................17

3.3.2 Source levels .....................................................................................................................................................................21

3.4 Terms related to propagation and scattering of underwater sound......................................................23

3.4.1 Propagation .......................................................................................................................................................................23

3.4.2 Scattering ............................................................................................................................................................................24

3.5 Terms for properties of underwater sound signals ..............................................................................................27

3.5.1 Sound signals ...................................................................................................................................................................27

3.6 Terms related to sonar equations ........................................................................................................................................29

3.6.1 General...................................................................................................................................................................................29

3.6.2 Sonar equations and sonar equation terms ..........................................................................................31

3.7 Terms related to underwater bioacoustics ..................................................................................................................35

3.7.1 Auditory frequency weighting ..........................................................................................................................35

3.7.2 Sound reception.............................................................................................................................................................38

3.7.3 Sound production ...................................................................... ...................................................................................42

Annex A (informative) Alphabetical index ..................................................................................................................................................43

Bibliography .............................................................................................................................................................................................................................50

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ISO 18405:2017(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO’s adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following

URL: w w w . i s o .org/ iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 3,

Underwater acoustics.
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ISO 18405:2017(E)
Introduction
0.1 Overview

Vocabulary is the most basic of subjects for standardization. Without an accepted standard for the

definition of terminology, the production of scientific and engineering publications in a technical

area, including the development of standards for measurement, processing or modelling in that area,

becomes a laborious and time-consuming task that would ultimately result in the inefficient use of time

and a high probability of misinterpretation.

Basic terminology of underwater acoustics is defined in 3.1, followed by levels in 3.2. These are

followed by definitions of terms associated with sources of sound (3.3), propagation and scattering

(3.4), underwater sound signals (3.5), and sonar equations (3.6). Finally, 3.7 defines basic bioacoustical

terminology used in underwater acoustics.
0.2 Approach

The underlying philosophy followed in preparing this document is to define quantities independently of

how they are measured.
0.3 Remark on exceptions to the ISO/IEC 80000 series

In this document, the ISO/IEC 80000 series is followed for the definitions of physical quantities,

including the level of a power quantity and level of a field quantity. Two exceptions are made to this

general rule, as follows.

— Inconsistencies between ISO 80000-1 and ISO 80000-3 make it necessary to choose between them

(for example, the term “field quantity” used in ISO 80000-3 is deprecated by ISO 80000-1:2009,

Annex C, which prefers the term “root-power quantity”). This document follows ISO 80000-3, which

makes it incompatible with ISO 80000-1.

— The term “sound pressure level” is defined by ISO 80000-8 in a way that does not reflect conventional

use of this term to mean the level of the mean-square sound pressure. This convention is reflected

in ISO 80000-8 by the notes in the “Remarks” column alongside the definition. These remarks are

inconsistent with the definition, making it necessary to choose between the definition and the

remarks. This document follows the “Remarks”, which makes it incompatible with the ISO 80000-8

definition of “sound pressure level”.
0.4 Remark on levels and level differences, and their reference values

Levels used in underwater acoustics are defined in 3.2. In its most general form, a level L of a quantity

Q is defined in the International System of Quantities (see ISO 80000-3) as the logarithm of the ratio of

the quantity Q to its reference value, Q . In formula form, this definition can be written as

L = log (Q/Q ).
Q r 0

The nature of the quantity (Q), its reference value (Q ) and the base of the logarithm (r) should all be

specified. Reference values for use in underwater acoustics are specified by ISO 1683.

Two types of level are in widespread use in underwater acoustics, the level of a field quantity (see

ISO 80000-3:2006, 3-21) and the level of a power quantity (see ISO 80000-3:2006, 3-22). In underwater

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ISO 18405:2017(E)

acoustics, it is conventional to express both types of level in decibels (dB). When expressed in decibels,

the level L of a field quantity F is
L = 20 log (F/F ) dB,
F 10 0

where F is the reference value of the field quantity. Similarly, the level L of a power quantity P is

0 P
L = 10 log (P/P ) dB,
P 10 0

where P is the reference value of the power quantity. This definition of L is a product of the three

0 P

factors 10, log (P/P ) and 1 dB. In words, this product is written in this document as “ten times the

10 0

logarithm to the base 10 of the ratio P/P , in decibels”. For levels of both field and power quantities,

the nature of the quantity (F or P) is implied by the name of the level, while the base of the logarithm is

implied by the use of decibel as the unit. For all levels, the reference value is stated explicitly. The use

by this document of the definitions of “level” and “decibel” from ISO 80000-3 results in inconsistencies

between this document and ISO 80000-1 because of inconsistencies between ISO 80000-3 and

ISO 80000-1:2009, Annex C.

Level differences [i.e. differences between levels of like quantities (see ANSI/ASA S1.1-2013, 10.44)] are

also expressed in decibels. For example, if P and P are power quantities of the same kind, and L and

1 2 P,1
L are their respective levels, the corresponding level difference is
P,2
ΔL = L – L = 10 log (P /P ) dB – 10 log (P /P ) dB = 10 log (P /P ) dB.
P P,1 P,2 10 1 0 10 2 0 10 1 2
Similarly, for like field quantities F and F , with respective levels, L and L ,
1 2 F,1 F,2
ΔL = L – L = 20 log (F /F ) dB – 20 log (F /F ) dB = 20 log (F /F ) dB.
F F,1 F,2 10 1 0 10 2 0 10 1 2

Examples of level difference are transmission loss, array gain, and hearing threshold shift.

Differences between levels of power quantities of different kinds are encountered in 3.6 and 3.7 in

connection with the response of underwater systems, and are also expressed in decibels. For example,

if A and B are two power quantities, with A being a measure of the response signal (output) of a system

and B a measure of the forcing signal (input), such that the system sensitivity is S = A/B, the sensitivity

level of that system is
N = L – L = 10 log (A/A ) dB – 10 log (B/B ) dB = 10log (S/S ) dB
S A B 10 0 10 0 10 0
where S , the reference value of the sensitivity, is equal to A /B .
0 0 0

An example of sensitivity level in underwater acoustics is target strength (reference value = 1 m ). If

this quantity were expressed instead as the difference between levels of field quantities, defined as the

square root of the respective power quantities, the reference value would then become 1 m.

0.5 Remark on reference values of root-power quantities

For every real, positive power quantity, P, there exists a root-power quantity, F , equal to the square

1/2

root of P (see ISO 80000-1:2009), that is, F = P . The level of this root-power quantity is

L = 20 log (F /F ) dB.
F,rp 10 rp 0
1/2

This level is equal to L if the reference value F is given by F = P . Selected power quantities and

P 0 0 0

their respective reference values are listed in columns 1 and 2 of Table 1. The corresponding root-

power quantities and their respective reference values are listed in columns 3 and 4 of Table 1. A field

quantity is “a quantity whose square is proportional to power when it acts on a linear system” (see

ISO 80000-3), so all root-power quantities are also field quantities. For example, the level of mean-

square sound pressure, with reference value 1 μPa , is equal to that of root-mean-square sound

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ISO 18405:2017(E)

pressure, with reference value 1 μPa. These two reference values are therefore used interchangeably

for sound pressure level.

Table 1 — Power quantities, their corresponding root-power quantities, and their reference

values, based on Reference [21]
Power quantity Reference value Corresponding root-power Reference value
1/2
(P) (P ) quantity (F = P )
0 0 0
1/2
(F = P )
Mean-square sound pressure 1 μPa Root-mean-square sound 1 μPa
pressure
Mean-square sound particle 1 pm Root-mean-square sound 1 pm
displacement particle displacement
2 2
Mean-square sound particle 1 nm /s Root-mean-square sound 1 nm/s
velocity particle velocity
2 4 2
Mean-square sound particle 1 μm /s Root-mean-square sound 1 μm/s
acceleration particle acceleration
2 1/2
Sound exposure 1 μPa s Root sound exposure 1 μPa s
1/2
Sound power 1 pW Root sound power 1 pW
1/2
Sound energy 1 pJ Root sound energy 1 pJ
2 2
Source factor 1 μPa m Root source factor 1 μPa m
Propagation factor 1 m Root propagation factor 1 m
0.6 Remark on the usage of “acoustic” and “sound” in this document

This document recognizes the interchangeability of the words “acoustic” and “sound” when the word

“sound” is used as part of a compound noun, and not otherwise.
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INTERNATIONAL STANDARD ISO 18405:2017(E)
Underwater acoustics — Terminology
1 Scope

This document defines terms and expressions used in the field of underwater acoustics, including

natural, biological and anthropogenic (i.e. man-made) sound. It includes the generation, propagation and

reception of underwater sound and its scattering, including reflection, in the underwater environment

including the seabed (or sea bottom), sea surface and biological organisms. It also includes all aspects

of the effects of underwater sound on the underwater environment, humans and aquatic life. The

properties of underwater acoustical systems are excluded.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1 General terms
3.1.1 General
3.1.1.1
sound

alteration in pressure, stress or material displacement propagated via the action of elastic stresses in an

elastic medium and that involves local compression and expansion of the medium, or the superposition

of such propagated alterations

Note 1 to entry: The medium in which the sound exists is often indicated by an appropriate adjective, e.g.

airborne, water-borne, or structure-borne.

Note 2 to entry: In the remainder of this document, the medium is assumed to be a compressible fluid.

Note 3 to entry: A sound wave is a realization of sound.

Note 4 to entry: The word “sound” may also be used as part of a compound noun, in which case, it is a synonym of

“acoustic”. For example, “acoustic pressure” and “acoustic power”’ are synonyms of sound pressure (3.1.2.1) and

sound power (3.1.3.14).
[SOURCE: Reference [23] and Reference [35]]
3.1.1.2
ambient sound
sound (3.1.1.1) that would be present in the absence of a specified activity
Note 1 to entry: Ambient sound is location-specific and time-specific.

Note 2 to entry: In the absence of a specified activity, all sound is ambient sound.

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ISO 18405:2017(E)
Note 3 to entry: Ambient sound includes ambient noise (3.1.5.11).

Note 4 to entry: Examples of specified activity include the act of measuring the underwater sound and the

radiation of sound by specified sound sources.

Note 5 to entry: Ambient sound can be anthropogenic (e.g. shipping) or natural (e.g. wind, biota).

3.1.1.3
soundscape

characterization of the ambient sound (3.1.1.2) in terms of its spatial, temporal

and frequency attributes, and the types of sources contributing to the sound field

3.1.1.4
reverberation

sound (3.1.1.1) resulting from cumulative scattering of sound by an aggregation, or ensemble, of

scatterers

Note 1 to entry: Reverberation commonly arises from scatterers in a volume or on a surface.

3.1.1.5
material element
sound particle
smallest element of the medium that represents the medium’s mean density

Note 1 to entry: The characteristic length scale of this element is of the order of several times the mean free

molecular path (see Reference [22]).
3.1.2 Acoustical field quantities
3.1.2.1
sound pressure
contribution to total pressure caused by the action of sound (3.1.1.1)

Note 1 to entry: Sound pressure is a function of time, which may be indicated by means of an argument t, as in

p(t), where p is sound pressure and t is time.
Note 2 to entry: Sound pressure is expressed in pascals (Pa).

Note 3 to entry: The term “sound pressure” is sometimes used as a synonym of “root-mean-square sound

pressure”. This use is deprecated.

Note 4 to entry: The term “sound pressure” is defined by IEC 60050 as the root-mean-square value of p(t). While

this IEC definition is not compatible with the present (ISO) definition, users of ISO standards might nevertheless

encounter the IEC definition, for example, in hydrophone calibration standards developed by the IEC.

Note 5 to entry: Weighted sound pressure is defined in 3.7.1.1.
[SOURCE: ISO 80000-8:2007, 8-9.1 and 8-9.2, modified]
3.1.2.2
sound pressure spectrum
Fourier transform of the sound pressure (3.1.2.1)

Note 1 to entry: Sound pressure spectrum is a function of frequency, which may be indicated by means of an

argument f, as in P( f ), where P is sound pressure spectrum and f is frequency.

Note 2 to entry: In formula form, Pf =−expi2π ft p(t) dt, where p(t) is the sound pressure as a function

() ()

of time, t. If P( f) is known, p(t) can be calculated using the inverse Fourier transform pt =+expi2π ft

() ()
P( f ) df. See ISO 80000-2:2009.
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ISO 18405:2017(E)

Note 3 to entry: Sound pressure spectrum is expressed in units of pascal per hertz (Pa/Hz).

Note 4 to entry: In general, P( f ) is a complex function of frequency.

Note 5 to entry: The definition of sound pressure spectrum applies to a single-event or transient sound pressure

signal, in which case, for the purpose of the integral over time in the formula for P( f), the sound pressure p(t)

is set to zero at all times before the signal starts and after it ends. It can also be applied to a finite segment of a

continuous sound pressure signal, in which case, the start and end times of the segment shall be specified.

3.1.2.3
zero-to-peak sound pressure
peak sound pressure
0-pk

greatest magnitude of the sound pressure (3.1.2.1) during a specified time interval, for a specified

frequency range
Note 1 to entry: Zero-to-peak sound pressure is expressed in pascals (Pa).

Note 2 to entry: A zero-to-peak sound pressure can arise from a positive or negative sound pressure.

[SOURCE: ISO/TR 25417:2007, 2.4, modified]
3.1.2.4
compressional pressure
sound pressure (3.1.2.1), p(t), when p(t) > 0, where t is time
Note 1 to entry: Compressional pressure is expressed in pascals (Pa).

Note 2 to entry: For shock waves, compressional pressure may be referred to as “blast overpressure”. See

Reference [33].
3.1.2.5
peak compressional pressure
pk,c

greatest compressional pressure (3.1.2.4) during a specified time interval, for a specified frequency range

Note 1 to entry: Peak compressional pressure is expressed in pascals (Pa).

Note 2 to entry: A peak compressional pressure can only arise from a positive sound pressure.

Note 3 to entry: For shock waves, peak compressional pressure may be referred to as “peak blast overpressure”.

3.1.2.6
rarefactional pressure

magnitude of sound pressure (3.1.2.1), |p(t)|, when p(t) < 0, where p is sound pressure and t is time

Note 1 to entry: Rarefactional pressure is expressed in pascals (Pa).
3.1.2.7
peak rarefactional pressure
pk,r

greatest rarefactional pressure (3.1.2.6) during a specified time interval, for a specified frequency range

Note 1 to entry: Peak rarefactional pressure is expressed in pascals (Pa).

Note 2 to entry: A peak rarefactional pressure can only arise from a negative sound pressure.

Note 3 to entry: Peak rarefactional pressure is always positive.
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ISO 18405:2017(E)
3.1.2.8
peak-to-peak sound pressure
pk-pk

sum of the peak compressional pressure (3.1.2.5) and the peak rarefactional pressure (3.1.2.7) during a

specified time interval, for a specified frequency range
Note 1 to entry: Peak-to-peak sound pressure is expressed in pascals (Pa).

Note 2 to entry: The start and end times used to determine the time interval for the peak compressional pressure

shall be the same as those used to determine the time interval for the peak rarefactional pressure.

3.1.2.9
sound particle displacement

displacement of a material element (3.1.1.5) caused by the action of sound (3.1.1.1)

Note 1 to entry: Sound particle displacement is a function of time, t, which may be indicated by means of an

argument t, as in δ(t).
Note 2 to entry: Sound particle displacement is expressed in metres (m).

Note 3 to entry: Sound particle displacement is a vector quantity. Spatial components of the sound particle

displacement may be indicated by assigning subscripts to the symbol. For example, in Cartesian coordinates,

δ = (δ , δ , δ ). By convention in underwater acoustics, the z axis is usually chosen to point vertically down from

x y z
the sea surface, with x and y axes in the horizontal plane.
[SOURCE: ISO 80000-8:2007, 8-10, modified]
3.1.2.10
sound particle velocity

contribution to velocity of a material element (3.1.1.5) caused by the action of sound (3.1.1.1)

Note 1 to entry: Sound particle velocity is a function of time, t, which may be indicated by means of an argument

t, as in u(t).

Note 2 to entry: For small-amplitude sound waves in an otherwise stationary medium, the sound particle velocity

and sound particle displacement (3.1.2.9) are related by
∂δδ
u =

where δ(t) is the sound particle displacement at time, t, and the partial derivative is evaluated at a fixed position. The

formula above is an approximation, with relative error of order |u/c|, where c is the speed of sound in the medium.

Note 3 to entry: Sound particle velocity is expressed in units of metre per second (m/s).

Note 4 to entry: Sound particle velocity is a vector quantity. Spatial components of the sound particle velocity

may be indicated by assigning subscripts to the symbol. For example, in Cartesian coordinates, u = (u , u , u ). By

x y z

convention in underwater acoustics, the z axis is usually chosen to point vertically down from the sea surface,

with x and y axes in the horizontal plane.
[SOURCE: ISO 80000-8:2007, 8-11, modified]
3.1.2.11
sound particle acceleration

contribution to acceleration of a material element (3.1.1.5) caused by the action of sound (3.1.1.1)

Note 1 to entry: Sound particle acceleration is a function of time, t, which may be indicated by means of an

argument t, as in a(t).
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ISO 18405:2017(E)

Note 2 to entry: For small-amplitude sound waves in an otherwise stationary medium, the sound particle

acceleration and sound particle velocity (3.1.2.10) are related by
a =

where u(t) is the sound particle velocity at time, t, and the partial derivative is evaluated at a fixed position. The

formula above is an approximation, with relative error of order |u/c|, where c is the speed of sound in the medium.

Note 3 to entry: Sound particle acceleration is expressed in units of metre per second squared (m/s ).

Note 4 to entry: Sound particle acceleration is a vector quantity. Spatial components of the sound particle

acceleration may be indicated by assigning subscripts to the symbol. For example, in Cartesian coordinates,

a = (a , a , a ). By convention in underwater acoustics, the z axis is usually chosen to point vertically down from

x y z
the sea surface, with x and y axes in the horizontal plane.
[SOURCE: ISO 80000-8:2007, 8-12, modified]
3.1.3 Acoustical power quantities
3.1.3.1
mean-square sound pressure

integral over a specified time interval of squared sound pressure (3.1.2.1), divided by the duration of the

time interval, for a specified frequency range
2 2 2

Note 1 to entry: In formula form, p = pt dt , where p(t) is the sound pressure, and t and t are

1 2
tt−

the start and end times, respectively. For a transient sound, the start and end times are sometimes chosen to

correspond to the start and end of the percentage energy signal duration (3.5.1.5).

Note 2 to entry: Mean-square sound pressure is expressed in units of pascal squared (Pa ).

Note 3 to entry: The square root of the mean-square sound pressure is a field quantity known as the root-mean-

square sound pressure. This field quantity may be denoted p .
rms
3.1.3.2
mean-square sound particle displacement

integral over a specified time interval of squared magnitude of the sound particle displacement (3.1.2.9),

divided by the duration of the time interval, for a specified frequency range
2 2 2

Note 1 to entry: In formula form, δδ= ttd , where δ(t) is the magnitude of the sound particle

tt−
displacement, and t and t are the start and end times, respectively.
1 2

Note 2 to entry: Mean-square sound particle displacement is expressed in units of metre squared (m ).

Note 3 to entry: The square root of the mean-square sound displacement is a field quantity known as the root-

mean-square sound displacement. This field quantity may be denoted δ .
rms
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ISO 18405:2017(E)
3.1.3.3
mean-square sound particle velocity

integral over a specified time interval of squared magnitude of the sound particle velocity (3.1.2.10),

divi
...

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