ISO 18405:2017

INTERNATIONAL ISO
STANDARD 18405
First edition
2017-04
Underwater acoustics — Terminology
Acoustique sous-marine — Terminologie
Reference number
©
ISO 2017
© 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
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved

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
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.
iv © ISO 2017 – All rights reserved

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
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
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
F
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
rp
1/2
root of P (see ISO 80000-1:2009), that is, F = P . The level of this root-power quantity is
rp
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
...