CLC/TS 61949:2008

SLOVENSKI STANDARD
01-oktober-2008
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Ultrasonics - Field characterization - In situ exposure estimation in finite-amplitude
ultrasonic beams (IEC/TS 61949:2007)
Ultraschall - Charakterisierung von Feldern - Schätzung der In-situ-Expositionswerte in
Ultraschallbündeln mit finiten Amplituden (IEC/TS 61949:2007)
Ultrasons - Caractérisation des champs - Estimation de l'exposition in situ dans les
faisceaux ultrasonores d'amplitude finie (CEI/TS 61949:2007)
Ta slovenski standard je istoveten z: CLC/TS 61949:2008
ICS:
17.140.50 Elektroakustika Electroacoustics
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL SPECIFICATION
CLC/TS 61949
SPÉCIFICATION TECHNIQUE
July 2008
TECHNISCHE SPEZIFIKATION
ICS 17.140.50
English version
Ultrasonics -
Field characterization -
In situ exposure estimation in finite-amplitude ultrasonic beams
(IEC/TS 61949:2007)
Ultrasons -  Ultraschall -
Caractérisation des champs - Charakterisierung von Feldern -
Estimation de l'exposition in situ Schätzung der In-situ-Expositionswerte in
dans les faisceaux ultrasonores Ultraschallbündeln mit finiten Amplituden
d'amplitude finie (IEC/TS 61949:2007)
(CEI/TS 61949:2007)
This Technical Specification was approved by CENELEC on 2008-05-01.

CENELEC members are required to announce the existence of this TS in the same way as for an EN and to
make the TS available promptly at national level in an appropriate form. It is permissible to keep conflicting
national standards in force.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Cyprus, the
Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2008 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TS 61949:2008 E
Foreword
The text of document 87/349/CDV, future edition 1 of IEC/TS 61949, prepared by IEC TC 87, Ultrasonics,
was submitted to the IEC-CENELEC Parallel Unique Acceptance Procedure and was approved by
CENELEC as CLC/TS 61949 on 2008-05-01.
The following date was fixed:
– latest date by which the existence of the CLC/TS
has to be announced at national level (doa) 2008-08-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the Technical Specification IEC/TS 61949:2007 was approved by CENELEC as a Technical
Specification without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60601-2-37 NOTE  Harmonized as EN 60601-2-37:2008 (not modified).
IEC 61828 NOTE  Harmonized as EN 61828:2001 (not modified).
IEC 62359 NOTE  Harmonized as EN 62359:2005 (not modified).
__________
- 3 - CLC/TS 61949:2008
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year
1) 2)
IEC 61161 - Ultrasonics - Power measurement - EN 61161 2007
Radiation force balances and performance
requirements
IEC 62127-1 2007 Ultrasonics - Hydrophones - EN 62127-1 2007
Part 1: Measurement and characterization of
medical ultrasonic fields up to 40 MHz

1)
Undated reference.
2)
Valid edition at date of issue.

IEC/TS 61949
Edition 1.0 2007-11
TECHNICAL
SPECIFICATION
Ultrasonics – Field characterization – In situ exposure estimation
in finite-amplitude ultrasonic beams

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
V
ICS 17.140.50 ISBN 2-8318-9463-8

– 2 – TS 61949 © IEC:2007(E)
CONTENTS
FOREWORD.3
INTRODUCTION.5

1 Scope.6
2 Normative references .6
3 Terms and definitions .6
4 List of symbols .10
5 Equipment required .11
6 Test methods .11
6.1 Establishing quasi-linear conditions.11
6.1.1 The local distortion parameter .11
6.1.2 Upper limit for quasi-linear conditions for σ .12
q
6.1.3 Range of applicability for quasi-linear conditions .12
6.2 Measurement procedure for estimated in situ exposure .13
6.2.1 Identification of quasi-linear conditions .13
6.2.2 Tables of limiting mean peak acoustic pressure.14
6.2.3 Measurement of acoustic quantities under quasi-linear conditions .14
6.2.4 Measurement of the scaling factor .14
6.2.5 Calculation of attenuated acoustic quantities .15
6.3 Uncertainties .16

Annex A (informative) Review of evidence .18
Annex B (informative) Review of alternative methods for managing finite-amplitude
effects during field measurement .21
Annex C (informative) Parameters to quantify nonlinearity .23
Annex D (informative) Tables of upper limits for mean peak acoustic pressure for
quasi-linear conditions .26

Bibliography.30

Figure 1 – Flow diagram for obtaining values of attenuated acoustic quantities.13

Table A.1 – Experimental evidence of nonlinear loss associated with the propagation
of ultrasound pulses under diagnostic conditions in water .19
Table A.2 – Theoretical evidence of nonlinear loss associated with the propagation of
ultrasound pulses under diagnostic conditions in water .19
Table B.1 – Methods for estimation of in-situ exposure in nonlinear beams.22
Table C.1 – Parameters for quantification of nonlinearity in an ultrasonic field .23
Table D.1 – The upper limit for mean peak acoustic pressure (MPa) associated with
quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 2,0 MHz .26

q awf
Table D.2 – The upper limit for mean peak acoustic pressure (MPa) associated with
quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 3,5 MHz .27

q awf
Table D.3 – The upper limit for mean peak acoustic pressure (MPa) associated with
quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 5,0 MHz .28

q awf
Table D.4 – The upper limit for mean peak acoustic pressure (MPa) associated with
quasi-linear conditions. σ ≤ 0,5. Acoustic working frequency, f = 7,0 MHz.29
awf
q
TS 61949 © IEC:2007(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ULTRASONICS –
FIELD CHARACTERIZATION –
IN SITU EXPOSURE ESTIMATION
IN FINITE-AMPLITUDE ULTRASONIC BEAMS

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• The subject is still under technical development or where, for any other reason, there is
the future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC 61949, which is a technical specification, has been prepared by IEC technical committee
87: Ultrasonics.
– 4 – TS 61949 © IEC:2007(E)
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
87/349/DTS 87/364A/RVC
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
This publication is being issued as a technical specification (according to 3.1.1.1 of the
IEC/ISO directives, Part 1) as a “prospective standard for provisional application” in the field
of finite-amplitude ultrasonic beams, because there is an urgent need for guidance on how
standards in this field should be used to meet an identified need.
This document is not to be regarded as an “International Standard”. It is proposed for
provisional application so that information and experience of its use in practice may be
gathered. Comments on the content of this document should be sent to the IEC Central
Office.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
TS 61949 © IEC:2007(E) – 5 –
INTRODUCTION
Acoustic waves of finite amplitude generate acoustic components at higher frequencies than
the fundamental frequency. This provides a mechanism for acoustic attenuation which is not
significant at lower acoustic pressure, and for which there is substantial experimental and
theoretical evidence (Tables A.1 and A.2). The generation of harmonic frequency
components, and their associated higher attenuation coefficient, can occur very strongly when
high amplitude pulses, associated with the use of ultrasound in medical diagnostic
applications, propagate through water. This fact is of importance when measurements of
acoustic pressure, made in water, are used to estimate acoustic pressure in another
medium, or when intensity derived from hydrophone measurements in water is used to
estimate intensity within another medium. In particular, errors occur in the estimation of the
acoustic pressure and intensity in situ, if it is assumed that the propagation of ultrasound
through water, and through tissue, is linear.
Standards for measurement of frequency-rich pulse waveforms in water are well established
(IEC 62127-1). Whilst means to quantify nonlinear behaviour of medical ultrasonic beams are
specified, no procedures are given for their use. Since that time IEC 60601-2-37 and
IEC 62359 have introduced “attenuated” acoustic quantities, which are derived from
measurements in water and intended to enable the estimation of in situ exposure for safety
purposes.
This Technical Specification describes means to allow “attenuated” acoustic quantities to be
calculated under conditions where the associated acoustic measurements, made in water
using standard procedures, may be accompanied by significant finite-amplitude effects. A
number of alternative methods have been proposed (Table B.1).The approach used in this
Technical Specification is aligned with the proposal of the World Federation for Ultrasound in
1)
Medicine and Biology [1] , that “Estimates of tissue field parameters at the point of interest
should be based on derated values calculated according to an appropriate specified model
and be extrapolated linearly from small signal characterization of source-field relationships.”
___________
1)
Figures in square brackets refer to the Bibliography.

– 6 – TS 61949 © IEC:2007(E)
ULTRASONICS –
FIELD CHARACTERIZATION –
IN SITU EXPOSURE ESTIMATION
IN FINITE-AMPLITUDE ULTRASONIC BEAMS

1 Scope
This Technical Specification establishes:
• the general concept of the limits of applicability of acoustic measurements in water
resulting from finite-amplitude acoustic effects;
• a method to ensure that measurements are made under quasi-linear conditions in order to
minimise finite-amplitude effects, which may be applied under the following conditions:
− to acoustic fields in the frequency range 0,5 MHz to 15 MHz;
− to acoustic fields generated by plane sources and focusing sources of amplitude gain
up to 12;
− at all depths for which the maximum acoustic pressure in the plane perpendicular to
the acoustic axis lies on the axis;
− to both circular and rectangular source geometries;
− to both continuous-wave and pulsed fields;
• the definition of an acoustic quantity appropriate for establishing quasi-linear conditions;
• a threshold value for the acoustic quantity as an upper limit for quasi-linear conditions;
• a method for the estimation of attenuated acoustic quantities under conditions of nonlinear
propagation in water.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 61161, Ultrasonics – Power – Radiation force balances and performance requirements
IEC 62127-1:2007 Ultrasonics – Hydrophones – Part 1: Measurement and characterization of
medical ultrasonic fields up to 40 MHz
3 Terms and definitions
For the purposes of this document, the following definitions apply.
3.1
acoustic attenuation coefficient
coefficient intended to account for ultrasonic attenuation of tissue between the source and a
specified point
Symbol: α
–1 –1
Unit: decibels per centimetre per megahertz, dB cm MHz
[IEC 62359, definition 3.1]
TS 61949 © IEC:2007(E) – 7 –
3.2
acoustic pressure
pressure minus the ambient pressure at a particular instant in time and at a particular point in
the acoustic field
Symbol: p
Unit: pascal, Pa
[IEC 62127-1, definition 3.33, modified]
3.3
acoustic working frequency
arithmetic mean of the frequencies, f and f , at which the acoustic pressure spectrum is 3 dB
1 2
below the peak value
Symbol: f
awf
Unit: Hertz, Hz
[IEC 62127-1, definition 3.3.2, modified]
3.4
attenuated acoustic pulse waveform
the temporal waveform of the instantaneous acoustic pressure calculated in a specified
attenuation model and at a specified point. See 3.1 of IEC 62127-1 for acoustic pulse
waveform
Symbol: p (t)
α
Unit: pascal, Pa
3.5
attenuated acoustic power
value of the acoustic output power calculated for a specified attenuation model and at a
specified point
Symbol: P
α
Unit: watt, W
[IEC 62359, definition 3.3]
3.6
attenuated peak-rarefactional acoustic pressure
the peak-rarefactional acoustic pressure calculated in a specified attenuation model and at
a specified point
Symbol: p
, α
r
Unit: pascal, Pa
[IEC 62359, definition 3.4, modified]
3.7
attenuated pulse-intensity integral
the pulse-intensity integral calculated for a specified attenuation model and at a specified
point
Symbol: I
, α
pi
–2
Unit: joule per metre squared, J m
[IEC 62359, definition 3.6, modified]

– 8 – TS 61949 © IEC:2007(E)
3.8
attenuated spatial-peak temporal-average intensity
the spatial-peak temporal-average intensity calculated in a specified attenuation model
Symbol: I
, α
spta
–2
Unit: watt per metre squared, W m
[IEC 62359, definition 3.7, modified]
3.9
attenuated temporal-average intensity
the temporal-average intensity calculated in a specified attenuation model and at a specified
point
Symbol: I
, α
ta
–2
Unit: watt per metre squared, W m
[IEC 62359, definition 3.8, modified]
3.10
beam area
area in a specified plane perpendicular to the beam axis consisting of all points at which the
pulse-pressure-squared integral is greater than a specified fraction of the maximum value of
the pulse-pressure-squared integral in that plane
Symbol: A
b
Unit: metre squared, m
[IEC 62127-1, definition 3.7, modified]
3.11
local area factor
the square root of the ratio of the source aperture to the beam area at the point of interest.
The relevant beam area, A , is that for which the maximum pulse-pressure-squared integral is
b
greater than 0,135 (that is, 1/e ) times the maximum value in the cross-section. If the beam
area at the ־6 dB level, A , is known, the beam area can be calculated as
b,–6dB
A = A /0,69: (0,69 = 3ln(10)/10).
,
b b –6dB
0,69A
SAeff
F = .
a
A
b,−6dB
Symbol: F
a
3.12
local distortion parameter
an index which permits the prediction of nonlinear propagation effects along the axis of a
focused beam. The local distortion parameter is calculated according to 6.1.1
Symbol: σ
q
3.13
mean peak acoustic pressure
the arithmetic mean of the peak-rarefactional acoustic pressure and the peak-
compressional acoustic pressure
Symbol: p
m
Unit: pascal, Pa
TS 61949 © IEC:2007(E) – 9 –
3.14
nonlinear threshold value
a value of any nonlinear propagation index Y, such that for Y≤τ the beam has quasi-linear

characteristics at the selected point and for Y>τ the beam has nonlinear characteristics at the

selected point
Symbol: τ
3.15
peak-rarefactional acoustic pressure
maximum of the modulus of the negative instantaneous acoustic pressure in an acoustic
field or in a specified plane during an acoustic repetition period. Peak-rarefactional acoustic
pressure is expressed as a positive number
Symbol: p
r
Unit: pascal, Pa.
[IEC 62127-1, definition 3.44, modified]
3.16
peak-compressional acoustic pressure
maximum positive instantaneous acoustic pressure in an acoustic field or in a specified
plane during an acoustic repetition period
Symbol: p
c
Unit: pascal, Pa.
[IEC 62127-1, definition 3.45, modified]
3.17
quasi-linear
a condition of the ultrasonic field between the source and a plane at a specified axial depth
for which, at every point, less than a specified, small proportion of the energy has transferred
from the fundamental frequency to other frequencies through nonlinear propagation effects.
3.18
scaling factor
the ratio between the mean peak acoustic pressure at a location close to the transducer to
the mean peak acoustic pressure at the same location under quasi-linear conditions, where
quasi-linearity is determined at the point of interest.
Symbol: S
3.19
source aperture
equivalent aperture for an ultrasonic transducer, measured within the –20 dB pulse-pressure-
squared-integral contour, in the source aperture plane
Symbol: A
SAeff
Unit: metre squared, m
[IEC 61828, definition 4.2.65, modified]

– 10 – TS 61949 © IEC:2007(E)
3.20
source aperture plane
closest possible measurement plane to the external transducer aperture that is perpendicular
to the beam axis
[IEC 61828, definition 4.2.67]
3.21
transducer aperture width
full width of the transducer aperture along a specified axis orthogonal to the beam axis
Symbol: L
TA
Unit: metre, m
[IEC 61828, definition 4.2.74, modified]
4 List of symbols
α  acoustic attenuation coefficient
A  beam area
b
A source aperture
SAeff
β  nonlinearity parameter for water, ≅ 3,5
c  speed of sound
f  acoustic working frequency
awf
F  local area factor
a
I attenuated pulse-intensity integral
, α
pi
I  reduced pulse-intensity integral
pi,q
I attenuated spatial-peak temporal-average intensity
,α
spta
I  reduced spatial-peak temporal-average intensity
spta,q
I attenuated temporal-average intensity
,α
ta
I  reduced temporal-average intensity
ta,q
L  discontinuity length
L transducer aperture width
TA
P  total acoustic output power
P attenuated acoustic power
α
p  acoustic pressure
p (t) attenuated acoustic pulse waveform
α
p (t) acoustic pulse waveform under quasi-linear conditions
q
p  peak-compressional acoustic pressure
c
p  peak-compressional acoustic pressure close to the source for scaling
c,s
p pre-correction peak-compressional acoustic pressure close to the source for
c,s,m
scaling
p  reduced peak-compressional acoustic pressure close to the source for scaling
c,s,q
p  mean peak acoustic pressure
m
p  peak-rarefactional acoustic pressure
r
p attenuated peak-rarefactional acoustic pressure
,α
r
p  reduced peak-rarefactional acoustic pressure
r,q
TS 61949 © IEC:2007(E) – 11 –
p  pre-correction peak-rarefactional acoustic pressure close to the source for
r,s,m
scaling
p reduced peak-rarefactional acoustic pressure close to the source for scaling
r,s,q
ρ  density
S  scaling factor
σ  local distortion parameter
q
t pulse duration
d
nonlinear threshold value
τ
τ nonlinear threshold for σ
q q
Y  any nonlinear index
z  axial distance of the point of interest from the transducer face
5 Equipment required
Measurements of the acoustic pulse waveform shall be carried out using hydrophones in
water, as specified in IEC 62127-1.
Measurement of acoustic output power shall be made using planar scanning by means of a
hydrophone. The methods described in this document do not apply to measurements of
acoustic output power using a radiation force balance as specified in IEC 61161.
6 Test methods
The following method shall be used for measurement of acoustic quantities, using
hydrophones in water, when these measurements are to be used for subsequent calculation
of attenuated peak-rarefactional acoustic pressure, attenuated pulse-intensity integral,
attenuated temporal-average intensity, and attenuated acoustic power.
6.1 Establishing quasi-linear conditions
6.1.1 The local distortion parameter
For the purpose of measurement at any axial point of interest, the local distortion
parameter, σ , is calculated from the measured pulse waveform in water from the following
q
expression
2πf β
awf
σ = zp (1)
q m
F
ρ
a
c
where
p is the mean peak acoustic pressure (p +p )/2
m r c
p is the peak-rarefactional acoustic pressure at the point of interest
r
p is the peak-compressional acoustic pressure at the point of interest
c
z is the axial distance of the point of interest from the transducer face
f is the acoustic working frequency

awf
β is the nonlinearity parameter for water, ≅ 3,5
F is the local area factor
a
NOTE 1 For 2< F <12, σ ≅σ , where σ is the nonlinear propagation parameter at the focus as defined in
a q m m
IEC 62127-1. Also see Annex C.

– 12 – TS 61949 © IEC:2007(E)
NOTE 2 F = 2 may be associated with an unfocussed field. In an unfocussed field from a circular source, the
a
maximum axial amplitude is twice the acoustic pressure amplitude at the source.
NOTE 3 Alternative quantities to σ , that have been proposed elsewhere, are summarized in Table C.1
q
NOTE 4 Under some conditions, a value for the local area factor may not be available conveniently. Under these
conditions a conservative value F = 2 may be used.
a
6.1.2 Upper limit for quasi-linear conditions for σ
q
The field conditions shall be defined as quasi-linear if σ ≤ τ . τ is the nonlinear threshold

q q
q
for σ
q
For the purpose of this document, τ = 0,5 .
q
NOTE τ = 0,5 is the condition for which approximately 10 % of the energy (5 % of the amplitude at the acoustic
q
working frequency) has been transferred from the fundamental spectrum due to nonlinear propagation: see
Annex A.
6.1.3 Range of applicability for quasi-linear conditions
The procedures to establish quasi-linear conditions are applicable at all depths for which the
maximum mean peak acoustic pressure in the plane perpendicular to the acoustic axis lies
on the axis. Furthermore, having established quasi-linear conditions at any particular axial
point, together with an associated scaling factor, these conditions and scaling factor may
be used for measurements at all axial positions between the transducer and the selected
point. The procedures do not establish quasi-linear conditions for axial positions further from
the transducer than the selected point.
More generally, a procedure to establish quasi-linear conditions for all axial points of interest
in any particular field may be easily applied. This may be carried out by selecting a
measurement point at an axial distance greater than those of all axial points of interest in the
field under consideration. For example, for the purpose of establishing field maxima of any
acoustic quantity in a spherically-focused field, a single measurement point at the focus
should be sufficient. For fields with astigmatic focusing created, for example, by rectangular
ultrasound sources such as those commonly used for medical diagnostic purposes, for which
two focal depths exist, quasi-linear conditions shall be established at the focus of greater
axial distance from the transducer.

TS 61949 © IEC:2007(E) – 13 –
6.2 Measurement procedure for estimated in situ exposure
Figure 1 shows the principle of the measurement procedure, which has four stages:
a) identification of quasi-linear conditions;
b) measurement under quasi-linear conditions;
c) measurement of the scaling factor;
d) calculation of attenuated acoustic quantities.

Start
Set non-linear Are measurement Attenuate
output
threshold value conditions quasi-linear?
NO
YES
Measure and record the
acoustic pressure
waveform at the point of
interest
Measure pre-correction
and quasi-linear mean
peak acoustic
pressures at the source
Calculate and apply the
scaling factor
Select a tissue
exposure model
Calculate attenuated
acoustic quantities
IEC  2297/07
Figure 1 – Flow diagram for obtaining values of attenuated acoustic quantities
6.2.1 Identification of quasi-linear conditions
A calibrated hydrophone is positioned at the point of interest. The output from the transducer
is adjusted until the calculated value of σ lies within the criterion set in 6.1.2.
q
NOTE 1 Output may be adjusted either by reducing the voltage applied to the transducer, or with appropriate
acoustic attenuators [20, 21]. Where voltage control is used, the device should not alter the output by changing the
number of active elements, nor the weighting applied to them. Furthermore, caution and care should be taken to
check for and avoid pulser and/or nonlinear electromechanical transducer effects.

– 14 – TS 61949 © IEC:2007(E)
NOTE 2 For multi-mode conditions and for multiple focal zones, quasi-linear conditions must be identified for
each separate beam.
6.2.2 Tables of limiting mean peak acoustic pressure
For practical purposes, it may be more convenient to apply a threshold to the measured mean
peak acoustic pressure. Examples of threshold values of mean peak acoustic pressure,
calculated using the expression for σ given in 6.1.1, are given, for σ = 0,5, in Tables D.1,
q q
D.2, D.3 and D.4. This approach is equivalent to that given in 6.2.1 for the values of acoustic
field quantities given. For the purposes of this technical specification, quasi-linear conditions
may be considered as being established if the measured mean peak acoustic pressure is
equal to, or less than, the appropriate value given in these tables.
6.2.3 Measurement of acoustic quantities under quasi-linear conditions
The acoustic pulse waveform under quasi-linear conditions p (t) is calculated from the
q
hydrophone voltage waveform, measured using the methods given in IEC 62127-1. The term
“reduced” and the suffix q is used to refer to measurements associated with these conditions.
For purposes of calculation, the following acoustic measures are derived from the acoustic
pulse waveform under quasi-linear conditions, p (t): the reduced peak-rarefactional
q
acoustic pressure p , the reduced pulse-intensity integral I , the reduced temporal-
r,q pi,q
average intensity I and the reduced spatial-peak temporal-average intensity I .
ta,q spta,q
The reduced acoustic output power P is the acoustic output power under quasi-linear

q
conditions.
6.2.4 Measurement of the scaling factor
A scaling factor S is calculated from measurements of mean peak acoustic pressure close
to the source. The hydrophone is located within the source aperture approximately on the
acoustic axis.
6.2.4.1 Criteria for positioning the hydrophone close to the transducer
The position of the source aperture plane, in which the hydrophone measurements close to
the transducer are made, shall conform, as far as may be practical, to the following
requirements.
a) The distance between the transducer and the hydrophone shall be small enough to reduce
to a minimum the effects due to non-linear propagation. An appropriate criterion, based on
the discontinuity length L, is z < 0,1 L, where
⎛ ⎞
ρ
c
⎜ ⎟
L = (2)
⎜ ⎟
2π p f β
r,s,m awf
⎝ ⎠
in which p is the pre-correction peak-rarefactional acoustic pressure, and β is the
r,s,m
nonlinearity parameter for water, ≅ 3,5.
b) The distance between the transducer and the hydrophone shall be large enough to
prevent direct interference between the acoustically-generated hydrophone voltage and
electromagnetic coupling, and also from signals created from multiply-reflected acoustic
pulses. An appropriate criterion is z > t c where t is the pulse duration.
d d
c) The hydrophone shall be located in a position where the spatial rate of change in the
mean peak acoustic pressure is low in either direction perpendicular to the acoustic
axis. An appropriate criterion is that movement of the hydrophone by a distance equal to
its diameter shall result in a change in the peak hydrophone voltage of no more than 10 %.
Such conditions exist at axial points very close to a pulsed transducer where there is no
overlap in time between the edge and forward components in the wave. The range of
depth, z, for which this applies is approximately

TS 61949 © IEC:2007(E) – 15 –
L
2 2
TA
( ) − (t c)
d
0 < z < (3)
2t c
d
where L is the shortest possible transducer aperture width.
TA
NOTE For long pulse and continuous wave fields, condition (b) is not applied. In addition, whilst the general
condition (c) applies, the criterion given in Equation 3 may not be applicable, and an appropriate position should be
established by observation.
6.2.4.2 Measurement procedure
The procedure may be applied to any field measurement for which the criterion of 6.1.2 is not
met. Values of acoustic field quantities under these conditions are referred to as pre-
correction values. They may be, but are not restricted to, values measured under maximum
possible output conditions.
With the hydrophone positioned as specified in 6.2.4.1, measurements of peak-
compressional acoustic pressure and peak-rarefactional acoustic pressure are made
under two output conditions, pre-correction and reduced.
The reduced output condition is identical to that established under 6.2.1, for which the peak-
compressional acoustic pressure is p and the peak-rarefactional acoustic pressure is
c,s,q
p .
r,s,q
The pre-correction output condition is established by removing any attenuation associated
with the reduced output condition, whether this has been applied by voltage control or by
acoustic attenuators. All other controls shall remain unaltered. Under these conditions the
peak-compressional acoustic pressure is p and the peak-rarefactional acoustic
c,s,m
pressure is p .
r,s,m
The mean peak acoustic pressures under reduced and pre-correction conditions are
calculated as follows:
(p + p )
c,s,q r,s,q
p = (4)
m,s,q
and
( )
p + p
c,s,m r,s,m
p =. (5)
m,s,m
The scaling factor S is calculated as follows:
p
m,s.m
S =. (6)
p
m,s,q
6.2.5 Calculation of attenuated acoustic quantities
A linear homogeneous tissue exposure model with acoustic attenuation coefficient α is
used for the calculation of attenuated quantities.
NOTE To be aligned with other standards, the value of the acoustic attenuation coefficient, α, of the linear
–1
homogeneous tissue exposure model shall be 0,3 dB(cm MHz) . The acoustic attenuation coefficient of water is
assumed to be negligible; this condition applies when the nonlinear effects are much greater than the absorption as
estimated by the Goldberg number [25]. However, the method described is general, and any appropriate

– 16 – TS 61949 © IEC:2007(E)
attenuation model could, in principle, be used. For example, absorption dependent on a frequency power law
y
–1 –y
[32,33] can be used where α(f ) = α|f | in which α is an absorption constant in dB(cm ·MHz ) and y is an
awfawf 0
exponent usually between 1 and 2 for tissue and equal to 2 for water. To extend the equations for pressure and
intensity below, α f can be replaced by α(f ).
0 awf awf
For consistency, it is recommended that calculations in this section use practical units of
–1
dB (cm MHz) for acoustic attenuation coefficient, cm for the depth to the point of interest,
and MHz for acoustic working frequency.
The attenuated acoustic pulse waveform p (t) at a point of interest at depth z shall be
α
calculated as follows:
(−α zf / 20)
awf
p (t) = S ⋅ p (t)10 . (7)
α q
The attenuated peak-rarefactional acoustic pressure p at a point of interest at depth z
r,α
shall be calculated as follows:
(−α zf / 20)
awf
p (z) = S ⋅ p (z)10. (8)
r,α r,q
The attenuated pulse-intensity integral I at a point of interest at depth z shall be
pi,a
calculated as follows:
2 (−α zf /10)
awf
I (z) = S ⋅ I (z)10. (9)
pi,α pi,q
The attenuated temporal-average intensity I at a point of interest at depth z shall be
ta,α
calculated as follows:
(−α zf /10)
awf
I (z) = S ⋅ I (z)10. (10)
ta,α ta,q
The attenuated acoustic power, P , at a point of interest at depth z, shall be calculated as
α
follows:
(−α zf /10)
awf
P (z) = S ⋅ P 10. (11)
α q
NOTE If the pre-correction output power, P, has been measured close to the transducer, then the relationship
(−α zf /10)
awf
P (z) = P ⋅10 is equivalent to Equation 11 and there is no need to measure P .
α q
The attenuated spatial-peak temporal-average intensity, I at a point of interest at depth z
,α
spta
shall be calculated as follows:
2 (−α zf /10)
awf
I (z) = S ⋅ I (z)10. (12)
spta,α spta,q
6.3 Uncertainties
Guidance on assessment of uncertainties associated with the use of hydrophones is given in
of IEC 62127-1, and in the ISO Guide to the expression of uncertainty in measurement.
In calculating the value of the local distortion parameter, consideration shall be given to
uncertainties in the measurement of acoustic working frequency, mean peak acoustic
pressure, distance and local area factor. Greatest uncertainties may be expected to be
associated with the local area factor. An overall uncertainty in σ of ±50 % may be tolerated.
q
For nonlinear indicators of the form σ = fn(f ,z), such as σ , spectral distortion changes only
awf q
TS 61949 © IEC:2007(E) – 17 –
slowly in the range σ < 1. Therefore a large uncertainty is acceptable for a test for σ = 0,5,
q
for the purpose of defining quasi-linear conditions.
Uncertainty in the measurement of the scaling factor depends upon the separate
uncertainties in the measurements of mean peak acoustic pressures at the source under
quasi-linear and pre-correction conditions. In view of the requirement to position the
hydrophone close to the transducer, and the requirement to make measurements at low peak
acoustic pressures, electrical noise may be of particular concern. Hydrophone noise and
electrical pick-up can be evaluated by blocking the acoustic beam with a closed-cell air-filled
foam or a very good absorber. For pulsed operation, coherent electrical pick-up caused by the
electrical firing of the transducer should be windowed out by placing the acoustic signal at the
start of the acquisition window. Errors arising from temporal stability of output between
measurements should be assessed. Positioning errors may be minimized if the two
measurements required to calculate the scaling factor are carried out without repositioning
the hydrophone. Other potential sources of error are listed in Annex I of IEC 62127-1.

– 18 – TS 61949 © IEC:2007(E)
Annex A
(informative)
Review of evidence
A.1 Excess energy loss in water at high acoustic amplitudes
Acoustic waves of finite amplitude generate acoustic components at frequencies higher than
the fundamental [2]. This provides a mechanism for acoustic attenuation that is absent from
waves of low peak acoustic pressure [3,4]. The generation of harmonic frequency
components, and their associated higher attenuation coefficient, can occur very strongly when
high amplitude pulses associated with diagnostic ultrasound equipment propagate through
water. This fact is of importance when in situ exposure is to be predicted from measurements
made in water using hydrophones.
There is ample theoretical and experimental evidence that the propagation in water of
ultrasound pulses at biomedical frequencies and pressure amplitudes has associated with it a
loss of acoustic energy in excess of that from the fundamental frequency propagation. A
summary of some of this evidence is given in Tables A.1 and A.2. The loss has been
demonstrated by means of a reduction in the radiation force on a target as the distance
between the transducer and the target is increased [5], demonstrating a loss in acoustic
power with distance. Similarly, the axial profile in water is dependent on pulse amplitude, and
the comparison of profiles at high and low pulse amplitude has been used to
...