IEC 60318-3:2014

IEC 60318-3 ®
Edition 2.0 2014-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Simulators of human head and ear –
Part 3: Acoustic coupler for the calibration of supra-aural earphones used
in audiometry
Électroacoustique – Simulateurs de tête et d'oreille humaines –
Partie 3: Coupleur acoustique pour l’étalonnage des écouteurs supra-auraux
utilisés en audiométrie
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IEC 60318-3 ®
Edition 2.0 2014-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Simulators of human head and ear –

Part 3: Acoustic coupler for the calibration of supra-aural earphones used

in audiometry
Électroacoustique – Simulateurs de tête et d'oreille humaines –

Partie 3: Coupleur acoustique pour l’étalonnage des écouteurs supra-auraux

utilisés en audiométrie
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX M
ICS 17.140.50 ISBN 978-2-8322-1930-0

– 2 – IEC 60318-3:2014  IEC 2014
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Construction . 5
4.1 General . 5
4.2 Cavity dimensions . 6
4.3 Static pressure equalization . 8
4.4 Calibrated pressure type microphone . 8
5 Coupling of earphone to acoustic coupler . 9
6 Calibration . 9
6.1 Reference environmental conditions . 9
6.2 Method of calibration . 10
7 Maximum permitted uncertainty of measurements . 10
Bibliography . 12

Figure 1 – Dimensions of acoustic coupler . 7
Figure 2 – Coupling of earphone to coupler. 9

Table 1 – Height of the coupler as a function of the acoustic volume of the microphone . 8
Table 2 – Values of maximum permitted uncertainties U for a level of confidence of
max
approximately 95 % . 11

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS –
SIMULATORS OF HUMAN HEAD AND EAR –

Part 3: Acoustic coupler for the calibration
of supra-aural earphones used in audiometry

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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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.
International Standard IEC 60318-3 has been prepared by IEC technical committee 29:
Electroacoustics.
This second edition cancels and replaces the first edition published in 1998 and constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) adjustment of terms and wording to the other parts of IEC 60318,
b) introduction of maximum permitted uncertainties,
c) revised requirements for static pressure equalization.

– 4 – IEC 60318-3:2014  IEC 2014
The text of this standard is based on the following documents:
CDV Report on voting
29/796/CDV 29/811A/RVC
Full information on the voting for the approval of this standard 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.
A list of all the parts in the IEC 60318 series, published under the general title
Electroacoustics – Simulators of human head and ear can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability 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
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
ELECTROACOUSTICS –
SIMULATORS OF HUMAN HEAD AND EAR –

Part 3: Acoustic coupler for the calibration
of supra-aural earphones used in audiometry

1 Scope
This part of IEC 60318 specifies an acoustic coupler for the measurement of supra-aural
audiometric earphones in the frequency range from 125 Hz to 8 000 Hz.
The sound pressure developed by an earphone is not, in general, the same in the coupler as
in a person's ear. However, the acoustic coupler can be used as an objective and
reproducible means of measuring the output of supra-aural earphones. It can be used for
specifying reference equivalent threshold sound pressure levels (RETSPL) for the calibration
of audiometers.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61094-1, Measurement microphones – Part 1: Specifications for laboratory standard
microphones
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
3 Terms and definitions
For the purpose of this document, the following definition applies:
3.1
acoustic coupler
device for measuring the acoustic output of sound sources where the sound pressure is
measured by a calibrated microphone coupled to the source by a cavity of predetermined
shape and volume which does not necessarily approximate the acoustical impedance of the
normal human ear
4 Construction
4.1 General
The coupler consists essentially of a cylindrical cavity whose acoustic transfer impedance is
determined by the volume of air in the cavity and its dimensions (see 4.2). A microphone with
a diaphragm having high acoustic impedance is located in the base of the cylindrical cavity.
The coupler shall be made of a material that has no negative influences on its performance.
For example it should be acoustically hard and dimensionally stable. The general construction

– 6 – IEC 60318-3:2014  IEC 2014
of the coupler and mounting of the microphone shall aim to reduce the response to vibration
of any earphone or to sound outside the cavity.
In the following, the specified tolerance shall be reduced by an amount equal to the actual
expanded measurement uncertainty of the test laboratory before deciding if a device conforms
to this specification.
4.2 Cavity dimensions
The critical dimensions (see Figure 1) of the coupler are those that determine the shape and
volume of the cavity terminated by the microphone, the static pressure equalization
mechanism, the upper edge (lip) and the 45° angle.
3 3
The effective acoustic volume of the coupler shall be 5 780 mm ± 130 mm .
The diameter d shall be 23,825 mm ± 0,080 mm.
shall be 18,55 mm ± 0,16 mm.
Diameter d
The height l shall be 1,950 mm ± 0,065 mm.
The angle β defining the slope of the external part of the coupler shall be 45° ± 5,5°.
The height h of the coupler shall be calculated according to the following formula:
– –
V V V
1 eq 2
h = (1)
πd
where
V is the effective acoustic volume;
V is the equivalent acoustic volume of the microphone;
eq
V is the volume of the front cavity of the microphone, given by
V = πd ⋅l (2)
2 2 2
It is recommended that a value of h be selected from Table 1, according to the value of the
equivalent acoustic volume of the microphone.
NOTE 1 It is recognized that certain combinations of coupler cavity and microphone could cause problems if the
tolerance of the coupler diameter is exploited towards the smallest allowed diameter and at the same time, the
tolerance of the microphone diameter is exploited towards the largest allowed diameter. Practical experience
however, has proven that those problems are very unlikely to occur.

IEC
Dimension Specification
mm
A 73
B 57,2
C 25,27
d 23,825
d 18,55
l 1,95
h See 4.2, Equation.(1), Table 1
H 7,5
Angle In degrees
α
ß
Y
The dimensions printed in bold are the critical dimensions that determine the shape
and volume of the cavity terminated by the microphone, the capillary leak, the upper
edge (lip) and the 45° angle, see 4.2
Key
1 hole for thermometer
2 microphone
3 ground shield
4 contact pin
5 venting mechanism, see 4.3.
Figure 1 – Dimensions of acoustic coupler

– 8 – IEC 60318-3:2014  IEC 2014
Table 1 – Height of the coupler as a function of the acoustic volume of the microphone
Equivalent acoustic volume Height of the coupler
of the microphone
mm mm
0 < V ≤ 50 11,72 ± 0,15
eq
50 < V ≤ 100 11,62 ± 0,15
eq
100 < V ≤ 150 11,50 ± 0,15
eq
150 < V ≤ 200 11,39 ± 0,15
eq
NOTE 2 If the coupler described in this standard is constructed with a fixed value of h + l = 13,41 mm regardless
of the equivalent volume of the microphone, then the coupler will be identical with the NBS 9A coupler, see [1] .
The nominal effective acoustic volume of the type 9A coupler is:
V = V + 5 640 mm
1 eq
4.3 Static pressure equalization
Any change in the static pressure within the cavity caused by assembly of the earphone to the
coupler and microphone shall decay toward the static ambient pressure with a time constant
of less than 1,5 s. If this necessitates the introduction of a controlled leak in the coupler, it
shall have the following characteristics.
a) It shall not alter the cavity volume by more than 20 mm ;
b) It shall attenuate external sound reaching the cavity, with the entrance of the cavity
blocked, by at least 16 dB at 100 Hz, increasing by 6 dB per octave for increasing
frequency.
NOTE Equalization can be realized, for example, by a capillary tube with a diameter of 0,6 mm ± 0,05 mm
containing a wire with a diameter of 0,4 mm ± 0,05 mm.
4.4 Calibrated pressure type microphone
The overall pressure sensitivity level of the microphone and associated measuring system
(e.g. preamplifier) over the specified frequency range shall be known with a maximum
measurement uncertainty not exceeding 0,2 dB for a level of confidence of 95 %.
The internal shape of the base of the coupler shall correspond to that of the laboratory
standard (LS) configuration specified in IEC 61094-1.
NOTE A WS1P microphone conforming to IEC 61094-4 [2], without protective grid but with a coupler ring that
converts the microphone to an LS1P configuration can be used. WS2P microphones conforming to IEC 61094-4 [2]
can also be used, provided that the LS1 configuration and the stipulated volume are preserved, for example by
using an adaptor.
The microphone used shall have a high acoustic impedance, the equivalent acoustic volume
being less than 200 mm at frequencies between 125 Hz and 8 000 Hz. The manufacturer of
the microphone shall state the equivalent acoustic volume.
If it is necessary to use a microphone for which the diameter of the free part of the diaphragm
is less than the diameter of the coupler cavity, the axes of the microphone and the cylindrical
cavity shall coincide The microphone to be used shall be stated by the manufacturer.
There shall be an effective seal between the coupler and the microphone. However, any
obstruction in the static pressure equalization device of the microphone shall be avoided.
_______________
Numbers in square brackets refer to the Bibliography.

5 Coupling of earphone to acoustic coupler
The earphone to be calibrated shall be applied to the acoustic coupler without acoustic
leakage with a force 4,5 N ± 0,5 N, not including the weight of the earphone itself (see
Figure 2). If, for a specific earphone, a different coupling force is specified this shall be
stated.
The earphone shall not rest on the sloping side of the acoustic coupler, but only on the upper
edge (or lip, see Figure 2).
In the case of earphones with a hard earcap, a thin film of sealing material or thin soft rubber
ring should be used on the lip in order to produce an effective seal between the earphone and
the upper edge of the coupler.
NOTE With some earphones, a special adapter can be used for coupling the earphone to the coupler, e.g. see [3].
IEC
Key
1 coupling force between 4 N and 5 N
2 earphone
3 lip
4 coupler
5 microphone
Figure 2 – Coupling of earphone to coupler
6 Calibration
6.1 Reference environmental conditions
The reference environmental conditions are the following:

– 10 – IEC 60318-3:2014  IEC 2014
– static pressure: 101,325 kPa
– temperature: 23 °C
– relati
...