EN 61094-3:1995
(Main)Measurement microphones - Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique
Measurement microphones - Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique
Applies to laboratory standard microphones meeting the requirements of IEC 61094-1. The principles of the method are applicable to other types of microphones. In particular, microphones which fulfil the requirements of IEC 61094-1, when fitted with a special adaptor, may also be calibrated according to this standard when the adaptor is removed. Specifies a primary method of determining the free-field sensitivity so as to establish a reproducible and accurate basis for the measurement of sound pressure under free-field conditions.[]This publication supersedes IEC 60486.
Meßmikrofone - Teil 3: Primärverfahren zur Freifeld-Kalibrierung von Laboratoriums-Normalmikrofonen nach der Reziprozitätsmethode
Microphones de mesure - Partie 3: Méthode primaire pour l'étalonnage en champ libre des microphones étalons de laboratoire par la méthode de réciprocité
S'applique aux microphones étalons de laboratoire répondant aux prescriptions de la CEI 61094-1. Les principes de la méthode sont applicables à d'autres types de microphones. En particulier, les microphones qui répondent aux prescriptons de la CEI 61094-1, quand ils sont munis d'un adaptateur spécial, peuvent aussi être étalonnés selon cette norme quand l'adapteur est enlevé. Spécifie une méthode primaire de détermination de l'efficacité en champ libre de manière à établir une base reproductible et exacte pour la mesure de la pression acoustique dans des conditions de champ libre.[]Cette norme remplace la CEI 60486.
Measurement microphones - Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique (IEC 61094-3:1995)
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Standards Content (Sample)
SLOVENSKI STANDARD
01-september-2002
Measurement microphones - Part 3: Primary method for free-field calibration of
laboratory standard microphones by the reciprocity technique (IEC 61094-3:1995)
Measurement microphones -- Part 3: Primary method for free-field calibration of
laboratory standard microphones by the reciprocity technique
Meßmikrofone -- Teil 3: Primärverfahren zur Freifeld-Kalibrierung von Laboratoriums-
Normalmikrofonen nach der Reziprozitätsmethode
Microphones de mesure -- Partie 3: Méthode primaire pour l'étalonnage en champ libre
des microphones étalons de laboratoire par la méthode de réciprocité
Ta slovenski standard je istoveten z: EN 61094-3:1995
ICS:
17.140.50 Elektroakustika Electroacoustics
33.160.50 Pribor Accessories
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
CEI
NORME
IEC
INTERNATIONALE
1094-3
INTERNATIONAL
Première édition
STANDARD
First edition
1995-11
Microphones de mesure
Partie 3:
Méthode primaire pour l'étalonnage en champ libre
des microphones étalons de laboratoire
par la méthode de réciprocité
Measurement microphone
Part 3:
Primary method for free-field calibration
of laboratory standard microphones
by the reciprocity technique
de reproduction réservés — Copyright — all rights reserved
© CEI 1995 Droits
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1094-3 ©IEC:1995 - 3 -
CONTENTS
Page
FOREWORD 5
Clause
1 Scope 7
2 Normative references 7
3 Definitions 7
3.1 Reciprocal microphone 7
3.2 Phase angle of free-field sensitivity of a microphone 9
3.3 Acoustic centre of a microphone 9
3.4 Equivalent point-transducer 9
3.5 Electrical transfer impedance 9
3.6 Acoustical transfer impedance 9
3.7 Principal axis of a microphone 9
3.8 Free-field conditions 9
4 Reference environmental conditions
5 Principles of free-field calibration by reciprocity 11
5.1 General principle
5.2 Basic expressions
5.3 Insert voltage technique
5.4 Free-field receiving characteristics of a microphone
5.5 Free-field transmitting characteristics of a microphone 15
5.6 Reciprocity procedure
5.7 Final expressions for the free-field sensitivity
6 Factors influencing the free-field sensitivity 19
6.1 General 19
6.2 Polarizing voltage
6.3 Ground shield reference configuration
6.4 Acoustic conditions
6.5 Position of the acoustic centre of a microphone
6.6 Dependence on environmental conditions 21
7 Calibration uncertainty components
7.1 General
7.2 Electrical transfer impedance 23
7.3 Attenuation of sound in air
7.4 Deviations from ideal field conditions 25
7.5 Polarizing voltage
7.6 Physical quantities
7.7 Uncertainty on free-field sensitivity level
Annexes
A Values for the position of the acoustic centres of microphones.
Values of the air attenuation coefficients 29
B
C Environmental influence on the sensitivity of microphones
1094-3 ©IEC:1995 - 5 -
INTERNATIONAL ELECTROTECHNICAL COMMISSION
MEASUREMENT MICROPHONES
Part 3: Primary method for free-field calibration
of laboratory standard microphones
by the reciprocity technique
FOREWORD
1) The IEC (Inte rnational Electrotecnical Commission) is a world-wide organization for standardization comprising all national
electrotechnical Committees (IEC National Committees). The object of the IEC is to promote international cooperation on
all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities,
the IEC publishes International Standards. 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-govemmental organizations liaising with the IEC also participate in this preparation. The 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 the IEC on technical matters, prepared by technical committees on which all the
National Committees having a special interest therein are represented, express, as nearly as possible, an international
consensus of opinion on the subjects dealt with.
They have the form of recommendations for inte rnational use published in the form of standards, technical repo rts or guides
3)
and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National committees undertake to apply IEC International Standards
transparently to the maximum extent possible in their national and regional standards. Any divergence between the IEC
Standard and the corresponding national or regional standard shall be clearly indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment
declared to be in conformity with one of its standards.
International Standard IEC 1094-3 has been prepared by IEC technical committee 29:
Electroacoustics.
of IEC 1094 cancels and replaces IEC 486 published in 1974.
This part
The text of this part is based on the following documents:
DIS Report on voting
29/294/DIS 29/311/RVD
Full information on the voting for the approval of this standard can be found in the report on voting
indicated in the above table.
IEC 1094 consists of the following parts, under the general title Measurement microphones:
– Part 1: 1992, Specifications for laboratory standard microphones
– Part 2: 1992, Primary method for pressure calibration of laboratory standard microphones by the
reciprocity technique
– Part 3: 1995, Primary method for free-field calibration of laboratory standard microphones by the
reciprocity technique
– Part 4: 1995, Specifications for working standard microphones.
Annexes A, B and C are for information only.
1094-3 ©IEC:1995 -
7 -
MEASUREMENT MICROPHONES
Part 3: Primary method for free-field calibration
of laboratory standard microphones
by the reciprocity technique
1 Scope
This part of IEC 1094 is applicable to laboratory standard microphones meeting the requirements of
IEC 1094-1. The principles of the method are applicable to other types of microphones. In pa rticular,
microphones which fulfil the requirements of IEC 1094-1, when fitted with a special adaptor, may also be
calibrated according to this standard when the adaptor is removed.
This part of IEC 1094 specifies a primary method of determining the free-field sensitivity so as to establish
a reproducible and accurate basis for the measurement of sound pressure under free-field conditions.
This part of IEC 1094 is intended for use by laboratories with highly experienced staff and specialized
equipment.
2 Normative references
The following normative documents contain provisions, which through reference in this text, constitute pro-
of IEC 1094. At the time of publication, the editions indicated were valid. All normative
visions of this part
documents are subject to revision, and parties to agreements based on this part of IEC 1094 are encou-
raged to investigate the possibility of applying the most recent editions of the normative documents
indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
IEC 27-2: 1972, Letter symbols to be used in electrical technology - Pa rt 2: Telecommunications and
electronics
IEC 50 (801): 1994, International Electrotechnical Vocabulary (lEV) - Chapter 801: Acoustics and electro-
acoustics
rt 1: Specifications for laboratory standard microphones
IEC 1094-1:1992, Measurement microphones - Pa
IEC 1094-2: 1992, Measurement microphones - Part 2: Primary method for pressure calibration of
laboratory standard microphones by the reciprocity technique
Acoustics -Attenuation of sound during propagation outdoors - Pa rt 1: Calculation of the
ISO 9613-1: 1993,
absorption of sound by the atmosphere
ISO: 1993, Guide to the expression of uncertainty of measurements
3 Definitions
For the purpose of this pa rt of IEC 1094, the following definitions apply in addition to the definitions given
in IEC 1094-1.
A linear passive microphone for which the open-circuit reverse and forward
3.1 reciprocal microphone:
transfer impedances (see 206 in IEC 27-2) are equal in magnitude.
1094-3 ©IEC:1995 - 9 -
3.2
phase angle of free-field sensitivity of a microphone: For a sinusoidal plane progressive wave
of given frequency, for a specified direction of sound incidence and for given environmental conditions, the
phase angle between the open-circuit voltage and the sound pressure that would exist at the position of the
acoustic centre of the microphone in the absence of the microphone.
Unit: degree or radian (° or rad)
3.3 acoustic centre of a microphone: For a sound-emitting transducer, for a sinusoidal signal of given
frequency and for a specified direction and distance, the point from which the approximately spherical
wavefronts, as observed in a small region around the observation point, appear to diverge.
NOTES
1 The acoustic centre of a reciprocal transducer when used as a receiver is coincident with the acoustic centre when used
as a transmitter.
2 This definition only applies to regions of the sound field where spherical, or approximately spherical wavefronts are observed.
3.4 Transducer which, when located at the position of an acoustic centre
equivalent point-transducer:
of a microphone, simulates the transmitting and receiving characteristics of that microphone for a given
direction and range of distance.
3.5 electrical transfer impedance: For a system of two acoustically coupled microphones, the quotient
of the open-circuit voltage of the microphone used as receiver by the input current through the electrical
terminals of the microphone used as transmitter.
Unit: ohm (0)
NOTE - This impedance is defined for the ground-shield configuration given in 7.2 of IEC 1094-1.
3.6 acoustical transfer impedance: For a system of two acoustically coupled microphones, the quotient
of the sound pressure acting on the diaphragm of the microphone used as receiver by the short-circuit
volume velocity produced by the microphone used as transmitter.
Unit: pascal second per cubic metre (Pa•s/m3)
3.7 principal axis of a microphone: Line through the centre of and perpendicular to the diaphragm of
the microphone.
3.8 free-field conditions: Free-field conditions prevail when a sound wave can propagate freely without
disturbances of any kind.
4 Reference environmental conditions
The reference environmental conditions are:
- temperature t = 23,0 °C;
static pressure = 101,325 kPa;
ps.r
- relative humidity Hr. = 50 %.
NOTE - The reference temperature is chosen to be 23,0 °C because practical considerations require that most calibrations
be carried out at, or near, this temperature.
1094-3 ©IEC:1995 - 11 -
5 Principles of free-field calibration by reciprocity
5.1 General principle
A reciprocity calibration of microphones may be carried out by means of three microphones, two of which
shall be reciprocal, or by means of an auxiliary sound source and two microphones, one of which shall be
reciprocal.
NOTE - If one of the microphones is not reciprocal, it can only be used as a sound receiver.
5.1.1 General principles using three microphones
Let two of the microphones be connected acoustically under free-field conditions. Using one of them as a
sound source and the other as a sound receiver, the electrical transfer impedance is measured. When the
acoustic transfer impedance of the system is known, the product of the free-field sensitivities of the two
coupled microphones can be determined. Using pair-wise combinations of the microphones (1), (2) and
(3), three such mutually independent products are available, from which an expression for the free-field
sensitivity of each of the three microphones can be derived.
5.1.2 General principles using two microphones and an auxiliary sound source
th
First, let e two microphones be connected acoustically under free-field conditions, and the product of the
free-field sensitivities of the two microphones be determined, see 5.1.1. Next, let the two microphones be
presented to the same sound pressure, set up by the auxiliary sound source under identical free-field
conditions. The ratio of the two output voltages will then equal the ratio of the two free-field sensitivities.
th free-field sensitivities of the two microphones, an expression for
Thus, from e product and the ratio of the
the free-field sensitivity of each of the two microphones can be derived.
NOTE - In order to obtain the ratio of free-field sensitivities, a direct comparison method may be used, and the auxiliary sound
source may be another transducer or a third microphone having mechanical or acoustical characteristics which differ from
those of the microphones being calibrated.
5.2 Basic expressions
Laboratory standard microphones and similar microphones are considered reciprocal and thus the two-port
equations of the microphones can be written as:
U = z11 i
+z12Q
(1)
p Z 1
21 + Z22
where
is the sound pressure at the acoustic terminals (diaphragm) of the microphone;
p
is the signal voltage at the electrical terminals of th
...
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