EN 61094-3:1995

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
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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 the microphone;
U
is the volume velocity through the acoustical terminals (diaphragm) of the microphone;
i is the current through the electrical terminals of the microphone;
Z11 = Ze is the electrical impedance of the microphone when the diaphragm is blocked;
Z22 = Za is the acoustic impedance of the microphone when the electrical terminals are unloaded;
the forward transfer impedances, Mp being the pressure
MpZa is equal to the reverse and
Z12 = Z21 =
sensitivity of the microphone.

1094-3 © I EC:1995 -
13 -
Equations (1) may then be rewritten as:
U = Ze i + Nip Za
(1a)
P = MP Za_i+Za ck.
which constitute the equations of reciprocity for the microphone.
When the sound pressure p is not uniform over the surface of the diaphragm, as will be the case at high
frequencies when the microphone is located in a plane progressive wave, the location of the acoustic
terminals is given through the equivalent point-transducer simulating the microphone. In this case, equation
(1) will also be valid for the real microphone through a special interpretation of p, see 5.4 and 5.5.
5.3 Insert voltage technique
voltage technique is used to determine the open-circuit voltage of a microphone when it is
The insert
rtain open-circuit voltage and internal impedance be
electrically loaded. Let a microphone having a ce
connected to a load impedance. To measure the open-circuit voltage, an impedance which is small
compared with the load impedance is connected in series with the microphone and a calibrating voltage
applied across it. Let a sound pressure and a calibrating voltage of the same frequency be applied alter-
nately. When the calibrating voltage is adjusted until it gives the same voltage drop across the load
impedance as results from the sound pressure on the microphone, the open-circuit voltage will be equal
in magnitude to the calibrating voltage.
5.4 Free-field receiving characteristics of a microphone
po . The equivalent circuit of
Let a microphone be placed in a progressive plane wave of sound pressure
0 is the sound pressure when the diaphragm is blocked and
the microphone is given in figure 1, where p'
p the actual sound pressure at the acoustic terminals of the microphone. Z a r is the acoustic radiation
impedance of the microphone.
Za,r
>
I °—I
Microphone
I Po
r
o
Figure 1 – Equivalent circuit for a receiving microphone in a sound field
Let go be related to po through:
^o
— - S(f,e)
Ro
where S(f,0), the scattering factor, is a function of frequency and angle of incidence of the sound wave
impinging on the diaphragm of the microphone.
S(f,9) depends on the geometrical configuration of the microphone.

1094-3 © I EC:1995 - 15 -
Asp = p'o - Z
a r g, the two-port equations of the microphone (la) can be written as:
S1= Zei
+MZa q
(2)
MP Zai*
p,= (Za+ Zr) si
and thus, from the basic definition, the free-field sensitivity is given by:
^
M - = M Z' S(f^e)
(3)
p
p, Za + Za,r
1^0
difference between the pressure and the free-field sensitivity is determined not
Equation (3) shows that the
only by the geometry of the microphone through the scattering factor S(f,0) but also by the relation between
the acoustic impedance of the microphone and the radiation impedance.
5.5 Free-field transmitting characteristics of a microphone
Let a microphone be used as a transmitter under free-field conditions. The equivalent circuit of the
microphone is given in figure 2.
i
o )1.
Microphone
U Z a, r
Figure 2– Equivalent circuit for a transmitting microphone under free-field conditions
As p = - Z g, the two-port equations of the microphone (1) can be written as:
a,r
I+ MP Za p
!1 = Z0
(4)
o=
MPZai+(Za+ Zr) q
so that:
M P Mr
SX= Za +Z r l i
s(te)
From the general principle of reciprocity, it can be deduced that, at a remote point, the equivalent point-
g S(f,0) = M1 land the sound pressure pc, at the distance
transducer will act as a simple source of strength -
between this point and the equivalent point-transducer will then be:
d
p,= j ieYd e i `af (5)
2dM 1
where
X = a + j(3 is the complex propagation coefficient.
NOTE - The derivation of equation (5) given above is based on a lumped parameter representation of the microphone (see
equation (1)). A more rigorous derivation can be obtained by using an integral form of representation of the equations of the
microphone.
1094-3 ©IEC:1995 - 17 -
5.6 Reciprocity procedure
Let the microphones (1) and (2) with the free-field sensitivities Mf
and Mf 2 be situated in a free-field facing
each other and with coincident principal axes. A current i,
through the electrical terminals of microphone
(1) will produce a sound pressure po given by equation (5) at a distance
d from the acoustic centre of the
microphone under free-field conditions. When introducing microphone (2) into the sound field and assuming
no interaction takes place between the two microphones, the open-circuit voltage of microphone (2) will be:
Pf -Yd
^2
!12 Al
= M M
r.2 Ro 1, 1 1 ,2 11
,
2 d12 e
d12
being the distance between the acoustic centres of microphone (1) and (2).
Thus, the product of the free-field sensitivities is given by:
2d 112
Y d
,2
e
(6)
1141,1A41,2,
pf
NOTE - The
right hand side of equation (6) is the ratio of the electrical transfer impedance of the system and the acoustical
transfer impedance between the positions of the acoustic centres in the absence of the microphones.
5.7
Final expressions for the free-field sensitivity
5.7.1
Method using three microphones
Let the electrical transfer impedance U2/i, (see 5.6) be denoted by Z^
, 72 with similar expressions for other
pairs of microphones. The final expression for the free-field sensitivity of microphone (1) is:
(7)
Similar expressions apply for microphones (2) and (3).
If only the modulus of the free-field sensitivity is of interest, only the modulus of each electrical transfer
impedance need be determined and a can be substituted for y.
The phase angle of the
free-field sensitivity for each microphone can be determined from the phase angles
of the electrical transfer impedances and y combined, with physical considerations to resolve the 180°
phase ambiguity in this expression.
5.7.2
Method using two microphones and an auxiliary sound source
If only two microphones and an auxiliary sound source are used, the final expression for the free-field
sensitivity is:
1/2
ii11'1
eYd,2
2 d72
M Z (8)
1 ,1 Pf e,12
^
f,2
where the ratio of the two free-field sensitivities is measured by comparison against the auxiliary source,
see 5.1.2.
If only the modulus of the free-field sensitivity is of interest, only the modulus of the electrical transfer
impedance and the ratio of the free-field sensitivities need be determined, and a can be substituted for y.
The phase angle of the free-field sensitivity for each microphone can be determined from the phase angle
of the electrical transfer impedance, the ratio of free-field sensitivities and y combined, with physical
considerations to resolve the 180° phase ambiguity in this expression.

1094-3 ©IEC:1995 - 19 -
6 Factors influencing the free-field sensitivity
6.1 General
The free-field sensitivity of a laboratory standard microphone depends on polarizing voltage and environ-
mental conditions. Further the definition of the free-field sensitivity implies that ce
rtain requirements be
fulfilled by the measurements. It is essential during a calibration that these conditions are controlled
sufficiently well so that the resulting uncertainty comp
...