EN 61094-8:2012

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Elektroakustika - Merilni mikrofoni - 8. del: Metode za primerjalno umerjanje v odprtem prostoru delovnih mikrofonov/Electroacoustique - Microphones de mesure - Partie 8: Méthodes pour l'étalonnage en champ libre par comparaison des microphones étalons de travailElectroacoustics - Measurement microphones - Part 8: Methods for free-field calibration of working standard microphones by comparison33.160.50PriborAccessories17.140.50ElektroakustikaElectroacousticsICS:Ta slovenski standard je istoveten z:EN 61094-8:2012SIST EN 61094-8:2013en01-marec-2013SIST EN 61094-8:2013SLOVENSKI
STANDARD
EUROPEAN STANDARD EN 61094-8 NORME EUROPÉENNE
EUROPÄISCHE NORM November 2012
CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Management Centre: Avenue Marnix 17, B - 1000 Brussels
© 2012 CENELEC -
All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61094-8:2012 E
ICS 17.140.50
English version
Electroacoustics -
Measurement microphones -
Part 8: Methods for free-field calibration of working standard microphones by comparison (IEC 61094-8:2012)
Electroacoustique -
Microphones de mesure -
Partie 8: Méthodes pour l'étalonnage en champ libre par comparaison des microphones étalons de travail (CEI 61094-8:2012)
Elektroakustik -
Messmikrofone -
Teil 8: Verfahren zur Ermittlung des Freifeld-Übertragungskoeffizienten von Gebrauchs-Normalmikrofonen nach der Vergleichsmethode (IEC 61094-8:2012)
This European Standard was approved by CENELEC on 2012-10-24. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights. Endorsement notice The text of the International Standard IEC 61094-8:2012 was approved by CENELEC as a European Standard without any modification. SIST EN 61094-8:2013

- 3 - EN 61094-8:2012 Annex ZA (normative)
Normative references to international publications with their corresponding European publications
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.
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
IEC 61094-1 - Measurement microphones -
Part 1: Specifications for laboratory standard microphones EN 61094-1 -
IEC 61094-2 - Electroacoustics - Measurement
microphones -
Part 2: Primary method for the pressure calibration of laboratory standard microphones by the reciprocity technique EN 61094-2 -
IEC 61094-3 - Measurement microphones -
Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique EN 61094-3 -
IEC 61094-4 - Measurement microphones -
Part 4: Specifications for working standard microphones EN 61094-4 -
IEC 61094-5 - Measurement microphones -
Part 5: Methods for pressure calibration of working standard microphones by comparison EN 61094-5 -
IEC 61094-6 - Measurement microphones -
Part 6: Electrostatic actuators for determination of frequency response EN 61094-6 -
IEC/TS 61094-7 - Measurement microphones -
Part 7: Values for the difference between free-field and pressure sensitivity levels of laboratory standard microphones - -
ISO/IEC Guide 98-3 - Uncertainty of measurement -
Part 3: Guide to the expression of uncertainty in measurement (GUM:1995) - -
ISO 26101 - Acoustics - Test methods for the qualification of free-field environments
- -
IEC 61094-8 Edition 1.0 2012-09 INTERNATIONAL STANDARD NORME INTERNATIONALE Measurement microphones – Part 8: Methods for determining the free-field sensitivity of working standard microphones by comparison
Microphones de mesure – Partie 8: Méthodes pour la détermination de l'efficacité en champ libre par comparaison des microphones étalons de travail
INTERNATIONAL ELECTROTECHNICAL COMMISSION COMMISSION ELECTROTECHNIQUE INTERNATIONALE V ICS 17.140.50 PRICE CODE CODE PRIX ISBN 978-2-83220-380-4
– 2 – 61094-8 © IEC:2012 CONTENTS
FOREWORD . 4 1 Scope . 6 2 Normative references . 6 3 Terms and definitions . 7 4 Reference environmental conditions . 8 5 Principles of free-field calibration by comparison . 8 5.1 General principle . 8 5.2 General principles using sequential excitation . 8 5.3 General principles using simultaneous excitation . 8 6 General requirements . 9 6.1 The test space . 9 6.2 Methods of establishing the free-field . 9 6.2.1 General . 9 6.2.2 Using a test space with sound absorbing surfaces . 9 6.2.3 Time selective methods for obtaining the free-field sensitivity . 10 6.3 The sound source . 10 6.4 Reference microphone . 11 6.5 Monitor microphone . 12 6.6 Test signals . 12 6.7 Configuration for the reference microphone and microphone under test . 13 7 Factors influencing the free-field sensitivity . 13 7.1 General . 13 7.2 Polarizing voltage . 13 7.3 Acoustic centre of the microphone . 13 7.4 Angle of incidence and alignment with the sound source . 14 7.5 Mounting configuration . 14 7.6 Dependence on environmental conditions. 14 8 Calibration uncertainty components . 14 8.1 General . 14 8.2 Sensitivity of the reference microphone . 15 8.3 Measurement of the microphone output . 15 8.4 Differences between the sound pressure applied to the reference microphone and to the microphone under test . 15 8.5 Influence of indirect sound . 15 8.6 Influence of signal processing . 16 8.7 Influence of microphone characteristics and measurement system performance . 16 8.7.1 Microphone capacitance . 16 8.7.2 Measurement system non-linearity . 16 8.7.3 Validation of calibration system . 16 8.8 Uncertainty on free-field sensitivity level . 16 Annex A (informative)
Basic substitution calibration in a free-field chamber . 18 Annex B (informative)
Time selective techniques . 22 Bibliography . 30 SIST EN 61094-8:2013

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Figure A.1 – Illustration of source and receiver setup in a free-field room,
where the monitor microphone has been integrated into the loudspeaker . 18 Figure A.2 – Practical implementation in a hemi-anechoic room
with a source flush-mounted in the floor . 19 Figure A.3 – Examples of loudspeaker sources . 21 Figure B.1 – Illustration of set-up for measurement with time selective techniques . 23
Table 1 – Calibration options for the reference microphone
and associated typical measurement uncertainty . 12 Table 2 – Typical uncertainty components . 17
– 4 – 61094-8 © IEC:2012 INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________
MEASUREMENT MICROPHONES –
Part 8: Methods for determining the free-field sensitivity
of working standard microphones by comparison
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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any services carried out by independent certification bodies. 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. International Standard IEC 61094-8 has been prepared by IEC technical committee 29: Electroacoustics. The text of this standard is based on the following documents: CDV Report on voting 29/752/CDV 29/759/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. SIST EN 61094-8:2013

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A list of all the parts in the IEC 61094 series, published under the general title Measurement microphones 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it contains colours which are considered to be useful for the correct understanding of its contents. Users should therefore print this document using a colour printer.
– 6 – 61094-8 © IEC:2012 MEASUREMENT MICROPHONES –
Part 8: Methods for determining the free-field sensitivity
of working standard microphones by comparison
1 Scope This part of the IEC 61094 series is applicable to working standard microphones meeting the requirements of IEC 61094-4. It describes methods of determining the free-field sensitivity by comparison with a laboratory standard microphone or working standard microphone (where applicable) that has been calibrated according to either: – IEC 61094-3, – IEC 61094-2 or IEC 61094-5, and where factors given in IEC/TS 61094-7 have been applied, – IEC 61094-6, – this part of IEC 61094. Methods performed in an acoustical environment that is a good approximation to an ideal free-field (e.g. a high quality free-field chamber), and methods that use post processing of results to minimise the effect of imperfections in the acoustical environment, to simulate free-field conditions, are both covered by this part of IEC 61094. Comparison methods based on the principles described in IEC 61094-3 are also possible but beyond the scope of this part of IEC 61094. NOTE 1 This part of IEC 61094 is also applicable to laboratory standard microphones meeting the requirements of IEC 61094-1, noting that these microphones also meet the electroacoustic specifications for working standard microphones. NOTE 2 This part of IEC 61094 is also applicable to combinations of microphone and preamplifier where the determined sensitivity is referred to the unloaded output voltage of the preamplifier. NOTE 3 Other devices, for example, sound level meters can be calibrated using the principles of this part of IEC 61094, but are not within the scope of this standard. 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 IEC 61094-2, Electroacoustics – Measurement microphones – Part 2: Primary method for pressure calibration of laboratory standard microphones by the reciprocity technique IEC 61094-3, Measurement microphones – Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique IEC 61094-4, Measurement microphones – Part 4: Specifications for working standard microphones SIST EN 61094-8:2013

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IEC 61094-5, Measurement microphones – Part 5: Methods for pressure calibration of working standard microphones by comparison
IEC 61094-6, Measurement microphones – Part 6: Electrostatic actuators for determination of frequency response IEC/TS 61094-7, Measurement microphones – Part 7: Values for the difference between free-field and pressure sensitivity levels of laboratory standard microphones ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995) ISO 26101, Acoustics – Test methods for the qualification of free-field environments 3 Terms and definitions For the purpose of this document, the terms and definitions given in IEC 61094-1 and IEC 61094-3, as well as the following apply. 3.1
reference microphone laboratory standard microphone or working standard microphone where the free-field sensitivity has been previously determined 3.2
microphone under test device under test working standard microphone to be calibrated by comparison with a reference microphone Note 1 to entry: Other devices, for example, sound level meters, can be calibrated using the principles of this part of IEC 61094, but are not within the scope of this standard. 3.3
monitor microphone
microphone used to detect changes in sound pressure in the test environment
3.4
microphone reference point point specified on the microphone or close to it, to describe the position of the microphone Note 1 to entry: The microphone reference point may be at the centre of the diaphragm of the microphone. 3.5
reference direction inward direction toward the microphone reference point and specified for determining the acoustical response and directional response Note 1 to entry: The reference direction may be specified with respect to an axis of symmetry. 3.6
angle of incidence angle between the reference direction and a line between the acoustic centre of a sound source and the microphone reference point Note 1 to entry: Angle of incidence is expressed in degrees. SIST EN 61094-8:2013

– 8 – 61094-8 © IEC:2012 4 Reference environmental conditions The reference environmental conditions are: temperature 23,0 °C static pressure 101,325 kPa relative humidity 50 % 5 Principles of free-field calibration by comparison 5.1 General principle When a calibrated reference microphone and a microphone under test are exposed to the same free-field sound pressure, either simultaneously or sequentially, and under the same environmental conditions, then the ratio of their free-field sensitivities for those conditions is given by the ratio of their open-circuit output voltages. Then, both the modulus and phase of the free-field sensitivity of the microphone under test can be calculated from the known free-field sensitivity of the reference microphone. However, determination of the phase of the free-field sensitivity requires the definition of consistent reference phases at the acoustic centres of the microphones. At some frequencies, the measured free-field sensitivity of a microphone is strongly dependent on the mounting configuration and results for the microphone cannot be considered in isolation to the mounting configuration used (see 6.7).
The principle of the method also allows the microphone under test to be attached to measuring equipment, e.g. a particular preamplifier, and the sensitivity may be referred to the unloaded output of that measuring equipment. 5.2 General principles using sequential excitation In order for the two microphones to be sequentially exposed to essentially the same sound pressure, the output of the sound source and the environment conditions should not change.
Where there is potential for changes in the sound field, this shall be detected and corrected for, for example by using a monitor microphone. Examples of practical arrangements are given in Annex A. NOTE In principle it is possible to substitute a number of microphones under test sequentially into the sound field once the reference sound field has been established, but this places greater demands on the stability and spatial uniformity of the sound source and can increase the measurement uncertainty. 5.3 General principles using simultaneous excitation Simultaneous exposure of the reference and one or more microphones under test to the sound field overcomes the issue of the sound field changing with time, but requires identification of different points in the sound field where the sound pressures are the same. This may be achieved by configuring the test space and sound source to ensure a symmetrical sound field. If the effects of perturbations in the sound source are to be eliminated, it is essential that the output voltages from the microphone under test and the reference microphone be measured simultaneously when determining the open-circuit output voltage ratio.
In simultaneous comparison calibration, it is important that the presence of the reference microphone does not disturb the field incident on the microphone under test, and vice versa. The requirement for the source to provide two or more points in the sound field where the sound pressure is expected to be the same, places severe demands on the stability of the source’s directional characteristics. It may only be possible to achieve this by relaxing uncertainty requirements or by developing a source especially for this purpose. SIST EN 61094-8:2013

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6 General requirements 6.1 The test space The test space shall be as free as possible of any effects that cause instabilities in the sound field, for example between measurements with the microphone under test and the reference microphone. These include changing environment conditions, air flows, thermal gradients and electro-magnetic disturbances. The test space shall have a level of background noise and vibration that enables the measurements to meet the signal-to-noise requirements of the measurement system used. In practice steps should be taken to reduce the background noise as much as possible.
NOTE Heat sources in the test space can lead to some of the types of disturbance described above. 6.2 Methods of establishing the free-field
6.2.1 General There are two general approaches that can be taken in making free-field measurements. The first is to create an environment that attempts to establish a free field by using a test space with sound absorbing surfaces to prevent reflections of the sound coming directly from the source. The second is to use signal processing methods that enable the removal of signal content corresponding to indirectly received sound, thus simulating a free-field environment. There are many ways to implement both of these approaches. They can also be used in combination for the most demanding measurements. 6.2.2 Using a test space with sound absorbing surfaces Options for realising a true free-field environment range from free-field rooms (also known as anechoic chambers) to smaller scale enclosures and test boxes.
A free-field room typically has its surfaces covered with sound absorbing material, configured to present a gradually changing acoustic impedance to an incident sound wave. Often this is in the form of wedges that protrude into the room, though other configurations can be used. The depth of this absorbent layer, as well as its shape and design, determines the lowest frequency where sound absorption is effective. A hemi-anechoic room, where one of the room surfaces is formed by a reflecting plane, can also be used. In this case the sound source should be mounted flush with the reflecting surface, so that the surface acts as an ‘infinite’ baffle. Secondary sound radiation, from the edges of the sound source or its mounting, are thus avoided.
NOTE 1 Although edge diffraction from the sound source is eliminated, diffraction from the boundaries of the reflecting plane will still be present. The room shall have an identified region where the sound field can be assumed to contain only plane progressive wave emanating from sound source (i.e. approximates a free sound field). The sound source and measurement positions shall be located within this region. For low frequencies long wedges with very high sound absorption are required, leading to the need for a very large room to enable measurements to be made at a sufficient distance from the wedge tip. Free-field calibration using a room with sound absorbing surface therefore becomes impractical and an alternative method may be needed. One approach is to mount the microphone, complete with its pressure equalisation vent mechanism, inside a small enclosure, within which a low frequency sound pressure can be generated. Although there is no acoustic propagation, the sensitivity determined in such a field will nevertheless be a good approximation to the free-field sensitivity, because diffraction effects are minimal when the sound wavelength is significantly greater than the dimensions of the microphone. SIST EN 61094-8:2013

– 10 – 61094-8 © IEC:2012 NOTE 2 For WS1 microphones at reference environmental conditions, diffraction effects will contribute less than 0,1 dB to the free-field sensitivity level below 500 Hz. For WS2 and WS3 microphones the contribution will be even smaller.
NOTE 3 By using alternative techniques at low frequency, a practical low frequency limit for a free-field room of around 500 Hz will suffice. NOTE 4 Even an alternative calibration method for low frequency will be limited to frequencies above the low frequency limit of the test or reference microphone, or by the ability to calibrate the reference microphone at low frequencies. Free-field calibration can also be carried out in smaller scale test boxes. However their limited dimensions and depth of absorbent lining will restrict the frequency range over which they will be effective and their overall performance. When the measurement method used assumes that a free field exists, the performance of the room shall be quantified in this respect. A method is described in ISO 26101. 6.2.3 Time selective methods for obtaining the free-field sensitivity
The use of time selective methods provides a possibility to measure the free-field sensitivity of a microphone in conditions that might otherwise be unsuitable for direct free-field calibration. With a suitable test arrangement it can be possible to distinguish between the component of the output signal resulting from the directly received acoustic wave and that received indirectly, as a result of reflection. Reflected sound travels a longer path to reach the microphone and therefore takes a greater time to do so. If the direct wave propagation and any settling effects within the microphone occur before the arrival of the first reflection, some form of time-selective technique or time gating can be used to consider the response to the direct sound only, thus simulating what would occur in an ideal free field.
NOTE Methods based on this approach for establishing the free-field response are sometimes referred to as quasi-free-field techniques. Time selective techniques often have their own low frequency limitations, which need to be considered along with test space limitations noted above.
A variety of time-selective techniques have been developed and examples are described in Annex B. 6.3 The sound source The sound source typically consists of a loudspeaker fitted in an enclosure or baffle. However alternative types of sound source may be deployed. Examples of sound sources can be found in Annex A. NOTE 1 A reciprocal microphone may be driven electrically and used as a sound source. The sound source shall be capable of generating plane progressive waves at the measurement position. In practice the sound source may not radiate plane waves, but at a sufficiently long distance from the source, wave fronts can be considered plane across the region occupied by the reference or microphone under test.
If the sound source is used for simultaneous calibration, the directivity pattern shall also be known to enable a suitable choice of measurement points to be determined. The directivity pattern shall be stable over the time period of a test.
If more than one measurement position is used, it may be desirable to use a sound source having an omni-directional directivity pattern in the frequency range of use. To fulfill the plane wave requirement along the length of the test object, measurements shall be made within the region where the field is purely progressive. SIST EN 61094-8:2013

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The further requirements listed below may have greater or lesser importance depending on the calibration method adopted. The sound source shall be capable of generating sufficient sound pressure level at the test location(s) at all the frequencies of interest. Sound pressure levels typically between 70 dB and 80 dB are usually sufficient, but the chosen level will depend on the sensitivity of the microphones to be tested and the signal-to-noise ratio requirement of the measurement system. The sound source shall produce a stable output over the time period of a test.
The stability of a loudspeaker sound source should be monitored by some means during the course of a calibration. Options for monitoring the sound source include the use of an auxiliary monitor microphone and using the repeatability in results. At higher output levels, the loudspeaker may exhibit instabilities. The stability of the sound source shall therefore be established for the type of test signal used. Use of the minimum electrical input signal that provides an adequate signal-to-noise ratio in the measurement setup is also recommended. The sound source shall not produce distortion components that may generate a significant response from the microphone under test and/or reference microphone at frequencies other than the test frequency.
NOTE 2 The use of suitable band pass filters can reduce this effect with sinusoidal or narrow band test signals. NOTE 3 Distortion can also be a problem for impulsive stimuli when high peak output levels are required.
The size of the sound source shall be small relative to the distance to the measurement position(s), so that sound radiated or diffracted from off-axis elements of the source or its mounting does not cause significant deviations from ideal free-field behaviour of a point source, as the measurement distance changes. It may be necessary to use a number of sound sources each covering different parts of the frequency range. 6.4 Reference microphone
The reference microphone shall be a laboratory standard (LS) microphone or working standard (WS) microphone having a known free-field sensitivity and corresponding uncertainty at the desired range of calibration frequencies.
Table 1 shows the available calibration options and the typical measurement uncertainty for the free-field sensitivity, for the reference microphone types available.
– 12 – 61094-8 © IEC:2012 Table 1 – Calibration options for the reference microphone
and associated typical measurement uncertainty
Reference microphone type Calibration method Reference Typical expanded uncertainty (k = 2)
in dB 1 kHz 10 kHz LS
Primary free-field calibration IEC 61094-3 0,25 0,10 Primary pressure calibration with the addition of a free-field to pressure sensitivity level difference IEC 61094-2 and IEC/TS 61094-7 0,12 0,4 Secondary pressure calibration with the addition of a free-field to pressure sensitivity level difference IEC 61094-5 and IEC/TS 61094-7 0,15 0,5 LS and WS
Secondary free-field calibration
This part of IEC 61094 0,2 0,5 Electrostatic actuator calibration with the addition of a free-field to actuator response level difference IEC 61094-6 0,3 0,6
Where possible the reference microphone configuration should be chosen to match that of the microphone under test. An LS1P reference microphone shall be used without protection grid (where available). Working standard microphones may be used with or without protection grid, noting that removal of the protection grid is likely to yield the lowest uncertainty. If a protection grid is used, the reference free-field sensitivity or quoted uncertainty shall allow for this. 6.5 Monitor microphone A monitor microphone shall be used to detect changes in the sound field, if required to achieve the desired level of measurement uncertainty. The monitor microphone shall be permanently located in a sound field close to the sound source.
The monitor microphone shall not perturb the sound field reaching the microphone being measured. This usually requires the use of a small microphone (for example a WS3 microphone), to avoid diffraction effect that could distort plane wave propagation. It shall therefore be validated that the choice of monitor microphone and its location do not influence the results unduly, and that any influence is accounted for in the measurement uncertainty. 6.6 Test signals The test signal will be determined largely by details of the application and calibration method. In particular signal processing methods may require specific types of signal to be used. The source characteristics and mode of operation can also affect the choice of test signal.
Test signals can include:
– pure tone,
– swept-sine or stepped-sine, – wide-band white noise or pink noise,
– narrow-band noise (e.g. third-octave-band noise), – pseudo-random or periodic noise (e.g. maximum length sequences), – warble tones
(e.g. frequency modulated (FM) tones), SIST EN 61094-8:2013

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– tone bursts or noise bursts,
– chirps,
– impulses (e.g. clicks, sparks etc.). NOTE The test signal used can also place particular requirements on sound source, such as frequency response or dynamic range.
6.7 Configuration for the reference microphone and microphone under test The microphone shall be mounted on a semi-infinite cylindrical rod having the same diameter as the body of the microphone. Any deviation from this configuration, including guide wires or other hardware used to support the mounting rod, may influence the free-field sensitivity of the microphone, and any such effects shall be allowed for in the measurement uncertainty. Alternatively if the free-field sensitivity of the microphone under test is to be determined in a specific mounting configuration, then this configuration shall be used to mount the microphone under test during calibration. The preamplifier shall be integrated with the mounting rod and shall provide the reference ground-shield mechanical configuration appropriate for the type of microphone being tested, as specified in IEC 61094-1 or IEC 61094-4.
If the instruction manual specifies a maximum mechanical force to be applied to the central electrode contact of the microphone, this limit shall not be exceeded. The requirement to use the reference ground-shield configuration does not apply to combinations of microphone and preamplifier used as an integral system. If adapters are used to convert a preamplifier for use with different sized microphones, the adapter used shall also convert the ground-shield configuration accordingly. 7 Factors influencing the free-field sensitivity 7.1 General The free-field sensitivity of a measurement microphone depends on the operational and environmental conditions, as well as the geometrical configuration used in the calibration, hence the need to specify these parameters in defining the sensitivity. In addition it is necessary to ensure that these parameters are sufficiently controlled in the calibration process, so that the resulting uncertainty components can be taken into account in the uncertainty budget (see Table 2). In addition, the calibration process itself adds further components of uncertainty that are not directly connected with the operation of the microphone. These are listed in Clause 8. 7.2 Polarizing voltage If the microphone under test requires an external polarizing voltage, the manufacturer’s recommendations shall be followed. The actual polarizing voltage used during the calibration shall be stated, along with the reported free-field sensitivity. If the microphone is pre-polarized, care shall be taken not to apply an external polarizing voltage. 7.3 Acoustic centre of the microphone The definition of the free-field sensitivity of a microphone refers to the sound pressure at the acoustic centre of the microphone, before the microphone is introduced into the field. When comparing microphones their acoustic centres shall be positioned at the measurement points. SIST EN 61094-8:2013

– 14 – 61094-8 © IEC:2012 Alternatively, a microphone reference point defined by the manufacturer (for example at the centre of the diaphragm or protection grid) shall be specified for aligning the microphones, and the difference between this and the acoustic centre shall be treated as an uncertainty on the distance to the sound source, and therefore on the sound pressure. NOTE 1 The microphone acoustic centre is a function of frequency and the distance from the sound source. NOTE 2 At sufficiently large distances from the sound source, the acoustic centre can be considered constant. NOTE 3 A method for determining the acoustic centre is given in IEC 61094-3. 7.4 Angle of incidence and alignment with the sound source The free-field sensitivity of a microphone is a function of the angle of incidence, particularly at high frequencies. Some means of setting the orientation of the microphone in a repeatable manner shall be used. In addition, the co-axial alignment of the microphone with the sound source can cause errors in both the angle of incidence and applied sound pressure. Some means of setting this alignment in a repeatable manner shall be used. 7.5 Mounting configuration The component of the free-field sensitivity derived from diffraction is strongly influenced by the geometric configuration of the microphone and its mounting. The microphone shall therefore be calibrated in a specified mounting configuration. Where no such configuration is specified, a cylinder of the same diameter as the microphone body shall be used.
7.6 Dependence on environmental conditions The free-field sensitivity of the microphone depends on static pressure, temperature and humidity. This dependence can be determined by comparison with a well-characterized laboratory standard microphone over a range of conditions. The sensitivity of the reference microphone shall be corrected to the actual environmental conditions during the test.
Alternatively, when reporting the result of a calibration, the free-field sensitivity may be referred to the reference environmental conditions if reliable correction data are available. The actual conditions during the calibration shall be reported. 8 Calibration uncertainty components 8.1 General In addition to the factors which affect the free-field sensitivity mentioned in Clause 7, further uncertainty components are introduced by the method, the equipment and the degree of care under which the calibration is carried out.
Factors which affect the calibration in a known way shall be measured or calculated with as high an accuracy as is practical in order to minimize their influence on the resulting uncertainty.
The components of uncertainty considered below relate to general requirement of free-field calibration. Some components may not be relevant, or additional components may need to be considered, in specific implementations. SIST EN 61094-8:2013

61094-8 © IEC:2012 – 15 –
8.2 Sensitivity of the reference microphone
The uncertainty in the sensitivity of the reference microphone directly affects the uncertainty in the sensitivity of the microphone under test. The reference microphone sensitivity may be derived by applying free-field-to-pressure differences according to IEC/TS 61094-7 to a pressure reciprocity calibration according to IEC 61094-2. In this case the uncertainty of both elements shall be taken into account. If the reference microphone requires an external polarization voltage then any difference between the voltage applied when it was calibrated and the voltage applied when used as the reference microphone shall be allowed for in the uncertainty calculation. 8.3 Measurement of the microphone output Uncertainties of a random, or time-varying nature in the measurement of the outputs of the microphones, directly affects the uncertainty in the sensitivity of the microphone under test. Uncertainties of a systematic nature in the measurement of the outputs of the microphones may affect the uncertainty in the sensitivity of the microphone under test or may be reduced if the same system is used for both the test and reference microphones. 8.4 Differences between the sound pressure applied to the reference microphone and to the microphone under test
As stated in 5.1 the basis of a comparison method is that the test and reference microphones are exposed to a sound field having the same modulus, phase and angle of incidence. Any factor causing these parameters to alter will result in calibration uncertainty. This includes: –
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