SIST EN 61094-8:2013
(Main)Electroacoustics - Measurement microphones - Part 8: Methods for free-field calibration of working standard microphones by comparison
Electroacoustics - Measurement microphones - Part 8: Methods for free-field calibration of working standard microphones by comparison
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 freefield 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.
/
Electroacoustique - Microphones de mesure - Partie 8: Méthodes pour l'étalonnage en champ libre par comparaison des microphones étalons de travail
La CEI 61094-8:2012 est applicable aux microphones étalons de travail satisfaisant aux exigences de la CEI 61094-4. Elle décrit des méthodes pour la détermination de l'efficacité en champ libre par comparaison utilisant un microphone étalon de laboratoire ou un microphone étalon de travail (s'il y a lieu) qui a été étalonné d'après les indications d'un des documents suivants: CEI 61094-3; CEI 61094-2; CEI 61094-5; CEI 61094-6; ou a présente partie de la CEI 61094. Les méthodes employées dans un environnement acoustique constituant une bonne approximation d'un champ libre idéal (par exemple une chambre en champ libre de haute qualité) et les méthodes utilisant un post-traitement des résultats pour réduire au maximum l'effet des imperfections de l'environnement acoustique, afin de simuler les conditions d'un champ libre, sont toutes abordées dans la présente partie de la CEI 61094. Des méthodes de comparaison reposant sur les principes décrits dans la 61094-3 peuvent également être utilisées, mais elles sortent du domaine d'application de la présente partie de la CEI 61094.
Elektroakustika - Merilni mikrofoni - 8. del: Metode za primerjalno umerjanje v odprtem prostoru delovnih mikrofonov
Ta del skupine standardov IEC 61094 se uporablja za delovne mikrofone, ki izpolnjujejo zahteve iz standarda IEC 61094-4. Opisuje metode za ugotavljanje občutljivosti v odprtem prostoru s primerjavo z laboratorijskim standardnim mikrofonom ali delovnim mikrofonom (kjer je to primerno), ki je bil umerjen v skladu s:
– standardom IEC 61094-3,
– standardom IEC 61094-2 ali IEC 61094-5, pri čemer so bili uporabljeni faktorji iz standarda IEC/TS 61094-7,
– standardom IEC 61094-6,
– tem delom standarda IEC 61094.
Ta del standarda IEC 61094 zajema metode, ki se izvajajo v akustičnem okolju, ki je dober približek idealnemu prostemu zvočnemu polju (npr. visokokakovostna gluha komora), in metode, pri katerih se rezultati naknadno prilagodijo, da se čim bolj zmanjša učinek nepravilnosti v akustičnem okolju, s čimer se simulirajo pogoji prostega zvočnega polja. Možne so tudi primerjalne metode, ki temeljijo na načelih iz standarda IEC 61094-3, vendar ne spadajo na področje uporabe tega dela standarda IEC 61094.
General Information
Relations
Standards Content (Sample)
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
SIST EN 61094-8:2013
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.
SIST EN 61094-8:2013
EN 61094-8:2012 - 2 - Foreword The text of document 29/752/CDV, future edition 1 of IEC 61094-8, prepared by IEC/TC 29 "IEC TC 29, Electroacoustics" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN 61094-8:2012. The following dates are fixed: • latest date by which the document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 2013-07-24 • latest date by which the national standards conflicting with the document have to be withdrawn (dow) 2015-10-24
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
- -
SIST EN 61094-8:2013
SIST EN 61094-8:2013
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
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale ®
Warning! Make sure that you obtained this publication from an authorized distributor.
Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé. SIST EN 61094-8:2013 colourinside
– 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
61094-8 © IEC:2012 – 3 –
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
SIST EN 61094-8:2013
– 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
61094-8 © IEC:2012 – 5 –
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.
SIST EN 61094-8:2013
– 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
61094-8 © IEC:2012 – 7 –
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
61094-8 © IEC:2012 – 9 –
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 sensi
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.