IEC 61265:2018
(Main)Electroacoustics - Instruments for measurement of aircraft noise - Performance requirements for systems to measure sound pressure levels in noise certification of aircraft
Electroacoustics - Instruments for measurement of aircraft noise - Performance requirements for systems to measure sound pressure levels in noise certification of aircraft
IEC 61265:2018 specifies requirements for the electroacoustical performance of systems of instruments used to measure sound for the purposes of aircraft noise certification, and for other comparisons among aircraft models, and provides methods by which tests can be made periodically to verify that the performance continues to conform to the requirements within stated limits.
In general, a sound measurement system for this purpose comprises a combination of instruments extending from a microphone, including its windscreen and other accessories, through data recording and processing devices to a suitable output. Different measurement systems, regardless of their composition, perform the necessary functions in different ways and operate on either analogue or digital principles.
IEC 61265:2018 cancels and replaces the first edition published in 1995. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous edition:
a) addition of guidance for measurements for aircraft other than large transport aeroplanes;
b) addition of microphones used in ground plane measurement systems;
c) addition of weighted sound level measurements other than one-third-octave band measurements, for certain aircraft types;
d) revision and clarification of requirements for digital audio recording;
e) addition of requirements for evaluation of measurement uncertainty.
Electroacoustique - Instruments pour la mesure du bruit des aéronefs - Exigences relatives aux systèmes de mesure des niveaux de pression acoustique pour la certification acoustique des aéronefs
IEC 61265:2018 spécifie les caractéristiques électroacoustiques des systèmes instrumentaux utilisés pour la mesure des bruits dans le cadre de la certification acoustique des aéronefs, ainsi que pour l'établissement d'autres comparaisons entre des modèles d'aéronefs, et fournit des méthodes d'essais de vérification périodique qui permettent de s'assurer du maintien des caractéristiques en conformité avec les exigences, à l'intérieur de tolérances spécifiées.
En général, un système de mesure utilisé à cet effet se compose d'une combinaison d'instruments allant du microphone, y compris son écran anti-vent et ses autres accessoires, en passant par des dispositifs d'enregistrement et de traitement des données, jusqu'à un dispositif de sortie approprié. Les différents systèmes de mesure, indépendamment de leur composition, réalisent les fonctions nécessaires de différentes façons et fonctionnent suivant des principes analogiques ou numériques.
IEC 61265:2018 annule et remplace la première édition parue en 1995. Cette édition constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) ajout de recommandations pour les mesures d'aéronefs autres que des gros avions de transport;
b) ajout de microphones utilisés dans les systèmes de mesure à plan de sol;
c) ajout de mesures de niveau acoustique pondéré, autres que les mesures par bandes de tiers d'octave, pour certains types d'aéronefs;
d) révision et clarification des exigences applicables à l'enregistrement audio numérique;
e) ajout d'exigences pour l'évaluation de l'incertitude de mesure.
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IEC 61265 ®
Edition 2.0 2018-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Instruments for measurement of aircraft noise – Performance
requirements for systems to measure sound pressure levels in noise
certification of aircraft
Électroacoustique – Instruments pour la mesure du bruit des aéronefs –
Exigences relatives aux systèmes de mesure des niveaux de pression
acoustique pour la certification acoustique des aéronefs
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IEC 61265 ®
Edition 2.0 2018-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Instruments for measurement of aircraft noise – Performance
requirements for systems to measure sound pressure levels in noise
certification of aircraft
Électroacoustique – Instruments pour la mesure du bruit des aéronefs –
Exigences relatives aux systèmes de mesure des niveaux de pression
acoustique pour la certification acoustique des aéronefs
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.140.50; 49.020 ISBN 978-2-8322-5695-4
– 2 – IEC 61265:2018 © IEC 2018
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Purpose . 11
5 Requirements . 12
5.1 General . 12
5.1.1 Output data . 12
5.1.2 Time-pressure history . 12
5.1.3 Measurements of sound from aircraft ground power units . 12
5.1.4 Measurements of aircraft operations on the ground . 12
5.2 Measurement uncertainty . 12
5.2.1 Relationship with regulatory requirements . 12
5.2.2 Performance verification . 13
5.2.3 Periodic tests . 13
5.3 Reference environmental conditions . 13
5.4 Sound calibrator. 13
5.5 Microphone system . 13
5.5.1 Pressure and free-field type microphones . 13
5.5.2 Grazing incidence microphone configuration . 14
5.5.3 Ground plane microphone configuration . 15
5.5.4 Measurement configurations . 16
5.6 Measurement system exclusive of the microphone . 16
5.6.1 Frequency response . 16
5.6.2 Level linearity . 16
5.6.3 Floating-range measurement systems . 17
5.7 Spectrum analysis system . 17
5.7.1 Third-octave analysis . 17
5.7.2 Time constants . 17
5.7.3 Time offset . 17
5.7.4 Anti-aliasing. 18
5.8 Readout device resolution . 18
5.9 Sensitivity to various environments . 18
5.9.1 General . 18
5.9.2 Atmospheric pressure . 18
5.9.3 Air temperature . 18
5.9.4 Humidity . 19
5.9.5 Electromagnetic compatibility . 19
Annex A (informative) Methods of testing the electroacoustical performance of a
measurement system . 20
A.1 General . 20
A.2 Operating conditions for test . 20
A.3 Sound calibrator. 20
A.4 Microphone system frequency response . 20
A.5 Frequency response of the measurement system exclusive of the
microphone . 21
A.6 Linear operating range of the measurement system exclusive of the
microphone . 21
A.7 Spectrum analysis system . 21
Annex B (informative) Relationship between tolerance interval, corresponding
acceptance interval and the maximum permitted uncertainty of measurement . 22
Bibliography . 24
Figure 1 – Illustration of sound incidence angles from the principal axis of the
microphone . 15
Figure B.1 – Relationship between tolerance interval, corresponding acceptance
interval and the maximum permitted uncertainty of measurement . 22
Table 1 – ICAO measurement protocols (informative) . 11
Table 2 – Maximum difference between the free-field sensitivity level of a microphone
used in grazing-incidence microphone configuration at normal incidence and at
specified sound incidence angles . 15
– 4 – IEC 61265:2018 © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS – INSTRUMENTS FOR MEASUREMENT OF
AIRCRAFT NOISE – PERFORMANCE REQUIREMENTS FOR
SYSTEMS TO MEASURE SOUND PRESSURE LEVELS
IN NOISE CERTIFICATION OF AIRCRAFT
FOREWORD
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61265 has been prepared by IEC technical committee 29:
Electroacoustics.
This second edition cancels and replaces the first edition published in 1995. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) addition of guidance for measurements for aircraft other than large transport aeroplanes;
b) addition of microphones used in ground plane measurement systems;
c) addition of weighted sound level measurements other than one-third-octave band
measurements, for certain aircraft types;
d) revision and clarification of requirements for digital audio recording;
e) addition of requirements for evaluation of measurement uncertainty.
The text of this International Standard is based on the following documents:
CDV Report on voting
29/958/CDV 29/980A/RVC
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC 61265:2018 © IEC 2018
INTRODUCTION
IEC 61265 provides requirements for the electroacoustical performance of instruments for
measurement of the sound produced by aircraft in flight or on the ground, or by an aircraft
engine installed on an outdoor test stand, for the purposes of demonstrating compliance with
aircraft noise certification limits established by relevant national aviation authorities and for
other comparisons among aircraft models. The instruments can be components of a complete
measurement system. Methods are also indicated by which the performance of such
instruments can be tested periodically.
Measurement and data-analysis procedures for aircraft noise certification are described in
Volume I of Annex 16 to the Convention on International Civil Aviation, with further guidance
and descriptions of acceptable "equivalent procedures" given in the Environmental Technical
Manual prepared by the Committee on Aviation Environmental Protection (CAEP) of the
International Civil Aviation Organization (ICAO). Together these documents are referred to in
this document as "ICAO Annex 16". The procedures include measurement and analysis of the
sound from aircraft in operation, and, in some circumstances, of the sound from static engines
and engines under test, under given operating and atmospheric conditions.
Several of the requirements given in this document differ from the requirements of
IEC 61672-1 for sound level meters, especially concerning the frequency and directional
response, linear operating range and sensitivity to various environments. Many of these
differences are due to the requirement for uniform response at a wide range of angles of
sound arrival as an aircraft moves through the certification test flight. If the output signal from
a measurement system conforming to this document is processed to yield an overall sound
pressure level from all frequency bands, the level derived can differ from that obtained from a
sound level meter conforming to IEC 61672-1.
Systems in accordance with this document are used to perform measurements meeting the
requirements of ICAO Annex 16 or a certifying authority’s specific procedures to demonstrate
that a given aircraft complies with the limits for noise level near the ground over the course of
a test flight. Uncertainty of each measurement is considered when establishing the test
procedures, and it is not the intent of this document to duplicate the confidence interval
analysis inherent in the noise flight test procedure.
ELECTROACOUSTICS – INSTRUMENTS FOR MEASUREMENT OF
AIRCRAFT NOISE – PERFORMANCE REQUIREMENTS FOR
SYSTEMS TO MEASURE SOUND PRESSURE LEVELS
IN NOISE CERTIFICATION OF AIRCRAFT
1 Scope
This document specifies requirements for the electroacoustical performance of systems of
instruments used to measure sound for the purposes of aircraft noise certification, and for
other comparisons among aircraft models, and provides methods by which tests can be made
periodically to verify that the performance continues to conform to the requirements within
stated limits.
In general, a sound measurement system for this purpose comprises a combination of
instruments extending from a microphone, including its windscreen and other accessories,
through data recording and processing devices to a suitable output. Different measurement
systems, regardless of their composition, perform the necessary functions in different ways
and operate on either analogue or digital principles.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60942, Electroacoustics – Sound calibrators
IEC 61260-1, Electroacoustics – Octave-band and fractional-octave-band filters – Part 1:
Specifications
IEC 61672-1, Electroacoustics – Sound level meters – Part 1: Specifications
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
calibration record
time-pressure history or recorded value obtained during a calibration procedure, consisting of
a measurement system output while its acoustic input is provided by a sound calibrator
– 8 – IEC 61265:2018 © IEC 2018
3.2
effective perceived noise level
EPNL
EPNdB
single number evaluator of the subjective effects of aircraft noise on human beings,
expressed in decibels (EPNdB) as described in ICAO Annex 16, Appendix I
Note 1 to entry: This note applies to the French language only.
3.3
floating-range measurement system
measurement system incorporating automatic level range switching
3.4
free-field microphone
microphone having substantially uniform frequency response to plane progressive sound
waves arriving at angles substantially normal to the plane of the microphone diaphragm
3.5
free-field sensitivity
quotient of the root-mean-square voltage at the output of a microphone
system by the root-mean-square sound pressure that would exist at the position of the
microphone in the absence of the microphone, for a sinusoidal plane progressive sound wave
of specified frequency, at a specified sound-incidence angle
Note 1 to entry: The free-field sensitivity is expressed in volts per pascal.
3.6
free-field sensitivity level
twenty times the logarithm to the base ten of the ratio of the free-field
sensitivity to the reference sensitivity of one volt per pascal
Note 1 to entry: The free-field sensitivity level is expressed in decibels.
Note 2 to entry: The free-field sensitivity level can be determined by subtracting the sound pressure level (re
20 μPa expressed in dB) of the sound incident on the microphone from the voltage level (re 1 V expressed in dB) at
the output of the microphone system, and adding 93,98 dB to the result.
3.7
grazing incidence
condition where sound from the source of interest impinges on the microphone primarily at an
incidence angle parallel to the plane of the microphone diaphragm
3.8
L
ASmax
maximum value obtained during a given time period or event of A-frequency-weighted, S-time-
weighted sound pressure level
3.9
L
AE
sound exposure level (SEL) of the time integral of squared A-weighted sound pressure over a
given time period or event, with reference to the square of the reference sound pressure of 20
µPa, and a reference duration of one second, as provided by an integrating sound level meter
in accordance with ICAO Annex 16, Appendix 4
Note 1 to entry: L is expressed in decibels.
AE
EXAMPLE L is the sound energy of an acoustic event lasting one second that is equal to the sound energy of
AE
the measured event over the time interval that the A-weighted sound pressure level is within 10 dB of its maximum
value. It is typically 7 dB to 12 dB higher than the L during the same interval.
Amax
3.10
level difference
measured one-third-octave band output signal level on any level range, adjusted for the
difference between the settings of the level range controls on the level range and the
reference level range, minus the level of the corresponding electrical input signal
Note 1 to entry: The level difference is expressed in decibels.
3.11
level non-linearity
level difference on any level range, at a selected one-third-octave mid-band frequency, minus
the reference level difference, all input and output signals being relative to the same
reference quantity
Note 1 to entry: The level non-linearity is expressed in decibels.
3.12
level range
setting of the controls provided in a measurement system for the recording and level
measurement or spectral analysis of a sound pressure input signal
Note 1 to entry: The level range is expressed in decibels.
3.13
linear operating range
range of levels of steady sinusoidal electrical signals applied to the input of a measurement
system exclusive of the microphone system, extending from a lower boundary to an upper
boundary, over which the level non-linearity is within limits specified in 5.6.2
Note 1 to entry: The linear operating range applies to a stated level range and is expressed in decibels.
3.14
measurement system
combination of instruments used for the measurement of sound pressure levels, including a
sound calibrator, microphone system, signal recording and conditioning devices, and one third
octave band spectrum analysis or A-frequency-weighted, S-time-weighted sound level
measurement as required for the aircraft type being evaluated
Note 1 to entry: The measurement system covered by this document does not include analysis means for
computing metrics such as EPNL (EPNdB) or SEL (L ).
AE
Note 2 to entry: Practical installations can include a number of microphone systems, the outputs from which are
recorded simultaneously by a multi-channel recording device. The signal recording and conditioning devices and/or
the spectrum analysis system can have separate channels in parallel, again with facilities for selection, either of
the input or of the output. For the purposes of this document, each combination of a single microphone system and
a single data-recording and data-analysis channel within the other instruments comprises a separate, complete
measurement system, and the requirements apply accordingly.
3.15
microphone
electroacoustic transducer converting audio-frequency air pressure variations to AC voltage
Note 1 to entry: Microphones used for aircraft noise certification are typically of the electrostatic or "condenser"
type which have an output impedance requiring a preamplifier to be located adjacent to the microphone rather than
in external equipment connected to the microphone by a cable.
3.16
microphone system
components of the measurement system which produce an electrical output signal in
response to a sound pressure input signal, and which generally include a microphone, a
preamplifier, power supply, extension cables, and other devices as necessary
– 10 – IEC 61265:2018 © IEC 2018
3.17
pressure sensitivity
quotient of the root-mean-square voltage at the output of a microphone
system by the root-mean-square sound pressure, for sinusoidal sound pressure of specified
frequency at the microphone diaphragm
Note 1 to entry: The pressure sensitivity is expressed in volts per pascal.
3.18
pressure sensitivity level
twenty times the logarithm to the base ten of the ratio of the pressure
sensitivity to the reference sensitivity of one volt per pascal
Note 1 to entry: The pressure sensitivity level is expressed in decibels.
3.19
pressure type microphone
microphone having substantially uniform frequency response to sound pressure at the
microphone diaphragm
Note 1 to entry: For frequencies whose wavelength is long compared to the diameter of the microphone
diaphragm, pressure type microphones typically have uniform frequency response to plane progressive waves
arriving at angles substantially in the plane of the microphone diaphragm, which is also described as grazing
incidence.
3.20
principal axis
line through the centre of and perpendicular to the diaphragm of the microphone
3.21
reference flight path
intended path through space of the aircraft being tested, as defined by the relevant
certification test protocol
3.22
reference frequency
specified frequency of the sinusoidal sound pressure signal produced by the sound calibrator
Note 1 to entry: The reference frequency is expressed in hertz.
3.23
reference level difference
level difference on the reference level range for a reference electrical input signal
corresponding to the reference sound pressure level
Note 1 to entry: The reference level difference is expressed in decibels.
3.24
reference level range
level range for verifying the acoustical sensitivity of the measurement system, and containing
the reference sound pressure level
Note 1 to entry: The reference level range is expressed in decibels.
3.25
reference sound pressure level
sound pressure level produced, under reference environmental conditions, in the cavity of the
coupler of the sound calibrator that is used to verify the overall acoustical sensitivity of a
measurement system
Note 1 to entry: The reference sound pressure level is expressed in decibels.
3.26
sound-incidence angle
angle between the principal axis of the microphone and a line from the sound source to the
centre of the diaphragm of the microphone
Note 1 to entry: When the sound incidence angle is 0°, the sound is said to be received at the microphone at
"normal incidence"; when the sound incidence angle is 90°, the sound is said to be received at "grazing incidence".
Note 2 to entry: The sound-incidence angle is expressed in degrees.
4 Purpose
The purpose of this document is to ensure that different measurement systems used for
aircraft noise certification have the same electroacoustical characteristics within stated
acceptance limits under specified reference environmental conditions. This document does
not provide recommendations for installation of microphones or microphone windscreens, nor
requirements for measurement and analysis procedures used in aircraft noise certification, but
gives only the performance specifications for the measurement systems used to provide
sound pressure levels over specific frequency ranges and weightings, averaged over a period
of time.
Certain of the requirements apply to the complete measurement system, including any means
of recording a time waveform of the sound pressure signal to be measured prior to analysis.
Other requirements apply specifically to the microphone, which generates an electrical signal
in response to the sound pressure received. Still further requirements apply only to the
instruments used to operate on that signal in order to provide an output in the form of
one-third-octave-band or weighted sound pressure levels.
The requirements of this document can be applied to the instruments used to measure the
sound produced by each category of aircraft covered by certification protocols such as [1]
(see Table 1). Three different measurement protocols are mentioned in [1] This document
does not provide measurement recommendations for aircraft subject to certification
requirements earlier than those in effect at the time of its publication.
Table 1 – ICAO measurement protocols (informative)
ICAO Aircraft category and mass Microphone Noise metric ICAO Annex 16
Annex 16 configuration Appendix
chapter
3, 4 or 14 Subsonic jet, and propeller Grazing incidence EPNdB 2
aircraft > 8 618 kg
8 Helicopters Grazing incidence EPNdB 2
10 Propeller aircraft ≤ 8 618 kg Ground plane L 6
ASmax
11 Helicopters ≤ 3 175 kg Grazing incidence L 4
AE
13 Tiltrotors Grazing incidence EPNdB 2
____________
Numbers in square brackets refer to the Bibliography.
– 12 – IEC 61265:2018 © IEC 2018
5 Requirements
5.1 General
5.1.1 Output data
The measurement system shall provide time-indexed one-third-octave-band or A-weighted
sound level measurement values as appropriate for the analysis method specified.
For measurements to be used to calculate values in effective perceived noise level (EPNL),
the measurement system shall provide time-average one-third-octave-band sound pressure
levels covering at least the range of mid-band frequencies from 50 Hz to 10 kHz.
For L or L measurements, the measurement system shall provide either
AE ASmax
one-third-octave levels, or A-weighted sound pressure levels as provided by instruments
conforming to IEC 61672-1. One-third-octave measurements can be converted to the required
L or L values in subsequent data-analysis procedures not covered by this document.
AE ASmax
5.1.2 Time-pressure history
The measurement system may record the unmodified time-pressure history at the
microphone, allowing the original waveform to be reproduced at any time for subsequent
processing and analysis, instead of storing the results immediately in one-third-octave or
A-weighted values. Some certification requirements specify mandatory recording of
time-pressure history along with integrated values. Even when such recording is not required,
it is strongly recommended. When time-pressure history is recorded, it shall be an
uncompressed, time indexed pulse-code modulation waveform file, along with recordings of
calibration events, so that the measurement can be reconstructed in the future to
accommodate other weighting or analysis protocols.
5.1.3 Measurements of sound from aircraft ground power units
Measurements of sound levels from aircraft ground power units shall be made with sound
level meters in accordance with IEC 61672-1, with particular attention to calibration and the
potential influence of the sound scattering properties of the user holding the sound level
meter, if present.
5.1.4 Measurements of aircraft operations on the ground
Noise from aircraft operations on the ground, including testing of aircraft engines, is typically
measured using grazing incidence microphone systems (see 5.5 for additional provisions)
with the principal axis of the microphone oriented vertically, and evaluated using perceived
noise level (PNL) or A-weighted sound pressure level (L ) to approximate human
ASmax
perception. Free-field microphones as typically provided with sound level meters may also be
used, provided that the angle of sound arrival is within the angles for which free-field
sensitivity is within the acceptance limits of 5.5.2.2.
5.2 Measurement uncertainty
5.2.1 Relationship with regulatory requirements
Measurements of acoustic emission for aircraft certification are made in accordance with
regulatory requirements of ICAO Annex 16 or national certifying authorities, which establish a
noise limit for particular aircraft. Certifying authorities can require documentation of
measurement uncertainty to establish confidence that the noise measurement system
complies with this document. See Annex B for a discussion of the principles used in
establishing measurement uncertainty.
5.2.2 Performance verification
Each component of the system shall be calibrated, with traceability to relevant standards,
including any accessories such as preamplifiers used in the measurement. Passive electrical
accessories such as cables and connectors need not be included in the analysis. A
description of the calibration procedure shall include each step used to derive free-field
sensitivity in the directions over which measurements are made. The calibration record should
include an analysis of the error margin or uncertainty expected based on manufacturers’ data
and the uncertainty of the calibrations employed, to a coverage level of 95 %.
5.2.3 Periodic tests
Methods for periodic tests of the electroacoustical performance of a measurement system
giving results suitable for comparison with the requirements of this document are described in
Annex A. The results of any such tests shall be adjusted to correspond to reference
environmental conditions (see 5.3 for additional provisions).
5.3 Reference environmental conditions
Reference environmental conditions for specifying the performance of a measurement system
are:
• air temperature 23 °C;
• static air pressure 101,325 kPa;
• relative humidity 50 %.
5.4 Sound calibrator
A sound calibrator compliant with at least the Class 1 or Class 1/C requirements of IEC 60942
shall be used to check the overall acoustical sensitivity of the measurement system. The
reference sound pressure level produced in the cavity of the coupler of the sound calibrator
shall be calculated for the reference environmental conditions of 5.3, using
manufacturer-supplied information on the influence of atmospheric air pressure and
temperature if required.
National aircraft certification authorities can permit the use of calibrators meeting lower
accuracy requirements, if the additional uncertainty is stated.
5.5 Microphone system
5.5.1 Pressure and free-field type microphones
Microphones used for measurements in accordance with this document are omnidirectional
pressure transducers, responding to an instantaneous difference between the incident
pressure from the sound field and that of an internal volume of air which is coupled to the
sound field through a long acoustic time constant. Microphones are typically referred to as
"pressure type" when their transfer function is linear with respect to frequency for sound
pressure at the diaphragm, or "free-field type" when the transfer function is linear with respect
to frequency for an incident plane progressive wave, which is typically found in "free-field"
conditions. Either pressure type or free-field type microphones can be found that meet the
performance requirements of 5.5.2 and 5.5.3.
Microphones used in grazing incidence and ground plane microphone configuration are
typically referred to as "pressure type" microphones, with substantially uniform frequency
response to sound pressure integrated over the diaphragm surface.
Pressure type microphones have substantially uniform free-field sensitivity for distant sound
arriving at 90° to the principal axis, and exhibit increased sensitivity at high frequencies for
angles closer to normal incidence. Aircraft noise certification standards are based on these
characteristics, so that corrections are not required for sound incidence angles within ± 30° of
– 14 – IEC 61265:2018 © IEC 2018
grazing incidence. Pressure type microphones should not be used where the principal axis of
the microphone is oriented toward the sound source.
Free-field type microphones are typically used in sound level meters, and are intended for use
where the microphone principal axis is aligned to face a sound source. Measurements made
by pointing the microphone toward the sound source, such as for measurement of ground
equipment, are typically made with free-field microphones. Provided that they meet the
performance requirements of 5.5.2 and 5.5.3, such microphones function over the frequency
range of interest as pressure type microphones and are appropriate for use for aircraft
certification measurements.
Microphones conforming with the requirements of IEC 61094-4 [2], type WS2P, may be used
for measurements in accordance with this document. Other types may be used if they meet
the relevant electroacoustic requirements of this document.
5.5.2 Grazing incidence microphone configuration
5.5.2.1 General
Microphones used for grazing incidence measurements in accordance with Appendix 2 or 4 of
ICAO Annex 16 are mounted at 1,2 m above ground level, oriented with the diaphragm facing
away from the ground and substantially in the plane defined by the reference flight path of the
aircraft and the measuring station.
5.5.2.2 Frequency response in the plane of measurement
The free-field sensitivity level of the microphone used in grazing incidence configuration, at
90° to the principal axis of the microphone, at frequencies over at least the range of one-third-
octave mid-band frequencies from 50 Hz to 5 kHz, shall be within ± 1,0 dB of that at the
reference frequency, and within ± 2,0 dB for mid-band frequencies of 6,3 kHz, 8 kHz and
10 kHz. This response can be verified by applying the manufacturer’s correction curve for
different angles of incidence to the response as measured in a closed coupler or with an
electrostatic actuator, as required by the manufacturer’s calibration procedure.
5.5.2.3 Directional response
At each one-third-octave mid-band frequency over the range from 50 Hz to 10 kHz, the
free-field sensitivity levels of the microphone used in grazing incidence configuration at
sound-incidence angles of 30°, 60°, 90°, 120° and 150° (see Figure 1) shall not differ from the
free-field sensitivity level at a sound-incidence angle of 0° ("normal incidence") by more than
the values shown in Table 2. The free-field sensitivity level differences at sound-incidence
angles between any two adjacent sound incidence angles in Table 2 shall not exceed the
acceptance limit for the greater angle.
NOTE The values shown in Table 2 typically correspond to the characteristics of a "half inch" or smaller
measurement microphone.
90°
120° 60°
150°
30°
Principal axis
0°
IEC
Figure 1 – Illustration of sound incidence angles
from the principal axis of the microphon
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