IEC 61260-1:2014

IEC 61260-1 ®
Edition 1.0 2014-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Octave-band and fractional-octave-band filters –
Part 1: Specifications
Électroacoustique – Filtres de bande d'octave et de bande d'une fraction
d'octave –
Partie 1: Spécifications
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IEC 61260-1 ®
Edition 1.0 2014-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electroacoustics – Octave-band and fractional-octave-band filters –

Part 1: Specifications
Électroacoustique – Filtres de bande d'octave et de bande d'une fraction

d'octave –
Partie 1: Spécifications
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX X
ICS 17.140.50 ISBN 978-2-8322-1417-6

– 2 – 61260-1 © IEC:2014
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references. 7
3 Terms and definitions . 8
4 Reference environmental conditions . 12
5 Performance requirements . 12
5.1 General . 12
5.2 Octave frequency ratio . 13
5.3 Reference frequency . 13
5.4 Exact mid-band frequencies . 13
5.5 Nominal mid-band frequencies . 13
5.6 Band-edge frequencies . 14
5.7 Time-averaged signal levels . 14
5.8 Filter attenuation . 14
5.9 Reference attenuation . 15
5.10 Relative attenuation . 15
5.11 Normalized effective bandwidth . 18
5.12 Effective bandwidth deviation . 19
5.13 Linear operating range . 19
5.14 Time-invariant operation . 20
5.15 Anti-alias filters . 20
5.16 Summation of output signals . 21
5.17 Overload indicator . 21
5.18 Filter decay time. 21
5.19 Maximum input signal . 21
5.20 Output terminals and terminating impedances . 22
5.21 Power supply check . 22
5.22 Sensitivity to various environments . 22
5.22.1 General . 22
5.22.2 Ambient air temperature and relative humidity . 22
5.23 Electrostatic-discharge and electromagnetic-compatibility requirements . 22
5.23.1 General . 22
5.23.2 Electrostatic discharges . 23
5.23.3 Immunity to power-frequency and radio-frequency fields . 23
5.23.4 Emission limits . 25
6 Instrument marking . 25
7 Instruction manual . 26
7.1 General . 26
7.2 Operation . 26
7.3 Testing . 27
Annex A (informative) Relationship between tolerance interval, corresponding
acceptance interval and the maximum-permitted uncertainty of measurement . 28
Annex B (normative) Maximum-permitted expanded uncertainties of measurement . 29
Annex C (informative) Examples of conformance assessment to specifications of this
standard . 30

61260-1 © IEC:2014 – 3 –
C.1 General . 30
C.2 Conformance criteria . 30
C.3 Example test results . 31
Annex D (informative) Base 2 filters . 33
Annex E (normative) Nominal mid-band frequencies . 34
E.1 Mid-band frequencies for octave-band and one-third-octave-band filters . 34
E.2 Mid-band frequencies for one-half-octave-band filters . 34
E.3 Mid-band frequencies for other bandwidths . 34
Annex F (informative) Normalized frequencies at breakpoints of acceptance limits on
minimum and maximum relative attenuation for one-third-octave-band filters . 36
Annex G (informative) Filter response to exponentially swept sinusoidal signals . 38
G.1 Exponential frequency sweep . 38
G.2 Response of set of band-pass filters to a sweep . 38
Annex H (informative) Measurement of filter decay time . 41
H.1 General . 41
H.2 Measurement of filter decay time . 41
H.2.1 Instruments with the capability to measure reverberation time . 41
H.2.2 Instruments without the capability to measure reverberation
time . 41
Bibliography . 43

Figure 1 – Minimum and maximum limits on relative attenuation as a function of f/f
m
for class 1 and class 2 octave-band filters . 17
Figure A.1 – Relationship between tolerance interval, corresponding acceptance
interval and the maximum-permitted uncertainty of measurement . 28
Figure C.1 – Examples of conformance assessment . 32
Figure G.1 – Relation between the logarithmic frequency scale and the linear time
scale due to the exponential sweep . 40

Table 1 – Acceptance limits on relative attenuation for octave-band filters . 15
Table 2 – Limits for radiated disturbance of class B Information Technology Equipment
(ITE) at a distance of 10 m . 25
Table 3 – Limits for conducted disturbance to the voltage of a public supply of electric
power . 25
Table B.1 – Maximum-permitted expanded uncertainties of measurement . 29
Table C.1 – Examples of conformance assessment . 31
Table E.1 – Mid-band frequencies for octave-band and one-third-octave-band filters in
the audio range . 35
Table F.1 – Acceptance limits on relative attenuation for one-third-octave-band filters . 37

– 4 – 61260-1 © IEC:2014
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROACOUSTICS –
OCTAVE-BAND AND FRACTIONAL-OCTAVE-BAND FILTERS –

Part 1: Specifications
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
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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
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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
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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 61260-1 has been prepared by IEC technical committee 29:
Electroacoustics.
This first edition of IEC 61260-1, future IEC 61260-2 and future IEC 61260-3, cancel and
replace the first edition of IEC 61260 published in 1995, and Amendment 1:2001. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the
IEC 61260:
a) the single document in the first edition of IEC 61260:1995 is in IEC 61260 series
separated into the three parts covering: specifications, pattern evaluation tests and
periodic tests;
b) the IEC 61260:1995 specified three performance categories: classes 0, 1 and 2. The
IEC 61260 series specifies requirements for class 1 and 2;
c) in the IEC 61260:1995, the design goals for the specification can be based on base-2 or
base 10 design. In IEC 61260 series only base-10 is specified;

61260-1 © IEC:2014 – 5 –
d) the reference environmental conditions have been changed from 20 °C / 65 % RH to
23 °C / 50 % RH;
e) IEC 61260:1995 specified tolerance limits without considering the uncertainty of
measurement for verification of the specifications. IEC 61260 series specifies acceptance
limits for the observed values and maximum-permitted uncertainty of measurements for
laboratories testing conformance to specifications in the standard.
The text of this standard is based on the following documents:
FDIS Report on voting
29/835/FDIS 29/839/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts in the IEC 61260 series, published under the general title
Electroacoustics – Octave-band and fractional-octave-band filters 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.
– 6 – 61260-1 © IEC:2014
INTRODUCTION
IEC 61260:1995 and its Amendment 1:2001 are now separated into the following three parts
of IEC 61260 series:
• Part 1: Specifications
• Part 2: Pattern evaluation tests (under consideration)
• Part 3: Periodic tests (under consideration)
For assessments of conformance to performance specifications, IEC 61260-1 uses different
criteria than were used for the IEC 61260:1995 edition.
IEC 61260:1995 did not provide any requirements or recommendations to account for the
uncertainty of measurement in assessments of conformance to specifications. This absence
of requirements or recommendations to account for uncertainty of measurement created
ambiguity in determinations of conformance to specifications for situations where a measured
deviation from a design goal was close to a limit of the allowed deviation. If conformance was
determined based on whether a measured deviation did or did not exceed the limits, the end-
user of the octave-band and fractional-octave-band filters incurred the risk that the true
deviation from a design goal exceeded the limits.
To remove this ambiguity, IEC Technical Committee 29, at its meeting in 1996, adopted a
policy to account for measurement uncertainty in assessments of conformance in International
Standards that it prepares.
This first edition of IEC 61260-1 uses an amended criterion for assessing conformance to a
specification. Conformance is demonstrated when (a) measured deviations from design goals
do not exceed the applicable acceptance limits and (b) the uncertainty of measurement does
not exceed the corresponding maximum-permitted uncertainty. Acceptance limits are
analogous to the tolerance limits allowances for design and manufacturing implied in the
IEC 61260:1995.
Actual and maximum-permitted uncertainties of measurement are determined for a coverage
probability of 95 %. Unless more-specific information is available, the evaluation of the
contribution of a specific filter or filter set to a total measurement uncertainty can be based on
the acceptance limits and maximum-permitted uncertainties specified in this standard.

61260-1 © IEC:2014 – 7 –
ELECTROACOUSTICS –
OCTAVE-BAND AND FRACTIONAL-OCTAVE-BAND FILTERS –

Part 1: Specifications
1 Scope
1.1 This part of the IEC 61260 series specifies performance requirements for analogue,
sampled-data, and digital implementations of band-pass filters. The extent of the pass-band
region of a filter's relative attenuation characteristic is a constant percentage of the exact mid-
band frequency for all filters of a given bandwidth. An instrument conforming to the
requirements of this standard may contain any number of contiguous band-pass filters
covering any desired frequency range.
1.2 Performance requirements are provided for two filter classes: class 1 and class 2. In
general, specifications for class 1 and class 2 filters have the same design goals and differ
mainly in the acceptance limits and the range of operational temperature. Acceptance limits
for class 2 are greater than, or equal to, those for class 1. Maximum-permitted expanded
uncertainties of measurement are also specified.
1.3 Performance requirements are given for designs where the octave frequency ratio and
the mid-band frequencies are powers of ten.
1.4 Band-pass filters conforming to the performance requirements of this standard may be
part of various measurement systems or may be an integral component of a specific
instrument such as a spectrum analyser.
1.5 This standard specifies the ranges of environmental conditions for operation of the
filters. The required range depends on whether the instrument containing the filters is
designed to be operated in a controlled environment or more generally in the field.
1.6 Band-pass filters conforming to the requirements of this standard are capable of
providing frequency-band-filtered spectral information for a wide variety of signals, for
example, time-varying, intermittent or steady; broadband or discrete frequency; and long or
short durations.
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 61000-4-2, Electromagnetic compatibility (EMC) – Part 4-2: Testing and measurement
techniques – Electrostatic discharge immunity test
IEC 61000-4-3:2006, Electromagnetic compatibility (EMC) – Part 4-3: Testing and
measurement techniques – Radiated, radio-frequency, electromagnetic field immunity test
IEC 61000-6-1:2005, Electromagnetic compatibility (EMC) – Part 6-1: Generic standards –
Immunity for residential, commercial and light-industrial environments

– 8 – 61260-1 © IEC:2014
IEC 61000-6-2:2005, Electromagnetic compatibility (EMC) – Part 6-2: Generic standards –
Immunity for industrial environments
IEC 61000-6-3:2006, Electromagnetic compatibility (EMC) – Part 6-3: Generic standards –
Emission standard for residential, commercial and light-industrial environments
Amendment 1:2010
IEC 61672-1, Electroacoustics – Sound level meters – Part 1: Specifications
CISPR 22:2008, Information technology equipment – Radio disturbance characteristics –
Limits and methods of measurement
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM: 1995)
ISO/IEC Guide 98-4:2012, Uncertainty of measurement – Part 4: Role of measurement
uncertainty in conformity assessment
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61000-4-2,
IEC 61000-4-3, IEC 61000-6-1, IEC 61000-6-2, and IEC 61000-6-3, as well as the following
apply.
3.1
band-pass filter
filter with a single transmission band (or pass-band with small relative attenuation) extending
from a lower band-edge frequency greater than zero to a finite upper band-edge frequency
3.2
octave frequency ratio
frequency ratio nominally equal to an octave or a frequency ratio of 2:1
Note 1 to entry: 5.2.1 gives the expression of the octave frequency ratio for this standard.
3.3
bandwidth designator
reciprocal of a positive integer, including 1, to designate the fraction of an octave band
Note 1 to entry: The bandwidth designator is used to designate the nominal bandwidth of the filters in a set of
filters, for example, for 1/b = 1/12, the filters are designated as one-twelfth-octave-band filters.
3.4
reference frequency
single frequency selected to normalize the attenuation response for all band-pass filters in a
filter set
Note 1 to entry: The reference frequency is expressed in hertz (Hz).
3.5
exact mid-band frequency
frequency that has a specified relationship to the reference frequency such that the ratio of
the exact mid-band frequencies of any two contiguous band-pass filters is the same for all
filters in a filter set of a specified bandwidth
Note 1 to entry: Exact mid-band frequency is expressed in hertz (Hz).

61260-1 © IEC:2014 – 9 –
3.6
nominal mid-band frequency
rounded mid-band frequency for the designation of band-pass filters
Note 1 to entry: Nominal mid-band frequency is expressed in hertz (Hz).
3.7
normalized frequency
for a band-pass filter, ratio of a frequency to the corresponding exact mid-band frequency
3.8
band-edge frequencies
frequencies at the lower and upper edges of the pass-band of a band-pass filter such that the
exact mid-band frequency is the geometric mean of the lower and upper band-edge
frequencies
Note 1 to entry: Band-edge frequencies are expressed in hertz (Hz).
3.9
normalized bandwidth of a filter
relative bandwidth for a given filter, the ratio of the upper band-edge frequency minus the
corresponding lower band-edge frequency to the exact mid-band frequency
3.10
octave-band filter
band-pass filter for which the ratio of upper band-edge frequency to lower band-edge
frequency is the octave frequency ratio
3.11
fractional-octave-band filter
band-pass filter for which the ratio of upper band-edge frequency to lower band-edge
frequency is the octave frequency ratio raised to an exponent equal to the applicable
bandwidth designator
Note 1 to entry: An octave-band filter is also a fractional-octave-band filter (1/b = 1/1).
3.12
signal level
time-average signal level
at any frequency, ten times the logarithm to the base ten of the ratio of a specified time-mean-
square signal to the square of a specified reference value
Note 1 to entry Time-averaged signal level is expressed in decibels (dB).
3.13
filter attenuation
at any frequency, for a band-pass filter, the input signal level minus the corresponding output
signal level
Note 1 to entry: Filter attenuation is expressed in decibels (dB).
3.14
reference attenuation
for all band-pass filters in an instrument, nominal filter attenuation in the pass-band for
determining relative attenuation
Note 1 to entry: Reference attenuation is expressed in decibels (dB).
3.15
relative attenuation
filter attenuation minus the reference attenuation

– 10 – 61260-1 © IEC:2014
Note 1 to entry: Relative attenuation is expressed in decibels (dB).
3.16
normalized response
at any normalized frequency, the anti-logarithm to the base ten of minus one-tenth of the
corresponding relative attenuation
3.17
normalized effective bandwidth
integral over normalized frequency of the normalized response of a band-pass filter to
constant-amplitude sinusoidal input signals, the normalized response being weighted with the
inverse of the normalized frequency
3.18
normalized reference effective bandwidth
normalized effective bandwidth for a band-pass filter having zero relative attenuation in the
passband and infinite relative attenuation at other frequencies
3.19
effective bandwidth deviation
ten times the logarithm to the base ten of the ratio of the normalized effective bandwidth of a
filter to the normalized reference effective bandwidth
Note 1 to entry: Effective bandwidth deviation is expressed in decibels (dB).
3.20
reference level range
one of the available level ranges specified for testing the electrical performance
characteristics of the band-pass filters in a filter set
3.21
reference input signal level
specified reference level of the input signal on the reference level range
Note 1 to entry: The reference input signal level is expressed in decibels (dB).
3.22
level linearity deviation
on any level range at the exact mid-band frequency, if not otherwise specified, an indicated
output signal level minus the anticipated output signal level
Note 1 to entry: Level linearity deviation is expressed in decibels (dB).
3.23
linear operating range
for a stated filter and a stated level range, the extent of steady sinusoidal input signal levels
over which level linearity deviations do not exceed the applicable limits of this standard
Note 1 to entry: Linear operating range is expressed in decibels (dB).
3.24
level range control
device for adjusting the sensitivity of a band-pass filter in response to changes in the level
of the input signal in order to maintain the overall operation of the filter within the linear
operating range
3.25
measurement range
for any exact mid-band frequency, the range from the lower boundary of the input signal
level for the linear operating range on the most-sensitive level range to the upper boundary
of the input signal level for the linear operating range on the least-sensitive level range

61260-1 © IEC:2014 – 11 –
Note 1 to entry: A measurement range is expressed in decibels (dB).
3.26
analogue filter
filter that operates continuously on an input signal to produce a filtered output
3.27
sampled-data filter
computational process that operates on samples of an input signal to produce a filtered
output
3.28
digital filter
subset of sampled-data filters that operates on digitized samples of input data
3.29
time-invariant operation
operational mode or capability of a system of band-pass filters such that the response to a
signal is independent of the time when the signal was applied
3.30
filter decay time
at a stated frequency, elapsed time required for the output signal level to decrease by 60 dB
after sudden cessation of the signal from the input to the filter
Note 1 to entry: Filter decay time is expressed in seconds (s).
3.31
reference orientation
orientation of a band-pass filter with respect to the principal direction of an emitter or receiver
of radio-frequency fields
3.32
group X band-pass filter
self-contained instrument that includes band-pass filtering facilities conforming to the
requirements of this standard and which specifies internal battery power for the normal mode
of operation and requiring no external connection to other apparatus to operate the instrument
3.33
group Y band-pass filter
self-contained instrument that includes band-pass filtering facilities conforming to the
requirements of this standard and which specifies connection to a public supply of electrical
power for the normal mode of operation and also requiring no external connection to other
apparatus to operate the instrument
3.34
group Z band-pass filter
instrument that includes band-pass filtering facilities conforming to the requirements of this
standard and requiring two or more items of equipment to be connected together by some
means for the normal mode of operation, with operation either from batteries or from a public
supply of electrical power
Note 1 to entry: If the items communicate by means of radio or optical methods, but are not connected by any
conductive device, the items are not connected in this context.
3.35
coverage probability
probability that the set of true quantity values of a measurand is contained within a specified
coverage interval
– 12 – 61260-1 © IEC:2014
[SOURCE: ISO/IEC Guide 98-4:2012, 3.2.8]
3.36
acceptance limit
specified upper or lower bound of permissible measured quantity values
[SOURCE: ISO/IEC Guide 98-4:2012, 3.3.8]
4 Reference environmental conditions
Reference environmental conditions are as follows:
• temperature  23 °C
• static pressure 101,325 kPa
• relative humidity 50 %
5 Performance requirements
5.1 General
5.1.1 Electrical response characteristics specified in this standard for fractional-octave-
band filters apply under the reference environmental conditions of Clause 4, if not otherwise
stated.
5.1.2 Any filter design realization may be utilized provided the resulting filters conform to
all applicable requirements of this standard.
5.1.3 Band-pass filters may be powered by batteries or from external power supply
systems.
5.1.4 The configuration of the filter shall be as specified in the Instruction Manual for one
of the normal modes of operation, including required accessories.
5.1.5 For filters enclosed in a sound level meter with detachable preamplifier, the signal
input to the filter may be, as specified by the supplier, the input of the preamplifier through a
suitable input device replacing the microphone, or the terminal where the signal from the
preamplifier normally is connected.
5.1.6 Acceptance limits in this standard include allowances for design, manufacturing and
aging.
5.1.7 In subsequent subclauses, acceptance limits are provided for allowable values of
measured deviations from design goals. Annex A describes the relationship between
tolerance interval, corresponding acceptance interval and the maximum-permitted uncertainty
of measurement.
5.1.8 For pattern-evaluation tests and periodic tests, the laboratory shall determine that
their actual expanded uncertainties, as the 95 % coverage intervals in accordance with
ISO/IEC Guide 98-3 and ISO/IEC Guide 98-4, do not exceed the maximum-permitted
expanded uncertainties specified in Annex B.
5.1.9 Conformance to the specifications is demonstrated when (a) the measured deviations
from the design goals do not exceed the applicable acceptance limits and (b) the
corresponding actual expanded uncertainties of measurements does not exceed the
corresponding maximum-permitted uncertainty of measurement given in Annex B.

61260-1 © IEC:2014 – 13 –
5.1.10 Annex C gives examples of evaluation of conformance to specifications of this
standard.
5.2 Octave frequency ratio
5.2.1 For this standard, the octave frequency ratio, G, shall be given by the following
expression
3/10
G = 10
(1)
5.2.2 The octave frequency ratio calculated from Formula (1) to six significant digits is
1,995 26. Filters designed according to this ratio are designated base-10 filters.
NOTE 1 Filters specified in this standard are by convention called octave-band and fractional-octave band filters.
NOTE 2 For technical reasons, some filters have been designed based on G = 2, exactly. Such filter designs are
called base-2 filters. The probability that a base-2 filter conforms to the requirements of this standard decreases as
the difference between the mid-band frequency and the reference frequency increases; see Annex D.
5.3 Reference frequency
For the purposes of this standard, the reference frequency, f , is 1 000 Hz, exactly.
r
5.4 Exact mid-band frequencies
5.4.1 When the denominator of the bandwidth designator is an odd number, the exact mid-
band frequencies, f , of any filter in a set of filters shall be determined from the following
m
expression
x/b
f = f G (2)
m r
where f is the reference frequency and 1/b is the bandwidth designator, for example 1/1 or
r
1/3 for octave-band or one-third-octave-band filters, respectively.
5.4.2 When the denominator of the bandwidth designator is an even number, exact mid-
band frequencies of any filter in a set of filters shall be determined from the following
expression
(2x+1)/(2b)
f = f G (3)
m r
where x in Formulas (2) and (3) is any integer, positive, negative or zero.
NOTE 1 The outputs of narrow-bandwidth fractional-octave-band filters that have exact mid-band frequencies
determined from Formula (2) or Formula (3) can be combined to approximate the band level indicated by a filter of
wider bandwidth with a corresponding exact mid-band frequency and corresponding band-edge frequencies.
NOTE 2 When the denominator of the bandwidth designator is an odd number, one of the filters in a complete
filter set can have a mid-band frequency of 1 000 Hz. When the denominator of the bandwidth designator is an
even number, the band-edge frequencies of an adjacent pair of filters in a complete filter set can be at 1 000 Hz
and none of the filters will have a mid-band frequency of 1 000 Hz.
5.5 Nominal mid-band frequencies
Octave-band and fractional-octave-band filters shall be identified, or labelled, by their nominal
mid-band frequencies. Annex E provides exact and nominal mid-band frequencies for octave-
band and one-third-octave-band filters for the usual range of audio frequencies. Annex E also
specifies a procedure for determining the nominal mid-band frequencies for fractional-octave-
band filters with other bandwidth designators.

– 14 – 61260-1 © IEC:2014
5.6 Band-edge frequencies
5.6.1 Lower and upper band-edge frequencies for a pass-band filter shall be determined
from the following expressions:
-1/(2b)
f = f G (4)
1 m
and
+1/(2b)
f = f G (5)
2 m
where
f is the lower band-edge frequency;
f is the upper band-edge frequency;
G is the octave frequency ratio given by Formula (1), and
f is an exact mid-band frequency determined from Formula (2) or Formula (3).
m
NOTE An exact mid-band frequency is the geometric mean of the corresponding band-edge frequencies as given
by f = f f .
m 1 2
1/b 3/10
5.6.2 A band-edge frequency ratio is given by f /f = G , for example 10 for octave-
2 1
1/10
for one-third-octave-band filters.
band filters and 10
+1/(2b) -1/(2b)
5.6.3 The normalized bandwidth of a filter is given by (f – f )/f = G – G .
2 1 m
5.7 Time-averaged signal levels
5.7.1 A time-averaged signal level, L, shall be determined according to the following
expression:
T
(1/ T ) V (t )dt
∫
L = 10lg dB (6)
V
where
V(t)is the instantaneous signal as a function of time t,
T is the elapsed time for integration and averaging, and
V is an appropriate reference value such as 1 µV if the signal is a voltage.
5.7.2 The reference value shall be the same for the level of input signals and output
signals.
5.8 Filter attenuation
5.8.1 For any normalized frequency, Ω = f/f , filter attenuation, A(Ω), shall be determined
m
from the following expression:
A(Ω) = L (Ω) – L (Ω) (7)
in out
where
L (Ω) is the time-averaged level of the input signal and
in
61260-1 © IEC:2014 – 15 –
L (Ω)is the corresponding time-averaged level of the output signal.
out
5.8.2 For measurement of filter attenuation, the resolution of the indications of the levels of
the input and output signals shall be 0,1 dB or smaller.
5.9 Reference attenuation
5.9.1 The instruction manual shall specify the reference attenuation in the pass-band. The
reference attenuation shall be nominally the same for all filters of all available filter
bandwidths in a set of filters.
5.9.2 Verification of the specified reference attenuation may require that the filters be
adjusted according to a procedure described in the instruction manual.
5.10 Relative attenuation
5.10.1 Relative attenuation, ΔA(Ω)at normalized frequency Ω = f/f , shall be determined
m
from the following expression:
ΔA(Ω) = A(Ω) – A (8)
ref
where
A is the reference attenuation.
ref
5.10.2 For class 1 or class 2 octave-band filters, in the pass-band from Ω to Ω , the
1 2
relative attenuation of any filter shall be within the acceptance limits in Table 1 for the
minimum and maximum relative attenuations at the specified octave-band normalized
frequencies. In the stop-bands for Ω < Ω and Ω > Ω , the relative attenuation shall be not
1 2
less than the minimum acceptance limits in Table 1.
Table 1 – Acceptance limits on relative attenuation for octave-band filters
Minimum and maximum acceptance limits on relative attenuation
Normalized frequency
in dB
Ω = f/f
m
Class 1 Class 2
-4
Ω ≤ G +70; +∞ +60; +∞
l
-3
G +60; +∞ +54; +∞
Ω
l
-2
Ω G +40,5; +
...