SIST EN ISO 11820:1999

SLOVENSKI STANDARD
SIST EN ISO 11820:1999
01-november-1999
Akustika – Merjenje dušilnikov na mestu delovanja (ISO 11820:1996)
Acoustics - Measurements on silencers in situ (ISO 11820:1996)
Akustik - Messungen an Schalldämpfern im Einsatzfall (ISO 11820:1996)
Acoustique - Mesurage sur silencieux in situ (ISO 11820:1996)
Ta slovenski standard je istoveten z: EN ISO 11820:1996
ICS:
17.140.01 $NXVWLþQDPHUMHQMDLQ Acoustic measurements and
EODåHQMHKUXSDQDVSORãQR noise abatement in general
SIST EN ISO 11820:1999 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------

SIST EN ISO 11820:1999

---------------------- Page: 2 ----------------------

SIST EN ISO 11820:1999

---------------------- Page: 3 ----------------------

SIST EN ISO 11820:1999

---------------------- Page: 4 ----------------------

SIST EN ISO 11820:1999

---------------------- Page: 5 ----------------------

SIST EN ISO 11820:1999

---------------------- Page: 6 ----------------------

SIST EN ISO 11820:1999
Is0
INTERNATIONAL
STANDARD 11820
First edition
1996-l 2-I 5
Acoustics - Measurements on silencers
in situ
Acoustique - Mesurages sur silencieux in situ
Reference number
IS0 11820: 1996(E)

---------------------- Page: 7 ----------------------

SIST EN ISO 11820:1999
IS0 11820: 1996(E)
Page
Contents
1 Scope .
..................................................................
2 Normative references
3 Definitions .
................................................
4 Corrections for background noise
4.1 Transmission sound pressure level difference. .
4.2 Insertion sound pressure level difference .
..................................................................
5 Installation conditions
6 Measuring instruments .
..........................................................
6.1 Acoustic instruments
6.2 Air flow, static pressure and temperature measuring
7
devices .
7
7 Test object and measuring conditions .
7
8 Measurement procedures .
............................................................................... 7
8.1 General
7
8.2 Acoustic measurements .
.............. 9
8.3 Flow, pressure and temperature measurements.
9 Evaluation . 10
.................. 10
9.1 Evaluation of sound pressure measurements.
Evaluation of flow measurements . 12
9.2
10 Information to be recorded . 13
14
11 Information to be reported .
Annexes
A Field corrections . 15
B Calibration of directional microphones and microphones
...................................... 17
equipped with a turbulence windscreen
C Bibliography . 18
0 IS0 1996
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
ii

---------------------- Page: 8 ----------------------

SIST EN ISO 11820:1999
IS0 11820: 1996(E)
0 IS0
Foreword
IS0 (the International Organization for Standardization) is a worldwide fed-
eration of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
technical committees. Each member body interested in a subject for which
a technical committee has been established has the right to be represented
on that committee. International organizations, governmental and non-
governmental, in liaison with ISO, also take part in the work. IS0 collab-
orates closely with the International Electrotechnical Commission (IEC) on
all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are cir-
culated to the member bodies for voting. Publication as an International
75 % of the member bodies casting
Standard requires approval by at least
a vote.
International Standard IS0 11820 was prepared by Technical Committee
lSO/TC 43, Acoustics, Subcommittee SC 1, Noise.
Annexes A to C of this International Standard are for information only.

---------------------- Page: 9 ----------------------

SIST EN ISO 11820:1999
IS0 11820:1996(E) @ IS0
Introduction
This International Standard gives a method for evaluating the acoustic
performance of silencers under plant-operating conditions. The attenuation
losses determined express the extent to which the level of sound power
passing through a duct, or across the internal cross-section of an aperture
or opening (e.g. in an enclosure or a building) is reduced by the use of a
silencer. Sound transmission via flanking elements is attributed to the
silencer performance unless the flanking element is not a part of the
silencer or of the related duct walls. The influences of flow noise and of
alterations to the operating conditions with and without a silencer are
included.
In laboratory measurements on ducted silencers in accordance with
IS0 7235, insertion losses, static pressure losses and regenerated sound
(flow noise) are determined under well-defined conditions. In practical ap-
plications both the sound field and flow field are less uniformly distributed.
This can lead to different attenuations and greater pressure losses. In ad-
dition, sound levels and rates of flow are mutually dependent. Therefore, in
this International Standard the regenerated sound is not measured separ-
ately but is treated as a property of the silencer in its operating installation
which limits the degree of attenuation in the particular application.

---------------------- Page: 10 ----------------------

SIST EN ISO 11820:1999
IS0 11820:1996(E)
INTERNATIONAL STANDARD 0 IS0
Measurements on silencers in sifu
Acoustics -
such as a gas turbine generator, scrubbing plant,
1 Scope
cooling tower, heating ventilation and air con-
ditioning (HVAC) plant, exhaust stack, air intake
1 .l This International Standard specifies measure-
duct, weapon, internal combustion engine, com-
ments on silencers in situ. It is applicable to measure-
pressor, etc.);
ments on silencers in practical applications for
acoustic analysis, acceptance tests and similar
b) all types of passive silencers (absorptive, reac-
evaluations. Results obtained in accordance with this
tive, reflection and blowdown silencer);
International Standard cannot be compared to per-
c) active silencers (involving amplifiers and loud-
formance data obtained from laboratory measure-
speakers) as far as the insertion loss of passive
ments on ducted silencers in accordance with
silencers is equivalent to the off/on conditions of
IS0 7235, partly because of different test conditions
active devices; and
(such as sound field distribution, flow, temperature and
mounting conditions) and partly because of different
d) other measures or means of effecting acoustic
definitions.
attenuation in air or other gases (e.g. components
installed in ducting, louvres, grilles and deflector
Depending on the method used, the measurement is
hoods).
either of
Additionally, this International Standard is applicable to
the determination of the effect of cleaning or refurbish-
- insertion loss Dis, or
ing silencers.
- transmission loss Dt,.
This International Standard is not applicable to closed
high-pressure systems (e.g. silencers in closed pipes)
The measurement method depends upon the type of
since measurements of structure-borne sound are not
silencer and the installation conditions (e.g. insertion
anticipated.
loss measurements must be carried out for blowdown
silencers).
1.3 Quantities to be measured include the following:
The subscripts denote the practical application of
NOTE 1
a) sound pressure levels in octave bands with centre
the silencer and the particular installation and operating
frequencies at least from 63 Hz to 4 kHz and, if
conditions: “s” stands for ‘%I situ”, “t” for transmission, and
“i” for insertion. possible and required, from 31,5 Hz to 8 kHz or in
one-third-octave bands with centre frequencies
Additional characteristic quantities, which could in-
from 50 Hz to 5 kHz and, if possible and required,
clude measurements taken using artificial sound
from 25 Hz to 10 kHz
sources or measurements taken to determine the di-
- at a point or points on the source side of a
rectivity of sound propagation from the silencer, may
silencer,
be agreed upon in accordance with this International
- at a point or points on the receiver side of a
Standard.
silencer;
1.2 This International Standard is applicable to
b) static and dynamic pressures, flow velocities and
temperatures at selected positions.
a) silencers which are installed either as a whole or
Operating data to be determined include flow rate,
in the form of individual baffles in the propagation
pressure and speed, which define the operating con-
path of sound (e.g. openings of ducts) originating
ditions of the machine or plant to be silenced.
from a sound source (machine, building, plant

---------------------- Page: 11 ----------------------

SIST EN ISO 11820:1999
@ IS0
IS0 11820: 1996(E)
Lp2 is the mean sound pressure level (ref.
2 Normative references
20 PPa), in decibels (in one-third-octave or
octave bands), on the source side of the
The following standards contain provisions which,
silencer, for all measuring points used to de-
through reference in this text, constitute provisions of
termine the airborne sound reaching the
this International Standard. At the time of publication,
silencer.
the editions indicated were valid. All standards are
subject to revision, and parties to agreements based
NOTE 2 Dtl,, is not self-standing but is an intermedi-
on this International Standard are encouraged to in-
ate step towards determining the transmission Boss
vestigate the possibility of applying the most recent
(see 9.1.3).
editions of the standards indicated below. Members of
IEC and IS0 maintain registers of currently valid fnter-
Mean sound pressure levels L, are defined by
national Standards.
IS0 374411994, Acoustics - Determination of sound
1 N
power levels of noise sources using sound press-
1 O”J’%j dB . . .
(2)
c
7
ure - Engineering method in an essentially free field
j=l
i I
over a reflecting plane.
where
IS0 5221 :I 984, Air distribution and air diffusion -
Rules to methods of measuring air flow rate in an air
L pj is the individual level;
handling duct.
N is the number of measuring points.
I EC 651: 1979, Sound level meters.
3.2 insertion sound pressure leve’l difference,
IEC 651:1979/Amd.l:1993, Amendment No. 1.
Difference, in decibels, in sound pressure levels
Dips:
measured at a point or averaged over a small meas-
I EC 804: 1985, Integrating-averaging sound level
urement area before and after installation of a silencer:
meters.
. . .
(3)
IEC 804:1985/Amd.l :I 989, Amendment No. 1.
IEC 804: 1985/Amd.2: 1993, Amendment No. 2.
LPI is the sound pressure level (ref. 20 PPa), in
decibels (in one-third-octave or octave
bands, see IEC 1260), of the sound source(s)
3 Definitions
measured at a point or averaged over a small
measurement area, after installation of the
For the purposes of this International Standard, the
silencer;
following definitions apply.
Lpll is the sound pressure level (ref. 20 PPa), in
decibels (in one-third-octave or octave
3.1 transmission sound pressure level differ-
bands), occurring at the same measuring
Difference, in decibels, between the mean
ence, Q,,:
point or over a small measurement area due
sound pressure levels on the source and receiver side
to the sound source(s) to which the silencer
of a silencer:
is to be attached, before installation of the
silencer.
P -
. . .
(1)
Dtps = $2 - $1
NOTE 3 As opposed to DtPS, Dips is restricted to a
point or a small area where certain directivity indices of
where
sound radiation with and without the silencer are effec-
tive. The reference to a small area with a diameter of
about half a wavelength rather than a point may be
L,, is the mean sound pressure level (ref.
useful to avoid strong effects of interference between
20 FPa), in decibels (in one-third-octave or
direct and reflected waves which may occur at certain
octave bands, see IEC 1260), on the receiver
points.
side of the silencer, for all measuring points
used to determine the airborne sound within
3.3 transmission loss, Dts: Difference, in decibels,
the duct or propagated from the aperture and
between the levels of the sound power incident upon
external surfaces of the silencer;
and transmitted from the silencer:

---------------------- Page: 12 ----------------------

SIST EN ISO 11820:1999
IS0 11820:1996(E)
@ IS0
. . . = 1 m2;
(4) SO
Dts = EW2 - LWI
K2 is the correction, in decibels, for the
where
field distribution in the incident and
reflected sound field on the source
Lwl is the level (ref. 1 pW), in decibels (in one-
side of the silencer (see annex A).
third-octave or octave bands), of the sound
NOTE 4 The definitions of the areas SI and S2
power propagated through the silencer into
include basic field corrections so that the correc-
an attached duct, into a room or into free
tions KI and IQ are generally small, typically less
space, to be determined from
than 3 dB in absolute value.
L,, = L,, +I0 Ig ($/so) dB+ K, n . - (5)
3.4 insertion loss, Dis: Difference, in decibels, be-
tween the levels of the transmitted sound power with
in which, depending on the installation con-
and without a silencer:
ditions (see clause 5 and 9.1):
=L,,-L, . . .
is the area of the measurement
Sl (8)
Dis
surface on the receiver side of the
where
silencer corresponding to the mean
sound pressure level LPI , or
Lwr is the sound power level (ref. 1 pW), in deci-
is one-quarter of the absorption A in
Sl
bels (in one-third-octave or octave bands)
a reverberant receiving room, where
with the silencer installed, to be determined
Sabine’s formula applies so that
from
S1 = (6 In lO)VI(cT,) I . . (6)
dB+K, . .(9)
4vI = $1 +I0 b (%/So)
in which
in which
v is the volume of the room;
c is the speed of sound (for
L is the mean sound pressure level
PI
air at room temperature
(ref. 20 uPa), in decibels (in one-
c = 340 m/s);
third-octave or octave bands) with
the silencer installed, averaged
is the reverberation time;
over all measurement points for
= 1 m2;
SO airborne sound carried by the duct
or propagated from the aperture;
K1 is the correction, in decibels, for the
and depending on the installation
field distribution in the transmitted
conditions (see clause 5 and 9.1):
sound field on the receiver side of
the silencer (see annex A);
is the area of the measurement
SI
surface behind the silencer cor-
Lw2 is the level (ref. 1 pW), in decibels (in one-
responding to the mean sound
third-octave or octave bands), of the sound
pressure level, L,, , or
power incident upon the silencer, to be de-
termined from
is one-quarter of the absorption A
SI
in a reverberant receiving room,
where Sabine’s formula applies so
Lw2 =LP2+101g(s2/so) dB+K,. .(7)
that
in which, depending on the installation con-
S, = (6 In IO)V/(C~‘~) . . . (10)
ditions (see clause 5 and 9.1):
in which Tr is the reverberation
S2 is the area of the measurement
time, and V and c are as defined in
surface on the source side of the
3.3;
silencer corresponding to the mean
sound pressure level Lp2 g or
= 1 m2;
SO
is one-quarter of the total silencer
s2
is the correction, in decibels, for
KI
intake area in a reverberant source
the sound field distribution behind
room, where Sabine’s formula ap-
the silencer (see annex A);
plies and sound pressure levels LP2
LwrI is the sound power level (ref. 1 pW), in
are measured at various positions-in
decibels (in one-third-octave or octave
the room but not very close to the
silencer or to any sound source;

---------------------- Page: 13 ----------------------

SIST EN ISO 11820:1999
IS0 11820:1996(E) @ IS0
bands) without the silencer, to be deter- pressure loss approximately equals the static pressure
difference between these rooms or the duct sections.
mined from
Where the cross-section of the silencer does not equal
L,, = $1 + 10 Ig (s** /so> dB + K,, (11)
that of the duct or aperture in which the silencer is in-
stalled and transition elements are part of the silencer,
in which
the pressure measurements are carried out beyond
the transition sections.
LPI1 is the mean sound pressure level
(ref. 20 FPa), in decibels (in one-
third octave or octave bands) with-
36 . static pressure difference, Aps: Where the inlet
out the silencer installed, averaged
and outlet areas of a ducted silencer differ, but the
over all measurement points for
temperature of the gas does not vary markedly , the
airborne sound carried by the duct
static pressu re difference nips is related to the total
or propagated from the aperture;
pressure loss APT by
and depending on the installation
conditions (see clause 5 and 9.1):
is the area of the measurement . . .
&I (14)
surface corresponding to the mean
sound pressure level LPI*, or
is one-quarter of the absorption A where
&I
in a reverberant receiving room,
where Sabine’s formula applies so is the density of the gas, in kilograms per
B
that
cubic metre;
SII = (6 In 10)V/(cTII) . . . (12)
is the volume flow of the gas, in cubic me-
4v
tres per second;
in which 7’rr is the reverberation
time, and V and c are as defined in
is the silencer upstream cross-sectional
SlJ
3.3;
area, in square metres;
= 1 m2;
SO
is the silencer downstream cross-sectional
Sd
is the correction, in decibels, for
KII
area, in square metres.
the sound field distribution in the
duct or in front of the aperture
without the silencer installed (see
annex A).
4 Corrections for background noise
NOTE 5 In most cases the areas SI and $1
are equal and the corrections KI and KII are
similar so that these terms cancel each other 4.1 Transmission sound pressure level
in the evaluation of the insertion loss Dis. For
difference (see 3.1)
special cases, see annex A.
Correct the measured sound pressure levels for back-
3.5 total pressure loss of silencer, A~J-: Difference,
ground noise (i.e. sound not coming from the source
between the mean total pressure upstream, pTU , and
and the duct or the aperture for which the silencer will
downstream, PTd , of the silencer:
operate) according to table 1. If the measuring con-
P -
ditions are such that a correction of 3 dB is not suf-
. . .
Ai’T = PTu - PTd (13)
ficient, then L,, cannot be determined using the
Where the inlet and outlet areas of a ducted silencer
method described in this International Standard. It is
are equal and there are no significant changes in tem-
then only possible to state that
perature or density of the gas along the silencer, the
total pressure loss is equal to the static pressure dif-
LPI < Lhl - 3 dB
ference.
Where a silencer is inserted between two rooms or in
where Lb,
is the measured mean sound pressure
a duct of large cross-sectional area, and where the
level (in one-third-octave or octave bands) on the re-
flow velocities are negligible in these, the total
ceiver side of the silencer.

---------------------- Page: 14 ----------------------

SIST EN ISO 11820:1999
IS0 11820: 1996(E)
@ IS0
mined are schematically illustrated in figure 1. This
- Corrections for background noise
Table II
figure shows 16 different installation configurations for
Values in decibels
transmission loss measurements and 4 for insertion
loss measurements. The source side may be
- a duct,
pressure level alone
- a room with a diffuse sound field,
- a room with a non-diffuse sound field, or
rements invalid
- a space with an acoustically free field.
The receiver side may be
- a duct,
- a room with a diffuse sound field,
- a room with a non-diffuse sound field, or
- a space with an acoustically free field.
4.2 Insertion sound pressure level difference
When a silencer acceptance test is to be based on this
(see 3.2)
International Standard, agreement shall be reached
between interested parties on the type of installation
Correct the measured sound pressure levels for back-
conditions to be considered, on the measurement
ground noise (i.e. sound not coming from the source
positions, and on the magnitude of the field correction
and the duct or the aperture for which the silencer will
terms K to be applied.
operate) according to table 1. If the measuring con-
ditions are such that a correction of 3 dB is not suf-
NOTE 7 Results obtained for a situation corresponding to
cannot be determined using the
ficient, then Dips No. 6 of figure 1 may be different from those determined in
accordance with IS0 140-I 0, and results obtained for a
method described in this International Standard. It is
situation corresponding to No. 8 of figure 1 may be different
then only possible to ascertain that
from those determined in accordance with IS0 140-5, de-
pending on the measurement surfaces chosen.
Dips ’ Lb11 - Lb1
In special situations, where measurements cannot be
where
made during operation of the actual sound source and
an artificial sound source is used instead, the type and
L’ pI is the sound pressure level (in one-third-
installation of this source shall be specified. For better
octave or octave bands, see IEC 1260) with
comparison with actual source spectra, the measure-
the silencer installed, under the influence of
ments shall be carried out in one-third-octave bands.
the extraneous sound;
Special correction terms K must be defined consider-
ing the effects of different sound field distributions,
L’ PI1 is the sound pressure level (in one-third- temperatures and flow conditions.
octave or octave bands) without the si-
NOTE 8 The actual correction terms depend on the par-
lencer, under the influence of the same ex-
ticular situation. Their determination requires the use of
traneous sound.
complex theoretical models beyond the scope of this Inter-
national Standard.
NOTE 6 The background noise defined in 3.12 of
IS0 72351991 includes regenerated sound, while the
extraneous sound considered in this International
Standard excludes the regenerated sound.
6 Measuring instruments
6.1 Acoustic instruments
5 Installation conditions
The instrumentation shall comply with the require-
ments of IS0 3744. Use class 1 sound level meters as
Potential installation conditions i n wh ich either the
specified in IEC 651 and IEC 804. Directional micro-
transmission loss or loss may be deter-
the insertion

---------------------- Page: 15 ----------------------

SIST EN ISO 11820:1999
DUCT ROOM, non-diffuse OPEN SPACE
ROOM, diffuse
\\
\
DUCT
/
X
/
m
8
'\
ROOM,
\
I
diffuse /
X
0
E
0
2
E
.-
ii
.-
12
E
c
21
ROOM, l-
-\
\
non-
I
/
X
/
diffuse
16
00
X
0
00
0 00 ‘\
OPEN 0 0
0 \
0
0 I
X I
00
SPACE 0
0 / X
00 /
00
X
X
+ B
18
I
/
X
0
:
0
Any 2
duct/
_L E
.-
t
room/
ii
space X -\
\ 2
-
I
/
X
X /
x x
?cl
/-
/ \
Enveloping surf ace
/ \
Single point
X
0 0
Measurement points on source side
00000

---------------------- Page: 16 ----------------------

SIST EN ISO 11820:1999
IS0 11820:1996(E)
@ IS0
- with artificial sound generation (e.g. a loud-
phones may be used in particular cases (see 8.2.3) if
speaker) with the plant inoperative and without
all requirements of these standards, apart from those
flow.
on directivity, are met.
conditions is
system including the re- Measurement under existing operating
In gene ral, t he measureme nt
the preferred method.
cording equi pment shall be ca *librated.
Different results are to be expected for the differing
NOTE 9 In cases where the same measuring equipment is
used on the source and receiver side and the sound press- acoustic excitations which can arise under different
ure level differences are evaluated, and in cases where di-
operating conditions and under artificial excitation. In-
rectional microphones are used which are calibrated
fluencing factors include the sound field distribution,
under laboratory conditions as described in annex B, it is
regenerated sound, flow gradients, temperature, turbu-
not necessary to calibrate the microphone in the field but it
lence and flanking transmission. The attenuation
is advisable and good practice to use a calibrator for
losses measured in accordance with this International
checking the measurement system performance.
Standard are only valid in conjunction with the relevant
operating conditions during the measurement period.
The noise caused by flow past the microphone shall
These shall be determined and reported.
be suppressed by appropriate devices.
For acceptance tests on the silencer, agreement shall
NOTE 10 One of the following may be used:
be reached on the nominal operating conditions or op-
- a foam ball windscreen,
eration under normal conditions for the sound source if
- a nose cone, provided the direction of flow is known to these conditions are relevant for typical or particularly
an accuracy of approximately f 15O, or
frequent sounds produced under operating conditions.
- a Friedrich tube or turbulence screen for use as de-
If a machine is covered by a specific International
scribed in IS0 5136.
Standard, then the main state of operation specified
therein shall be chosen.
6.2 Air flow, static pressure and temperature
measuring devices
8 Measurement procedures
The procedures described in IS0 5221 shall be used
for measurements of flow velocity and static or dy-
namic pressure, if practicable.
8.1 General
Pitot static tubes and manometers or pressure trans-
Before application of the procedures described in this
ducers are most useful to determine pressure differ-
International Standard, agreement shall be reached
ences. When the pressure difference is less than
between the interested parties concerning the measur-
about IO Pa or the angle of incidence of flow on the
ing conditions. These include the operating state of the
Pitot static tube is greater than IO’, major inaccuracies
plant into which the silencer is installed or is to be in-
may occur. In cases of flow without significant fluctu-
stalled, as well as the positions of the measuring
ation or rotational components, vane anemometers
points. In addition to sound propagated through open-
can be applied to determine velocity distributions.
ings, sound may also be radiated from the external
surfaces of the silencer. Both sound components can
ring the
Any type of thermometer capable of measu
be taken into consideration by means of an appropri-
temperature to within + 5 OC is acceptable.
ate selection of measuring points. It is not sufficient for
the clear specification of measurement results merely
to refer to this International Standard without including
details of such agreements.
7 Test object and measuring conditions
The test object is a silencer installed (or intended for 82 . Acoustic measurements
installation) in a duct or at an opening in a machine or
plant or in the wall of an enclosed space. The effect of 8.2.1 Measurements in ducts
the silencer shall be measured either
For transmission loss measurements, locate the
microphone close to the silencer but preferably not
- under the existing operating conditions of the
closer than 1 m. Ensure that the measurements are
plant or equipment, which gives rise to a certain
not influenced by the gas flow. Avoid locations close to
flow rate and sound level, or
the source or to bends or obstructions in the duct, if
practicable.

---------------------- Page: 17 ----------------------

SIST EN ISO 11820:1999
@ IS0
IS0 11820: 1996(E)
A measurement surface is usually defined as a sur- Align microphones fitted with a nose cone or turbu-
face oriented perpendicular to the direction of sound lence screen with the direction of flow.
propagation or parallel to the entrance cross-section of
Where high flow velocity conditions (e.g. > 30 m/s) and
the silencer [see figure 2a)]. A number of measure-
high sound pressure levels [e.g. > 120 dB (ref.
ment positions are required to determine the average
20 PPa)] are present within the duct, it is permissible
sound pressure level.
to mount the microphone flush with the wall.
NOTE 11 The number depends on the size of the duct and
NOTES
the measurement conditions (e.g. access and temperature).
12 On the design of the duct system, consideration should
Locate the measuring points evenly distributed on or be given to the provision of access for the microphones.
close to (within an eighth of a wavele
...