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IS0
I NTERN AT1 ON A L ORGAN IZATl ON FOR STA N DA RDl ZATl ON
IS0 RECOMMENDATION
R 495
GENERAL REQUIREMENTS FOR THE PREPARATION OF TEST CODES
FOR MEASURING THE NOISE EMITTED BY MACHINES
1st E D IT ION
August 1 966
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The copyright of IS0 Recommendations and IS0 Standards belongs
to IS0 Member Bodies. Reproduction of these documents, in any
country, may be authorized therefore only by the national standards
of that country, being a member of ISO.
organization
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 495, General Requirements .for the Preparation of Test Codes
for Measuring the Noise Emitted by Machines, was drawn up by Technical Committee
ISO/TC 43, Acoustics, the Secretariat of which is held by the British Standards Institution (BSI).
Work on this question by the Technical Committee began in 1960 and led, in 1963, to the
adoption of a Draft IS0 Recommendation.
In December 1963, this Draft 1SO Recommendation (No. 695) was circulated to all the
IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of an editorial
nature, by the following Member Bodies:
Finland Korea, Rep. of
Australia
Austria France Netherlands
Belgium Germany New Zealand
Brazil Greece Sweden
Canada Hungary Switzerland
Chile India United Kingdom
Colombia Israel U.S.A.
Czechoslovakia Italy U.S.S.R.
Denmark Japan Yugoslavia
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council,
which decided, in August 1966, to accept it as an IS0 RECOMMENDATION.
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ISO/R 495 - 1966 (E)
IS0 Recommendation R 495 August 1966
GENERAL REQUIREMENTS FOR THE PREPARATION OF TEST
CODES FOR MEASURING THE NOISE EMITTED BY MACHINES
1. SCOPE
This IS0 Recommendation is concerned with the procedures to be followed in the objective
measurement of the noise emitted by machines. These procedures are not necessarily applicable
to noise of an impulsive character.
The aim is to indicate the general principles by which specific test codes for noise measurements
may be formulated. These general rules give different methods for measuring noise.
The specific codes for the various types of machines will have to select the most suitable method
having regard to the size of the machine and its application. The codes themselves should contain
all the necessary particulars to enable a result to be obtained with the required accuracy.
2. GENERAL
The measurement of the physical characteristics of the noise of machines has four objects:
(a) Verification that the noise of a given machine conforms to a certain standard;
(b) To make a comparison between the noise emitted by machines built to the same
specification ;
(c) To make a comparison between the noise emitted by different machines;
(d) The determination of the noise received at a distance.
The four aims stated above can be attained through the determination of the radiated sound
Special installation conditions of the machine are however required
power and its directivity.
and these conditions cannot always be met, particularly for large machines. When this is the case,
the test code may call for the measurement of sound pressure level near the machine and when
appropriate the determination of the sound pressure level at the reference radius. From these
sound pressure level measurements it is possible in certain cases to make an approximate deter-
mination of the radiated sound power.
3. DEFINITIONS AND TERMS
3.1 Sound pressure level is defined by IS0 Recommendation R 131, Expression of the Physical
and Subjective Magnitudes of Sound or Noise.
3.2 Sound level is defined as the reading given by a sound level meter complying with specific-
ations of the International Electrotechnical Commission (IEC), Publications No. 123 for
sound level meters, or No. 179 for precision sound level meters.
P
3.3 Soundpower level is defined as 10 log,,
Po
where P is the sound power and Po is the reference sound power of 10-l2 W.
For the purposes of this IS0 Recommendation, the following definitions apply :
3.4 Machine. Any sound source of which the acoustical characteristics are to be measured.
3.5 Prescribed surface. A hypothetical surface surrounding the machine as given in the test code
and over which the measurements are assumed to be made. Its area S is calculated as laid
down in the test code.
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ISO/R 495 - 1966 (E)
3.6 Equivalent hemisphere. A hypothetical hemisphere having the same area Sas the prescribed
surface.
3.7
Reference radius. A radius (measured from the centre of the equivalent hemisphere), as
given in the test code, to which all the results of measurements made on machines of the
same category (tested according to the same test code) are reduced.
4. INSTALLATION AND OPERATION OF THE MACHINE
The acoustic radiation of the machine can depend on its installation, particularly when the machine
is one of small dimensions.
The test code should, therefore, define the conditions of installation of the machine. It should
also define the conditions of operation of the machine during the test.
5. QUANTITIES TO BE MEASURED
Depending on the purpose of the measurement, the nature of the noise source, and the character
of the sound field, one or both of the following quantities should be measured:
(a) Sound level (it is recommended that weighting A be adopted);
(b) Sound pressure level in frequency bands of given width (it is recommended that octave
bands be used; for narrower bands, the result should always be available, expressed
in octave bands).
Since in all cases a pressure microphone will be used, the result of the measurements will be band
sound pressure level or sound level values.
NOTES
1. When the spectral distribution of the noise is well known, a measurement of wide band sound pressure level
or sound level may suffice.
2. Any determination of the spectral distribution of the noise should be in frequency bands corresponding
to those specified in IS0 Recommendat ion R 266, Preferred Frequencies for Acoustical Measurements.
6. RECOMMENDED METHODS OF MEASUREMENT
The accurate determination of the acoustic power of a machine has to be performed either as a
far-field sound pressure level measurement under hemispherical radiation or free-field conditions,
or as a sound pressure level measurement in a reverberant room under diffuse-field conditions.
It is often impossible to take sound pressure level measurements under such ideal conditions. In
such cases sound pressure level measurements have to be performed under semi-reverberant
conditions, or near-field measurements made.
6.1 Determination of sound power.
The methods available for the determination of the radiated sound power and the directivity
are described in Annexes A, B and C.
6.1.1 Hemispherical radiation and free-field methods. The determination of the sound power
and the directivity requires the siting of measurement points sufficiently remote from the
machine for the conditions of propagation of progressive sound waves to be established.
There should be no reflected waves at the measurement points and this requires premises
with sufficiently absorbent walls and dimensions large compared with those of the
machine (see Annex A).
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ISO/R 495 - 1966 (E)
Any effect of the surroundings or near-field effects will be made apparent by measuring
over two surfaces and checking if the sound pressure varies inversely as the square
root of the ratio of surface areas. A ratio of areas of 2 or 4 is recommended, corres-
ponding to -3 and -6 dB respectively. The results of such check measurements
should be stated.
6.1.2 Diffuse-field method. As an alternative to the hemispherical radiation and free-field
methods, the use of a highly reverberant enclosure is recommended.
In this case, the sound field tends to become uniform and the relevant theory for sound
power determination is described in Annex B.
This method can provide no information on the directivity of the acoustic radiation.
Since the degree of diffusion is dependent upon the nature of the noise source, this
method is not recommended for machines the noise from which contains prominent
components.
6.1.3 Semi-reverberant method. A third method in which semi-reverberant conditions exist is
possible (see Annex C).
This method can provide only limited information on the directivity of the acoustic
radiation.
6.2 Near-field sound pressure level measurement
6.2.1 General. This method consists of carrying out sound pressure level measurements at a
number of points suitably distributed around the machine, sufficiently close to the
so that the measurements are not significantly affected by nearby reflecting
machine
surfaces or background noise.
6.2.2 Prescribed surface. A surface as simple as possible and whose area can be calculated
easily should be marked out round the machine conforming approximately to the external
casing. This surface and its average distance from the outer casing should be laid down
in each test code.
The number and disposition of the measuring points required depend on the irregularity
of the acoustic field.
When it is necessary to carry out measurements closer to the machine than is specified
in the test code, or to explore only part of the surface, these limitations should be clearly
stated in the test report.
6.2.3 Evaluation of the result. In the majority of cases, in particular when the directivity is
not pronounced or when for the type of machine under consideration it varies relatively
slightly from one machine to another, the arithmetic mean of the decibel readings over
the prescribed surface may be a sufficiently accurate measure of the mean square pressure.
NOTE. - For a spread of 5 dB in the decibel readings, the error in taking the mean of the decibels
is less than 0.7 dB and for a spread of 10 dB it is less than 2.5 dB. When the spread is greater than
10 dB, this method of averaging is not suitable.
6.2.4 Effect of environment. The effect of the surroundings of the machine may be determined
by first placing such a machine in a space without reflections and secondly in the test
environment so that a direct comparison can be made; every precaution should be taken
to ensure constant conditions of machine noise and measuring apparatus.
As accurate determination of the influence of the surroundings is not possible, it is
recommended that this influence be kept to a minimum.
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ISO/R 495 - 1966 (E)
6.2.5 Calculation of the sound pressure level at a reference radius. The sound pressure level
at a reference radius which is laid down in the specific test codes, can be calculated from
that measured on the prescribed surface as described in Annex D.
The reference radius should be the same for all machines covered by a given code. One
of the values 1, 3 or 10 m should be adopted.
It is recommended that the value chosen should be slightly greater than the radius
of the equivalent hemisphere corresponding to the largest machine covered by the
test code.
6.2.6
Appr
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