ASTM E1124-97(2004)
(Test Method)Standard Test Method for Field Measurement of Sound Power Level by the Two-Surface Method
Standard Test Method for Field Measurement of Sound Power Level by the Two-Surface Method
SCOPE
1.1 This test method covers the field, or in situ measurement of sound power level by the two-surface method. The test method is designed to minimize the effects of reverberant conditions, directivity of the noise source under consideration, and the effects of ambient noise from other nearby equipment operating at the same time.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E1124 – 97 (Reapproved 2004)
Standard Test Method for
Field Measurement of Sound Power Level by the Two-
Surface Method
This standard is issued under the fixed designation E1124; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2.2 constituent surface area—a portion of the conformal
surface.
1.1 Thistestmethodcoversthefield,or in situmeasurement
of sound power level by the two-surface method. The test
4. Summary of Test Method
method is designed to minimize the effects of reverberant
4.1 The average one-third or full octave band sound pres-
conditions, directivity of the noise source under consideration,
sure levels are measured over two different conformal surfaces
and the effects of ambient noise from other nearby equipment
which envelop the equipment. These conformal surfaces
operating at the same time.
should be selected to consist of rectangular, cylindrical, and
1.2 This standard does not purport to address all of the
hemispherical constituent surfaces so that the surface areas
safety concerns, if any, associated with its use. It is the
may be easily calculated. From the difference between the two
responsibility of the user of this standard to establish appro-
average sound pressure levels taken at each surface and from
priate safety and health practices and determine the applica-
the areas of the surfaces, the sound power level may be
bility of regulatory limitations prior to use.
calculated. The calculation accounts for both the effect of the
2. Referenced Documents reverberant field and the noise of other equipment. It is
permissible to define conformal surfaces that completely en-
2.1 ASTM Standards:
velope the source, yet only measure over a portion of the
C634 Terminology Relating to Building and Environmental
conformalsurfaceduetorestrictionsfromprocessconnections
Acoustics
or accessibility.
2.2 ANSI Standard:
S1.4 Specification for Sound Level Meters
5. Significance and Use
3. Terminology 5.1 The function and operation of equipment in the field
often preclude the measurement of the free-field sound pres-
3.1 Definitions—For definitions of terms used in this test
4 sure levels of a single piece of equipment in the absence of
method, refer to Terminology C634.
interfering sound from other equipment operating at the same
3.2 Definitions of Terms Specific to This Standard:
time. The two-surface method will provide, in most cases, a
3.2.1 conformal surface—the locus of points which lie at a
reliable estimate of the normal sound power levels of a
fixed distance from the reference surface of a piece of
specimen operating in an adverse environment.
equipment. Two conformal surfaces are used in this test
5.2 This test method is intended for use in the field in the
method. These are surfaces over which the measuring micro-
presence of what is normally regarded as interfering back-
phones are swept. They are located at two different distances
ground noise. This test method is based upon the work of
from the equipment. Fig. 1 shows a typical arrangement of
,
56 7
Hubner and Diehl, but differs from all other current sound
these surfaces for a generalized piece of equipment.
power measurement procedures by requiring simultaneous
measurement at both conformal surfaces and by resolving
This test method is under the jurisdiction of ASTM Committee E33 on
time-averaged sound pressure levels at both surfaces to within
EnvironmentalAcousticsandisthedirectresponsibilityofSubcommitteeE33.08on
Mechanical and Electrical System Noise.
Current edition approved April 1, 2004. Published April 2004. Originally
approved in 1986. Last previous edition approved in 1997 as E1124–97. DOI:
10.1520/E1124-97R04. Hubner,G.,“AnalysisofErrorsinMeasuringMachineNoiseUnderFreeField
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Conditions,” Journal of the Acoustical Society of America,Vol 54, No. 4, 1973, pp.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM 967–977.
Standards volume information, refer to the standard’s Document Summary page on Hubner, G., “Qualification Procedures for Free Field Conditions for Sound
the ASTM website. Power Determination of Sound Sources and Methods for the Determination of the
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St., Appropriate Environmental Correction,” Journal of the Acoustical Society of
4th Floor, New York, NY 10036. America, Vol 61, No. 2, 1977, pp. 456–464.
4 7
Terminology C634 – 85 was the edition used during the development of this Diehl, G. M., Machinery Acoustics, J. Wiley and Sons, New York, NY, 1973,
test method. pp. 97–103.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1124 – 97 (2004)
FIG. 1 Configuration of Conformal Surfaces, General Case
0.1dB.Thesetwofeatures,simultaneousrecordingand0.1-dB 6. Operating Conditions
resolution, enable source sound power to be calculated when
6.1 Whenever possible, equipment under test must be oper-
the direct sound field of the source is actually lower in level
ating in a mode acceptable to all parties involved in the test.
than the ambient noise.
Otherwise operating conditions must at least be monitored in
5.3 The use of this test method is expected to be primarily
order that the test results are properly qualified in terms of
for the relative assessment of the sound power from similar
running speeds, flow rate, production rate, etc.
sources or for the prediction of sound levels in a plant based
upon measurements of similar sources in another plant. This
7. Apparatus
test method is believed to be capable of yielding a reasonably
7.1 Due to the amount of data which must be gathered and
good estimate of absolute power level with proper care of
processed, the following are considered to be the minimum
application and full conformance to the provisions of this
equipment necessary to meet the requirements of this test
procedure.
procedure.
5.4 Thetwo-surfacemethodisapplicableonlywhenthetwo
7.1.1 Microphones, that are matched in terms of frequency
measurement surfaces can be physically selected to produce
and pressure response. Begin by calibrating each data channel,
positive values of the difference in average sound pressure
using the same calibrator on each channel. Connect both
level.That is, the inner surface sound pressure level minus the
microphone channels to the cables, connectors, amplifiers, and
outer surface sound pressure level must be at least+0.1 dB.
recorder to be used in data gathering. Then arrange the
This limitation applies to each frequency band and each
microphones side by side in the presence of broad band
constituent surface area investigated. Only the frequency band
ambient noise and record for 60 s on both channels. The
in which a zero or negative difference occurs is it considered
differences in the averaged sound pressure levels in each
invalid and usually adjacent bands will be valid. In practice,
frequency band are calibration corrections which may be
only rarely will all three one-third octave bands of a given
applied to either channel prior to any calculation.
octaveyieldinvaliddataatallconstituentareas.Therefore,less
7.1.2 Magnetic Tape Recorder,two-channelinstrumentation
than complete results are permissible when one-third octave
grade having a frequency response of 61 dB over the
analysis is used and full octave results are reported.
frequency range of interest.
5.5 The two-surface method may not produce results when 7.1.2.1 It is recognized that even high-quality Amplitude
testing some very large machines in very reverberant rooms or Modulation (AM) tape recorders cannot maintain channel-to-
in rooms having a volume less than about 20 times the space channel frequency response within 0.1 dB. It is believed,
enclosed by an envelope around the larger dimensions of the however, that the requirement for determining the corrections
machine. In such cases, the sound pressure level close to the in 7.1.1 based on 60-s average readings sufficiently compen-
machine may not decrease in any regular way with increasing sates for expected instabilities, channel-to-channel. If digital
distancefromamachinesurface,makingitimpossibletoselect frequency modulation (FM) or pulse code modulation (PCM)
two measurement surfaces producing positive differences of tape recorders are used, the procedure of 7.1.1 should still be
sound pressure level. used.
E1124 – 97 (2004)
NOTE 1—The frequency response and accuracy of the acoustical
that will yield a measurable drop in average sound pressure
instruments are different from the interchannel resolution of the tape
level between the two surfaces for the frequency range of
recorder. Both the frequency response discussed in 7.1.2 and the accuracy
interest. As stated in Section 5, merely a 0.1-dB difference in
of the acoustical calibrators are distinctly different from the 0.1-dB
average sound pressure levels constitutes a measurable drop.
resolution discussed in 5.2.
However, the surfaces should be chosen so as to maximize the
7.1.3 Microphone Mounting Fixture—Asuggestedfixtureis
difference since the overall accuracy of the estimated sound
shown in Fig. 2.
power levels will be thereby improved. Obviously, the closer
7.1.4 Spectrum Analyzer, real-time one-third or full octave,
the inner surface is to the equipment, the easier it will be to
having a resolution of 0.1 dB with a digital display or printing
obtainalargepositivedifference,butpossiblenear-fieldeffects
capabilities.
dictate an inner surface farther from the equipment. Such
NOTE 2—Real-time analyzers having a resolution of 0.25 dB may also
near-field effects cannot be quantified by this test method nor
be used. However, because of the requirement for a positive sound level
can their effect on the calculated power levels be determined,
difference, as discussed in 5.4, these analyzers may yield less complete
so that this procedure can only suggest that the inner surface
resultscomparedwithwhatcouldbeobtainedwithananalyzerwithbetter
microphone be always at least 0.15 m, and for larger machines
resolution. In addition, the precision of the results will be reduced if only
at least 0.3 m, from the equipment surface thereby avoiding
differences greater than 0.25 dB can be obtained.
most of these effects.
7.2 Optional equipment may include:
8.1.2 If the locations of the two conformal surfaces are too
7.2.1 Programmable Calculator or Desktop Computer.
close together, measurable differences in average sound pres-
7.2.2 Data Processing, direct from output of real-time
sure levels will be difficult to obtain. On the other hand, no
analyzer.
advantage is gained by using progressively larger outer sur-
8. Procedure
faces once the outer surface microphone is in the fully
reverberant field since the sound level, and therefore the
8.1 Selection of Measurement Surfaces:
8.1.1 Conduct a preliminary survey of the sound field to differential, will be constant. No clear optimum ratio between
estimate the two optimum conformal measurement surfaces thesetwosurfaceareascanbeprescribedforallequipment.As
FIG. 2 Example of Suggested Measurement System
E1124 – 97 (2004)
a guide, however, experience has shown that an area ratio of and lower pedestal. Less than 100% coverage was used and
about 1.4 to 2.0, between the outer and inner surfaces, is a was accounted for as discussed in 9.4.
reasonable range that may be used in most cases.
8.1.6 No optimum distances from the equipment surface to
8.1.3 Select simple geometric shapes for conformal sur-
either conformal surface can be prescribed for all equipment.
faces. Fig. 1 shows an example of a generalized situation. In
However, for sources whose smallest dimension is 1 m, it is
Fig. 1, even though the equipment itself can be approximated
recommendedtheinnersurfacedistancebeatleast0.2m.Also,
by rectangular or cylindrical surfaces which just enclose the
for sources whose smallest dimension is 3 m, it is recom-
equipment, the reference surface is chosen so that the two
mended the outer surface distance be less than 2 m.
conformalmeasurementsurfacesareconvex.Itmaybehelpful
8.2 Data Acquisition:
to imagine the major equipment reference surfaces to be
8.2.1 Obtain simultaneous measurements of the sound pres-
defined by a membrane stretched over the equipment after the
surelevelatthetwomicrophonepositionsalongalinenormal,
removal of minor projections, gages, tubes, and cables not
that is perpendicular to, the inner conformal surface. See 7.1.3
expected to be noise sources themselves. Ideally, the sound
for a suggested microphone mounting fixture. Determine the
intensityvectorwouldbenormaltobothmeasurementsurfaces
atallpoints.Althoughthiscannotbedeterminedusingthistest averagesoundpressurelevelovereachconstituentsurfacearea
using a continuous uniform microphone sweep as indicated in
method, it may be helpful if the surveyor will attempt to
visualize the expected sound field and so might adjust the Fig. 4.
selection of conformal surfaces accordingly.
8.2.2 If the inner and outer measurement surfaces are
8.1.4 It is permissible to subdivide the conformal surfaces
subdivided into smaller constituent areas for the survey, the
intoseveralconstituentsurfaceareasforeaseofdatacollection
average sound pressure levels over the entire inner and outer
or because of inaccessibility. Any number of constituent
conformal surfaces are determined by summing the values
surface areas may be used to cover the conformal surface.
obtained for the respective constituent areas, as shown in 9.3.
Since the conformal surfaces will be measured simultaneously
8.2.3 The microphone sweeping speed shall be sufficiently
withtheinnerandoutermicrophones,careshouldbetakenthat
slow, continuous, and uniform that when the data are continu-
the constituent surface area boundaries define related regions
ouslyrecorded,arepresentativeaveragesoundpressurelevelis
ontheinnerandoutersurfaces.Theseconstituentsurfaceareas
obtainedforeachconstituentareasweptbythemicrophone(s).
willnotnecessarilybecomposedofgeometricallysimilarinner
Areasonable averaging period is usually between 30 and 60 s
andoutersurfacesbecauseoftheusuallycomplexshapeofthe
for each constituent area. A reasonable sweeping speed is
equipment sources themselves.
usually about 0.5 m/s.
8.1.5 Fig. 3 is an example of the application of these
8.2.4 Fig. 5 illustrates an alternate data
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