1、Designation: C 423 08Standard Test Method forSound Absorption and Sound Absorption Coefficients bythe Reverberation Room Method1This standard is issued under the fixed designation C 423; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisi
2、on, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This test method cov
3、ers the measurement of soundabsorption in a reverberation room by measuring decay rate.Procedures for measuring the absorption of a room, theabsorption of an object, such as an office screen, and the soundabsorption coefficients of a specimen of sound absorptivematerial, such as acoustical ceiling t
4、ile, are described.1.2 Field MeasurementsAlthough this test method prima-rily covers laboratory measurements, the test method describedin 4.1 can be used for making field measurements of theabsorption of rooms (see also 5.5). A non-standard method tomeasure the absorption of rooms in the field is de
5、scribed inAppendix X2.1.3 This test method includes information on laboratoryaccreditation (see Annex A1), asymmetrical screens (see An-nex A2), and reverberation room qualification (see Annex A3).1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its us
6、e. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C 634 Terminology Relating to Building and Environmen-tal AcousticsE 548 Gu
7、ide for General Criteria Used for EvaluatingLaboratory Competence3E 795 Practices for Mounting Test Specimens DuringSound Absorption Tests2.2 ANSI Standards:S1.6 Preferred Frequencies, Frequency Levels, and BandNumbers for Acoustical Measurements4S1.11 Specification for Octave-Band and Fractional-Oc
8、tave-Band Analog and Digital Filters4S1.26 Method for the Calculation of the Absorption ofSound by the Atmosphere42.3 ASTM Adjuncts:Historical Applications Note on Sound Absorber53. Terminology3.1 Except as noted in 3.3, the terms and symbols used inthis test method are defined in Terminology C 634.
9、 Thefollowing definition is not currently included in TerminologyC 634:3.1.1 sound absorption average, SAAa single numberrating, the average, rounded off to the nearest 0.01, of thesound absorption coefficients of a material for the twelveone-third octave bands from 200 through 2500 Hz, inclusive,me
10、asured according to this test method.3.1.1.1 DiscussionThe sound absorption coefficients shallbe rounded off to the nearest 0.01 before averaging. If theunrounded average is an exact midpoint, round to the nexthigher multiple of 0.01. For example, report 0.625 as 0.63.3.2 In previous versions of thi
11、s test method a single numberrating, called the noise reduction coefficient (NRC), wasdefined as follows:9Round the average of the sound absorption coefficientsfor 250, 500, 1000, and 2000 Hz to the nearest multipleof 0.05. If the unrounded average is an exact midpoint,round to the next higher multi
12、ple of 0.05. For example,0.625 and 0.675 would be reported as 0.65 and 0.70, respectively.9The noise reduction coefficient shall be reported in order toprovide comparison with values reported in the past see12.1.3).3.3 Definition of Term Specific to This StandardThefollowing term has the meaning not
13、ed for this test method only:3.3.1 output interval, Dt, T, sof a real-time analyzer, thetime between successive outputs; this time is not necessarilythe same as the integration time.1This test method is under the jurisdiction ofASTM Committee E33 on Buildingand Environmental Acoustics and is the dir
14、ect responsibility of SubcommitteeE33.01 on Sound Absorption.Current edition approved March 1, 2008. Published March 2008. Originallyapproved in 1958. Last previous edition approved 2007 as C 423 07a.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Servic
15、e at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn.4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.5A drawing of this spec
16、imen is available at a nominal charge from ASTMInternational Headquarters. Order Adjunct No. ADJC0423.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Summary of Test Method4.1 Measurement of the Sound Absorption of a Room:4.1.1 A
17、band of random noise is used as a test signal andturned on long enough (about the time for 20 dB decay in thetest band with the smallest decay rate) for the sound pressurelevel to reach a steady state. When the signal is turned off, thesound pressure level will decrease and the decay rate in eachfre
18、quency band may be determined by measuring the slope ofa straight line fitted to the sound pressure level of the averagedecay curve. The absorption of the room and its contents iscalculated, based on the assumptions that the incident soundfield is diffuse before and during decay and that no addition
19、alenergy enters the room during decay, from the Sabine formula:A 5 0.9210Vdc(1)where:A = sound absorption, m2or Sab,V = volume of reverberation room, m3or ft3,c = speed of sound (calculated according to 11.13), m/s orft/s, andd = decay rate, dB/s,These conditions must be fulfilled if the measurement
20、 is tohave meaning. The sound absorption calculated according toEq 1 is sometimes called the Sabine absorption.4.1.2 In general, sound absorption is a function of frequencyand measurements are made in a series of frequency bands.4.2 Measurement of a Sound Absorption CoeffcientTheabsorption of the re
21、verberation room is measured as outlined in4.1 both before and after placing a specimen of material to betested in the room. The increase in absorption divided by thearea of the test specimen is the dimensionless sound absorptioncoefficient. In inch-pound units it is reported with the dimen-sionless
22、 “unit” sabin per square foot, Sab/ft2.4.3 Measurement of the Sound Absorption of an Object Suchas an Offce Screen, a Theater Chair, or a Space AbsorberThe absorption of the reverberation room is measured asoutlined in 4.1 both before and after placing one or severalidentical objects in the room. Th
23、e increase in absorptiondivided by the number of objects is the absorption in squaremeters per object or sabins per object.5. Significance and Use5.1 Measurement of the sound absorption of a room is partof the procedure for other acoustical measurements, such asdetermining the sound power level of a
24、 noise source or thesound transmission loss of a partition. It is also used in certaincalculations such as predicting the sound pressure level in aroom when the sound power level of a noise source in the roomis known.5.2 The sound absorption coefficient of a surface is aproperty of the material comp
25、osing the surface. It is ideallydefined as the fraction of the randomly incident sound powerabsorbed by the surface, but in this test method it is operation-ally defined in 4.2. The relationship between the theoreticallydefined and the operationally measured coefficients is undercontinuing study.5.3
26、 Diffraction effects6usually cause the apparent area of aspecimen to be greater than its geometrical area, therebyincreasing the coefficients measured according to this testmethod. When the test specimen is highly absorptive, thesevalues may exceed unity.5.4 The coefficients measured by this test me
27、thod should beused with caution because not only are the areas encountered inpractical usage usually larger than the test specimen, but alsothe sound field is rarely diffuse. In the laboratory, measure-ments must be made under reproducible conditions, but inpractical usage the conditions that determ
28、ine the effectiveabsorption are often unpredictable. Regardless of the differ-ences and the necessity for judgment, coefficients measured bythis test method have been used successfully by architects andconsultants in the acoustical design of architectural spaces.5.5 Field MeasurementsWhen sound abso
29、rption measure-ments are made in a building in which the size and shape of theroom are not under the operators control, the approximation toa diffuse sound field is not likely to be very close. This mattershould be considered when assessing the accuracy of measure-ments made under field conditions.
30、(See Appendix X2 for aprocedure that can be used in the field with less sophisticatedinstrumentation.)6. Interferences6.1 Changes in temperature and relative humidity during thecourse of a measurement may have a large effect on the decayrate, especially at high frequencies and at low relative humidi
31、-ties. The effects are described quantitatively in ANSI S1.26.These effects of temperature and relative humidity changesshall be minimized as follows:6.1.1 During all measurements of decay rate The averagetemperature shall be no less than 10 C; Deviations from theaverage temperature shall not exceed
32、 5 C. The averagerelative humidity in the room shall be no less than 40%.Deviations from the average relative humidity shall not exceed6 5% in the measured relative humidity value.6.1.2 All decay rates in the 1000 Hz one-third octave bandand above shall be adjusted by subtracting the decay rate duet
33、o air absorption from the decay rate calculated according to11.4. For these calculations, assume the values calculated forthe mid-band frequency apply to the complete one-third-octaveband. The air absorption shall be calculated according to ANSIS1.26 using its standard air absorption values at the c
34、enterfrequency of each one third octave band, respectively. Use Eq2 below:dair5 mc (2)where:dair= decay rate due to sound absorption by the air, dB/s,6Chrisler, V., “Dependence of SoundAbsorption Upon theArea and Distributionof the Absorbent Material,” Journal of Research, National Bureau of Standar
35、ds, Vol13, 1934, p. 169: Northwood, T. D., Grisaru, M. T., and Medcof, M.A., “Absorptionof Sound by a Strip ofAbsorptive Material in a Diffuse Sound Field,” Journal of theAcoustical Society of America, Vol 31, 1959, p. 595: and Northwood, T. D.,“Absorption of Diffuse Sound by a Strip or Rectangular
36、Patch of AbsorptiveMaterial,” Journal of the Acoustical Society of America, Vol 35, 1963, p. 1173.C423082m = attenuation coefficient, dB/m, taken from ANSIS1.26, as described in 6.1.2.1, andc = speed of sound, m/s, calculated according to 11.13.6.1.2.1 The attenuation coefficients m shall be derived
37、 fromthe equations and calculation procedures of 5.1-5.3 and AnnexB of ANSI S1.26. Table 1 of ANSI S1.26 shall not be used.7. Reverberation Room7.1 DescriptionA reverberation room is a room designedso that the reverberant sound field closely approximates adiffuse sound field both in the steady state
38、, when the soundsource is on, and during decay, after the sound source hasstopped.7.2 Construction:7.2.1 The room is best constructed of massive masonry orconcrete materials, but other materials, such as well-dampedsteel, may be used. Lighter construction may be excessivelyabsorptive, especially at
39、frequencies below 200 Hz.7.2.2 The average absorption coefficient of the room sur-faces at each frequency, determined by dividing the absorptionof the empty room (measured according to Sections 10 and 11)by the area of the room surfaces, including both sides of thediffusers (see 7.4), shall be less
40、than or equal to 0.05 for theone-third octave bands centered at 250 through 2500 Hz, afterallowance has been made for atmospheric absorption accordingto ANSI S1.26. For the bands centered below 250 Hz, andabove 2500 Hz, the similarly determined coefficient shall beless than or equal to 0.10.7.2.3 Th
41、e room shall be isolated sufficiently to keep outsidenoises and structural vibrations from interfering with themeasurements.7.3 Size and ShapeThe volume of the room shall be noless than 125 m3. It is recommended that the volume be 200 m3or greater. No two room dimensions shall be equal nor shall the
42、ratio of the largest to the smallest dimension be greater than2:1. (See 11.12 on calculating room volume.)7.4 Sound Diffusion:7.4.1 Means shall be taken to ensure an approximation to adiffuse sound field both before and during decay. Experiencehas shown that a satisfactory approximation can be achie
43、vedwith a number of sound-reflective panels hung or distributedwith random orientations about the volume of the room. It isstrongly recommended that some of these panels be mountedon a rotating shaft or otherwise kept moving, presenting, ineffect, a room that continually changes its shape.7.4.2 The
44、goal is to achieve a rapid and continuous inter-change of energy between the directions of sound propagation,thereby increasing the probability that each surface area of theroom is exposed to sound of the same intensity.7.4.3 Laboratories are strongly encouraged to follow theprocedures in Appendix X
45、1 to determine the necessary area ofdiffusing panels to maximize the measured absorption coeffi-cients. If these procedures are followed, the data collected shallbe preserved and made available on request. If the proceduresin Appendix X1 are not followed, the surface area of thediffusing elements in
46、 the room (both faces) shall be at least25 % of the surface area of the reverberation room. (See NoteX1.1.)7.4.4 The reverberation room shall be qualified according toAnnex A3.7.5 Background NoiseThe level of the background noisein each measurement band, which includes both the ambientacoustical noi
47、se in the reverberation room and the electricalnoise in the measuring instruments, shall be at least 15 dBbelow the lowest level used to calculate decay rate (see 11.3).8. Instrumentation8.1 Sound SourceThe sound source shall be one or moreloudspeaker systems in a configuration such that the testfac
48、ility satisfies the qualifications of Annex A3. With adequatediffusion, loudspeakers facing into the trihedral corners of theroom will satisfy these requirements. The sound pressure levelproduced when the source is on and the sound in the rever-beration room is in the steady state shall be at least
49、45 dBabove the background noise in each measurement band.NOTE 1The value of 45 dB is the minimum value required by thismethod. In fact, the steady state may need more than 45 dB above thebackground noise to satisfy the requirements of 7.5 and 11.3.8.2 Test SignalThe test signal shall be a band of randomnoise with a continuous spectrum covering the range overwhich measurements are made. The frequency range of themeasurements shall include the one-third octave bands withmidband frequencies, as defined in ANSI S1.6, from 100 Hz to5000 Hz.8.3 MicrophonesThe micro
