Some standard content:
ICS17.140
National Standard of the People's Republic of China
GB/T20247-2006/1S0354:2003
Acoustics-Measurement of sound absorption in a reverberation room(ISO354.2003.IDT)
2006-05-08 Issued
Digital anti-injury
General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China Standardization Administration of China
2006-11-01 Implementation
Normative reference documents
Terms and definitions
Measurement principle
Frequency range
Test arrangement
Reverberation room and sound field diffusion
Temperature and relative humidity
Reverberation time measurement
Interrupted sound source method
Impulse response integration method
Reverberation time according to decay curve Interval value
8 Expression of results
8.1 Calculation method
8.2 Precision
8.3 Expression of results
9 Test report
Appendix A (Normative Appendix)
Appendix B (Normative Appendix)
Appendix C (Informative Appendix)
Diffusion of sound field in reverberation chamber
Installation of test specimens for sound absorption test
References
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GB/T20247-—2006/ISO354:2003
This standard is equivalent to ISO354:2003 "Acoustic reverberation chamber sound absorption measurement". GB/T20247-2006/ISO354:2003
When this standard is equivalent to an international standard, some terms and definitions shall be in accordance with GB/T3947-1996 "Terms of Acoustics". Appendix A and Appendix B of this standard are normative appendices, and Appendix C is an informative appendix. This standard is proposed by the Chinese Academy of Sciences.
This standard is under the jurisdiction of the National Technical Committee for Acoustics Standardization (SAC/TC17). The drafting units of this standard are: China Radio, Film and Television Design Institute, Institute of Acoustics, Chinese Academy of Sciences. The main drafters of this standard are: Chen Huaimin, Zhang Mingzhao, Luo Xuecong, Chen Jianhua, and Lv Yadong. GB/T20247-2006/S0354:2003
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When a sound source emits sound in a closed space, the reverberation sound will increase to a certain sound level, the sound source stops emitting sound, and the reverberation sound gradually decays. This decay depends on the sound absorption characteristics of the interface, air and objects in the closed space. Usually, the part of the incident sound energy absorbed by a surface is related to the incident angle of the sound. In order to link the reverberation time of halls, offices, factories, etc. with the noise reduction affected by sound absorption treatment, it is necessary to understand the sound absorption characteristics of each surface, and usually a proper average of all incident angles is adopted. Since the distribution of sound waves in a typical closed space contains a large number of unpredictable propagation directions, a uniform distribution state is adopted as the basic condition for the purpose of standardization. In addition, if the sound intensity is independent of the spatial position, the sound field distribution state at this time is called a diffuse sound field, and the sound is randomly incident on the room surface. The sound field in a properly designed reverberation room is similar to a diffuse sound field. Therefore, the sound absorption performance measured in the reverberation room is similar to the sound absorption performance measured under the basic conditions of the assumed standard.
This standard aims to promote the consistency of methods and conditions for measuring sound absorption in reverberation rooms. 1 Scope
GB/T20247-—2006/IS03542003
Sound absorption measurement in reverberation rooms
This standard specifies the method for measuring the sound absorption coefficient of acoustic materials used to treat interfaces such as walls or tops, or the sound absorption of objects such as furniture, people, and space absorbers in a reverberation room. This method is not suitable for measuring the sound absorption characteristics of low-damping resonators. The measurement results can be used for data comparison and design calculations related to indoor acoustics and noise control. 2 Normative referenced documents
The clauses in the following documents become clauses of this standard through reference in this standard. For all dated references, all subsequent versions are the same as the amendments (excluding errata) to this document. The latest version of this document is applicable to this standard. However, parties to agreements based on this standard are encouraged to study all undated references. The latest version applies to the vehicle standard. Common frequencies in the measurement are GB/T3240-1982 GB/T3241 octave and fractional octave filters (GB/T8241-1998eqvIEC6260:199 5) GB/T17247.1
eqvISO9613-1:199
Outdoor sound propagation attenuation
Terms and definitions
apply to this standard. The following terms and definitions
edenve
decay curve
B/117247.1-2000
Part: Calculation of atmospheric sound absorption
Describes the graph of the sound pressure level in the room decaying with time after the sound source stops emitting sound S
reverberation time
Reverberation time
After the sound has reached a steady state
is the time required for the average sound energy density to decay from its original value by one millionth (6B) pound, unit Note 1: It can be more accurately extrapolated to a shorter value to meet the definition of reverberation time of 60%B, Note 2: This definition is based on the assumption that the sound pressure level is linearly related to time and the background noise is low enough. 3.3
Interrupted noise method interruptednoisemethod The time required for the sound pressure level of the room to be excited is 60% of the time. A method of obtaining a decay curve by directly recording the decay of the sound pressure level after a broadband or narrowband sound source stops making sound. 3.4
Integrated impulse response method integrated impulse response method A method of obtaining a decay curve by integrating the square of the impulse response inversely with respect to time. 3.5
Impulse response impulse response
The instantaneous state of the sound pressure formed by a Dirac impulse sound emitted from one point in the room at another point Note: It is impossible to generate and radiate a true Dirac delta function pulse. However, in actual measurement, a sufficiently approximate instantaneous sound (such as a gunshot sound) can be used. Another optional measurement technique is to use a maximum length sequence signal (MLS) or other signal that determines the flat spectral characteristics and transform the measured response back into an impulse response. 1
GB/T20247-2006/ISO354:2003
Room sound absorption equivalent sound absorption area aofaroom The total sound absorption of each surface and object in the room plus the loss in the medium in the room. Note: The unit is m.
Note 2: The room sound absorption of an empty reverberation room is represented by A, and the room sound absorption of a reverberation room with a test specimen is represented by A. 3.7
fequivalent sound absorption area of the testspecimen Test specimen sound absorption
The difference in sound absorption of a reverberation room with and without a test specimen. Note: The unit is m
Area of thetest specimen
Area of thetest specimen
The area of the ground or wall covered by the specimen. Note 1: The unit is m.
Note 2: In the case where the specimen is surrounded by a structure (see Type E or Type J installation in Appendix B), the specimen area is the area surrounded by the structure3.9
tsound absorption coefficient
The ratio of the sound absorption of the specimen to the area of the specimen
Note 1: For a sound absorber with two exposed sides, the sound absorption coefficient is the ratio of the sound absorption of the specimen to the total area of the two sides of the specimen. -iiKAoNiKAca
Note 2: The sound absorption coefficient obtained by measuring the reverberation time may be greater than 1.0 (for example, due to the influence of diffraction), so it is not expressed as a percentage. Note 3: The subscript is to avoid confusion with the sound absorption coefficient defined as the ratio of non-reflected sound energy to incident sound energy, such as the case of a plane wave incident on a plane wall at a specific angle. This "geometric" sound absorption coefficient is always less than 1.0 and can therefore be expressed as a percentage. Measurement principle
The average reverberation time of the reverberation room with and without the test piece is measured separately: The sound absorption of the test piece A- is calculated from these reverberation time data using the Sabine formula (see 8.1.2). For a test piece with a uniformly covered surface (a flat sound absorber or a specified arrangement of objects), the sound absorption coefficient is the ratio of the sound absorption of the test piece A to the area of the test piece S (see 8.1.3).
If the test piece is composed of a number of identical objects, the sound absorption of a single object A is the ratio of the total sound absorption A to the number of objects (see 8.1.4).
Frequency range
The measurement should be carried out in 1/3 octaves, and its center frequency (Hz) is as follows in accordance with GB/T3240-1982: 100
Additional measurements outside this frequency range can be carried out in 1/3 octaves with a center frequency that complies with the provisions of GB/T3240-1982. In the low frequency range (below 100Hz), it is difficult to obtain accurate measurement results due to the low normal mode density of the reverberation room. 6 Test arrangementWww.bzxZ.net
6.1 Reverberation chamber and sound field diffusion
6.1.1 Reverberation chamber volume
GB/T20247-—2006/ISO354:2003 The volume of a reverberation chamber should not be less than 150m. The volume of a newly built reverberation chamber is recommended to be no less than 200m. Reverberation chambers with a volume exceeding 500m may not accurately measure the sound absorption in the high frequency band due to air absorption. 6.1.2 Reverberation chamber shape
The shape of the reverberation chamber should meet the condition of formula (1): lux<1.9vs
Where:
The maximum linear dimension of the room (for example, the maximum linear dimension of a rectangular room is the main diagonal), in meters (m): Zmax
VThe volume of the room, in cubic meters (m*). In order to achieve a uniform distribution of normal frequencies (especially in the low frequency band), the dimensions of any two sides of the room should not be in a small integer ratio. 6.1.3 Sound field diffusion
The gradually decaying sound field in the reverberation room should be sufficiently diffused. In order to achieve satisfactory diffusion, regardless of the shape of the reverberation room, it is usually necessary to set up fixed or suspended diffusers or rotating diffusers (see Appendix A). 6.1.4 Sound absorption
The 1/3 octave sound absorption A of an empty reverberation room calculated according to 8.1.2.1 should not exceed the value given in Table 1Table 1
Maximum sound absorption of an empty reverberation room with a volume of 200mFrequency/H2
Sound absorption/m
Rate/Hx
Sound absorption/m
If the volume V of the reverberation room is not 200m, the sound absorption values given in Table 1 should be multiplied by (V/200)2/500
The frequency characteristic diagram of the sound absorption of an empty reverberation room should be a smooth curve without obvious peaks or valleys. The difference between any 1/3 octave sound absorption and the average value of the sound absorption of its two adjacent 1/3 octave bands should not be greater than 15%. 6.2 Test piece
6.2.1 Planar sound absorber
6.2.1.1 The area of the test piece should be 10m~12m. If the volume V of the reverberation room is greater than 200m, the upper limit of the test piece area should be multiplied by V/200)a/3
The choice of the test piece area depends on the volume of the reverberation room and the sound absorption capacity of the test piece: the larger the room volume, the larger the test piece area should be. For test pieces with a small sound absorption coefficient, the upper limit of the test piece area requirement should be selected. 6.2.1.2 The test piece should be made into a rectangle with a width to length ratio of 0.7~1, and should not be less than 1m away from any room boundary, but at least 0.75m. The test piece boundary should not be parallel to the nearest room boundary. If necessary, heavier test pieces can be installed vertically along the wall and fall directly on the ground. In this case, the requirement that the test piece is at least 0.75m away from the room boundary can be ignored. 6.2.1.3 The test piece shall be installed in one of the ways specified in Appendix B, unless the manufacturer provides relevant instructions or the user specifies that the application details require a different installation method. The reverberation time of an empty reverberation chamber shall be measured without the test piece frame or side frame (except for the surrounding baffles in the case of Class J installation).
6.2.2 Discrete sound absorbers
6.2.2.1 Rectangular unit sound absorbing pads or panels shall be installed in the manner specified in Class J in Appendix B. 6.2.2.2 Discrete objects (such as seats, free-standing screens, people, etc.) shall be installed in the typical installation method in actual applications. For example, a seat or free-standing screen shall be placed on the ground, but not less than 1m from any other boundary of the room. Spatial sound absorbers shall be installed at least 1m away from any boundary of the room, the room diffuser, and the microphone. Office screens shall be installed as single objects 6.2.2.3 The test piece shall contain a sufficient number of single objects (generally at least three) to provide a measurable change in the room sound absorption greater than 1m, but not more than 12m. If the reverberation chamber volume V is greater than 200m, these two values should be multiplied by (V/200)3. The distance between the discrete objects3
GB/T20247—2006/ISO354.2003
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should be at least 2m and randomly arranged. If the test piece is only one object, at least three positions should be measured, each with a distance of at least 2m, and the measurement results should be averaged.
6.3 Temperature and relative humidity
6.3.1 Changes in temperature and relative humidity during the measurement process have a great influence on the measured reverberation time, especially in the high frequency band and when the relative humidity is low. This is quantitatively described in GB/T17247.1. 6.3.2 The measurements in the empty room and in the reverberation chamber after the test piece is placed should be carried out under conditions of almost the same temperature and relative humidity, so that the adjustment due to air absorption is not much different. In any case, the relative humidity in the reverberation chamber shall be at least 30% and at most 90% during the entire measurement period; the temperature shall not fall below 15°C. All measurements shall be corrected for changes in air absorption in accordance with 8.1.2.3. Allow the test piece to reach equilibrium in the temperature and relative humidity conditions in the reverberation chamber before testing. 7 Reverberation Time Measurements
7.1 Overview
7.1.1 Introduction
This standard describes two methods for measuring reverberation time curves: the interrupted source method and the impulse response integral. The decay curve measured by the interrupted source method is a statistical process. To obtain suitable repeatable data, several decay curves or several reverberation time values measured at the same transmitter/speaker position must be averaged. The impulse response integral of the room is a fixed number and will not be subject to statistical deviation, so averaging is not required. However, the impulse response integral method requires more advanced instrumentation and data processing capabilities than the interrupted source method. 7.1.2 Microphones and microphone positions
Different microphone positions should be set up. The distance between the positions is up to 41.5m. At least 2m from the sound source, the measuring microphone should be an omnidirectional microphone
The decay curves measured at different microphone positions should not be combined in any way. At least 1m from any surface in the room.
7.1.3 Sound source position
It should be emitted by an omnidirectional sound source. Different sound source positions should be set up, and the distance between the positions is at least 3m. The number of microphone positions in the reverberation room
7.1.4 Microphones
Therefore, the product of the number of microphone positions and the number of loudspeaker positions is at least 12. The decay curves measured by the spatially independent children are at least 12. It is allowed to use two or more sound sources at the same time, as long as they are each 3. The number of sound source positions is at least 2.
The maximum number of microphone positions in the room
1/3 octave band
If two or more sound sources are sounding at the same time, the attenuation of the spatial independent measurement shall not exceed 3dB.
The variation curve can be reduced to
7.2 Interrupted sound source method
7.2.1 Room sound excitation
Use the loudspeaker as the source. The signal to the loudspeaker is a broadband or narrowband noise signal with a continuous spectrum. When using broadband noise signals and the spectrum of the sound signal of the real-time analyzer, the difference between the sound pressure levels of two adjacent 1/3 octave bands in the reverberation room shall not exceed 6dB. When using narrowband noise signals H, its bandwidth shall be at least 143 octaves. The sound excitation time shall be long enough to produce a steady-state sound pressure level in all frequency bands to be measured before stopping. To this end, the acoustic excitation time should be at least half the estimated reverberation time.
The sound pressure level of the excitation signal should be high enough before decay so that the sound pressure level at the lower limit of the range of values in the affinity curve is at least 10 dB higher than the background noise level (see 7.4.1).
If the bandwidth of the signal is greater than 1/3 octave, the difference in reverberation time of adjacent bands will affect the lower part of the decay curve. If the reverberation time of adjacent bands differs by more than 1.5 times, the decay curve of the band with the shortest reverberation time should be measured separately using a 1/3 octave sound source.
7.2.2 Averaging
As explained in 7.1.1, multiple data measured at a certain microphone/speaker position must be averaged to reduce the measurement uncertainty caused by statistical deviations. At least 3 data should be averaged. If the repeatability of the interrupted sound source method is expected to be in the same range as the repeatability of the impulse response integration method, at least 10 data should be averaged (see 8.2). There are two averaging methods. The first is to use 4
Equation (2) to average the decay curves recorded at a certain microphone/speaker position. Where:
L( -10e[210%]
GB/T20247-2006/ISO3542003
（2）
The average sound pressure level at time t calculated for a total of N decays, in decibels (dB): the sound pressure level of the nth decay at time t, in decibels (dB). This method is generally called the "ensemble average method". The second averaging method is suitable for situations where the ensemble average method cannot be applied. First, the reverberation time value of a single decay curve is obtained, and then the obtained reverberation time value is arithmetic averaged. The decay curves recorded at different microphone/field speaker positions should not be averaged. Note: In theory, in laboratory measurements, averaging the reverberation time values can obtain results similar to the ensemble average method. When using a computer to control the instrument, The ensemble average method is always used. The decay curve obtained by averaging multiple decays will generally be smoother than that obtained by averaging a single decay, which will allow more reliable location of the range of values in the decay curve (in most cases this is done manually). 7.2.3 Recording System The recording system shall be an electronic recorder or other suitable system, including the necessary amplifiers and filters, for determining the average slope of the decay curve corresponding to the reverberation time. The instrument used to record (display and/or measure) the sound pressure level decay may use a) an exponentially continuous curve, or b) a linear average of successive discrete samples obtained by averaging the exponentially continuous output, or a linear average of successive discrete outputs, in some cases with considerable pauses in determining the average value. (or similar equipment)
The time constant of the exponential balance
see Note 2 should be low and as close to T/20 as possible. The averaging time of the linear balance should be
less than T/12
For instruments which record # as a series of discrete points, the sampling interval of the record should be less than the averaging time of the instrument (≤ I/12). Where the decay must be visually appraised, the time scale of the displayed graph should be adjusted so that the slope of the decay line is as close to 45° as possible.
Note, the sound pressure as a function of time is recorded in a graphical manner. A commercial level recorder of this grade is approximately equivalent to an index averaging instrument. Note 2 When using an averaging instrument, set the averaging time to a minimum and measure 20. Advantages are few: When using a linear averaging instrument, set the sampling time interval to a minimum of 12. No advantages. During the series of measurements, the corresponding averaging time can be set for each frequency band. The above method is not feasible. The measurement is too heavy. It is recommended to determine the averaging time or sampling time interval for all frequency bands according to the above requirements based on the shortest reverberation time.
The receiving equipment's measurement
Octave method The oscillator should comply with GB/T3241
7.3 Impulse response product
7.3.1 Direct method
The impulse response can be measured directly using a pulse sound source such as pistol shooting, balloon blasting, electric fireworks or other sound sources that can produce sufficient frequency width and energy (in accordance with the requirements of 7.2.1). Note: Loudspeakers are usually not suitable for generating high-frequency pulse signals with sufficient energy, but can only generate heavily filtered pulses. A proven method is to feed the loudspeaker system with the outdated impulse response of a bandpass filter (e.g. a 1/3 octave filter). 7.3.2 Indirect method
A special acoustic signal can be used to obtain the impulse response by special processing of the microphone signal. This will improve the signal-to-noise ratio. If the spectral characteristics of the sound source meet the requirements, swept frequency or pseudo-random noise (e.g. maximum length sequence MLS) can be used. Due to the improvement in the signal-to-noise ratio, the dynamic range of the sound source is much lower than that required in 7.31. If synchronous time averaging is performed (e.g. to improve the signal-to-noise ratio), it must be confirmed that the impulse response remains unchanged throughout the measurement process. The acoustic signal can be emitted by external hardware or software or a component of the measuring instrument.
The bandwidth of the acoustic signal should be greater than 1 /3 octave. The spectrum of the 1/3 octave to be measured should be relatively flat. In addition, the broadband noise spectrum can also be adjusted to provide an approximate pink noise spectrum with a 1/3 octave center frequency range from 100Hz to 5000Hz, so as to simultaneously measure the reverberation time of each 1/3 octave. The acoustic signal should make the decay curve of each frequency band meet the requirements for sound pressure level in 7.2.1 5
GB/T20247——2006/IS035420037.3.3 Recording system
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The recording system should include: a microphone and amplifier that meet the requirements of 7.1.2 and 7.2.3: capable of digitizing the recorded signal and completing tasks including impulse response Additional instruments for all necessary data processing such as integration and decay curve sampling. In the case of 7.3.2, the recording system will also include the necessary hardware and software to process the impulse response obtained from the recorded signal and to generate the test signal.
The impulse response should be filtered by 1/3 octave. The filtering process can be performed before or after the digitization of the impulse response, but in any case it must be performed before the integration process. Either analog filters or digital filters can be used. The filters should comply with the provisions of GB/T3241.
Note: The use of special test signals such as the maximum length sequence MLS requires not only more complex data processing but also deeper theoretical knowledge to obtain appropriate results. The specific details of this technology are beyond the scope of this standard. The user can refer to the relevant information for the standard range. 7.3.4 Integration of impulse response
Integrate the filtered impulse response in reverse. Theoretically, the result is equivalent to the average result of infinite decays obtained by the interrupted sound source method. Many commercial systems have integrated the reverse integration process, and users generally do not need to calculate the integration themselves. The basic operation process is as follows:
The decay curve of each frequency band is obtained by reverse integration of the square of the impulse response. In the ideal case without background noise, the square of the impulse response is integrated from the end point of the impulse response (t→) to the starting point of the impulse response. In this way, the decay as a function of time is shown in formula (3):
Where:
[p(t)dt=[pe(r)dt =
[p(t)dt-
Inverse integration of the square of the impulse response:
p(t)—impulse response sound pressure, in Pascals (Pa). (d)
To minimize the influence of background noise on the later stage of the impulse response, the following method is used for correction: ... (3)
If the sound pressure level of the background noise is known. Then the lower limit of integration is the intersection of the following two lines: one is the background noise level line, and the other is the slope that can represent the square decay curve of the impulse response. The upper limit of integration is still the starting point of the impulse response, and the decay curve is calculated by formula (4): E(t)
Where:
p(t)d(-)-C.
Length t, C is the optional correction value of the square of the impulse response integrated from t to 64
The C value is calculated under the assumption that the slope of the exponential decay curve of the sound energy is the same as that of the square decay curve of the impulse response from t to t, and the result is the most reliable. t. is the time corresponding to the sound pressure level 10dB higher than the time. If C is taken as zero, the limited starting point of integration will lead to a systematic underestimation of the reverberation time. In order to ensure that the reverberation time is underestimated by no more than 5%, the sound pressure level at the starting point of the reverse integration should be at least 15 dB below the maximum value of the square of the impulse response, plus the dynamic range of the reverberation time T estimate
7.4 Reverberation time value according to the decay curve 7.4.1 Value range
The decay curve of each frequency band specified in Chapter 5 should start at 5B below the starting sound pressure level. The value range should be 20 dB, and its lower limit should be at least 10 dB higher than the overall background noise of the measurement system. 7.4.2 Value method
When a computer-controlled recording system is used, calculating the minimum multiplication fit straight line over the entire value range is a convenient method for determining the reverberation time. Similar results can be obtained using other algorithms. When recording directly using a level recorder, a straight line should be drawn manually that is as close to the decay curve as possible. In the case of discrete point value selection, the number of points should be sufficient to allow the least squares fit method to be applied.
8 Result Expression
8.1 Calculation Method
8.1.1 Calculation of Reverberation Time T and T
GB/T20247-—2006/IS0354:2003 The reverberation time of each frequency band in the reverberation room is expressed by the arithmetic mean of all reverberation times measured in the cheek strap. The average values of the reverberation time of each frequency band measured in the empty reverberation room and with the test piece, T, and T·, should be calculated and expressed with two significant figures after the decimal point.
8.1.2Calculation of AFA2 and A
8.1.2.1 The sound absorption A (unit: m2) of an empty reverberation room should be calculated according to formula (5): A=55.3V
Wherein:
Volume of an empty reverberation room, in cubic meters (m*):-4Vm
The speed of sound propagation in the air under the condition of an empty reverberation room, in meters per second (m/s); The reverberation time of an empty reverberation room, in seconds (s): (5)
The sound intensity attenuation coefficient under the condition of an empty reverberation room, in units of per meter (m). Calculated according to GB/T17247.1 based on the air conditions of the empty reverberation room during the measurement process. The m value can be calculated by the attenuation coefficient α used in GB/T17247.1 as follows: m=
1olg(e)
Note: When the temperature is within the range of 15℃ to 30℃, the c value can be calculated by the formula c-331.45+0.6t, where c is the speed of sound in air, in meters per second (m/s), and t is the air temperature, in degrees Celsius (℃). 8.1.2.2 The sound absorption A of the reverberation chamber after the test piece is placed (unit: m) should be calculated according to formula (6): Az-55.3V
Where:
The propagation speed of sound in air under the reverberation chamber conditions after the test piece is placed, in meters per second (m/s); ·6
The reverberation time of the reverberation chamber after the test piece is placed, in seconds (s); The sound intensity attenuation coefficient under the reverberation chamber conditions after the test piece is placed, in meters per second (m-). It is calculated according to GB/T17247.1 based on the air conditions in the mixing chamber after the test piece is placed during the measurement. The m value can be calculated by the attenuation coefficient α used in GB/T17247.1 as follows:
1olg(e)
8.1.2.3 The sound absorption of the test piece At (unit: m). It should be calculated according to formula (7): At=A-A-55.3V(
8.1.3 Calculation of sound absorption coefficient α,
4V(mz-m)
The sound absorption coefficient α of a plane sound absorber or a specified arrangement of objects should be calculated as follows: A
S The area of the test piece, in square meters (m), see 3.8). 8.1.4 Calculation of sound absorption of discrete sound absorbers
For discrete sound absorbers, the result is usually expressed in terms of the sound absorption A of a single object, which should be calculated according to formula (9): 8)
GB/T20247--2006/IS0354:2003 Where:
Number of objects to be measured.
For a specified arrangement of objects, the result is expressed in terms of the sound absorption coefficient, calculated according to 8.1.3. 8.2 Precision
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8.2.1 Overview
The uncertainty of the entire sound absorption measurement is affected by two factors. The first is the uncertainty of the reverberation time measurement, which is particularly prominent when the interrupted sound source method is applied (see 8.2.2). The second factor causing uncertainty is the limitation of reproducibility, which is caused by the setting of the entire measurement process, including the reverberation room and the installation method. The changes caused by the laboratory setting are under investigation (see 8.2.3). 8.2.2 Repetition rate of reverberation time measurement
The relative standard deviation of the reverberation time T2 taken within the decay range of 20 dB can be estimated using formula (10): where:
E2o(T)
Example of Figure 1
Standard deviation of reverberation time value
T2
Measured reverberation time in seconds (s) ③Clear frequency center frequency in Hertz (H2): Number of decay curves.
Standard deviation of T2 measurement. 12 measurement points, each of which records three decays. ae
8.2.3 Reproducibility
The reproducibility of sound absorption measurements is under investigation. Figure 1 Example of standard deviation
8.3 Presentation of results
For all measurement frequency bands, the following results should be given in the measurement report in the form of tables and graphs: a) For planar sound absorbers, the sound absorption coefficient a, b) For single objects, the sound absorption of single objects A; c) For the specified object arrangement, the sound absorption coefficient α,. The sound absorption of the test piece should be rounded to 0.1m, and the sound absorption coefficient should be rounded to 0.01. Note: Note that the precision of the measurement results may be less than the precision indicated by the above decimal point rounding limit. 8
Vehicle rate/H2
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