GB/T 20247-2006 in English

《GB/T 20247-2006声学 混响室吸声测量》由TC17（全国声学标准化技术委员会）归口，主管部门为中国科学院。
本标准规定了在混响室内测量用于处理墙壁或顶部等界面的声学材料的吸声系数，或者如家具、人、空间吸声体等的吸声量的方法。该方法不适用于测量低阻尼共鸣器的吸声特性。

1 Scope

This standard specifies a method of measuring the sound absorption coefficient of acoustical materials used as wall or ceiling treatments, or the equivalent sound absorption area of objects, such as furniture, persons or space absorbers, in a reverberation room. It is not intended to be used for measuring the absorption characteristics of weakly damped resonators.
The results obtained can be used for comparison purposes and for design calculation with respect to room acoustics and noise control.

2 Normative References

The following documents contain provisions which, through reference in this text, constitute provisions of this standard. For dated references, all subsequent modification or revision (excluding correction contents) are not applicable to this standard. However, all concerned Parties are encouraged to study if the latest editions of these documents can be applied. For undated references, the latest edition of the normative document referred to applies.
GB/T 3240-1982 "Preferred Frequencies for the Acoustical Measurement"
GB/T 3241 "Octave-band and Fractional-octave-band Filters" (GB/T 3241-1998; eqv IEC 61260: 1995)
GB/T 17247.1 "Acoustics--Attenuation of Sound during Propagation Outdoors--Part 1: Calculation of the Absorption of Sound by the Atmosphere" (GB/T 17247.1-2000; eqv ISO 9613-1: 1993)

3 Terms and Definitions

For the purpose of this standard, the following terms and definitions apply.
3.1
Decay curve
Graphical representation of the decay of the sound pressure level in a room as a function of time after the sound source has stopped
3.2
Reverberation time
T
Time, in seconds (s), that would be required for the average sound energy density to decrease by one millionth (60dB) from the original value after the sound has already been stable and the sound source has stopped
Note 1: The definition of reverberation time with a decrease by 60dB of the sound pressure level can be fulfilled by linear extrapolation of shorter evaluation ranges.
Note 2: This definition is based on the assumptions that, in the ideal case, there is a linear relationship between the sound pressure level and time, and that the background noise level is sufficiently low.
3.3
Interrupted noise method
Method of obtaining decay curves by direct recording of the decay of the sound pressure level after exciting a room with broadband or band-limited noise
3.4
Integrated impulse response method
Method of obtaining decay curves by reverse-time integration of the squared impulse responses
3.5
Impulse response
Temporal evolution of the sound pressure observed at a point in a room as a result of the emission of a Dirac impulse at another point in the room
Note: It is impossible in practice to create and radiate true Dirac delta functions. But short transient sounds (e.g. from shots) may offer close enough approximations for practical measurements. An alternative measurement technique, however, is to use a period of maximum-length sequence type signal (MLS) or another deterministic, flat-spectrum signal and to transform the measured response back to an impulse response.
3.6
Equivalent sound absorption area of a room
The total equivalent sound absorption area by the surfaces and objects in a room plus the loss in the medium in this room
Note 1: The area is measured in square metres.
Note 2: For the empty reverberation room, this quantity is denoted by A1; for the reverberation room containing the test specimen, it is denoted by A2.
3.7
Equivalent sound absorption area of the test specimen
AT
Difference between the equivalent sound absorption area of the reverberation room with and without the test specimen
Note: The area is measured in square metres.
3.8
Area of the test specimen
S
Area of the floor or wall covered by the test specimen
Note 1: The area is measured in square metres.
Note 2: In the case of a test specimen surrounded by a structure (type E mounting or type J mounting in Appendix B), it is the area enclosed by the structure.
3.9
Sound absorption coefficient
αs
Ratio of the equivalent sound absorption area of a test specimen divided by the area of the test specimen
Note 1: For absorbers where both sides are exposed, the sound absorption coefficient is the equivalent sound absorption area of the test specimen divided by the area of the two sides of the test specimen.
Note 2: The sound absorption coefficient αs evaluated from reverberation time measurements can have values larger than 1.0 (e.g. because of diffraction effects), and αs is not, therefore, expressed as a percentage.
Note 3: The use of the subscript “s” is to avoid confusion with the sound absorption coefficient defined as the ratio of non-reflected-to-incident sound energy if a plane wave strikes a plane wall at a particular angle of incidence. That “geometric” sound absorption coefficient is always smaller than 1.0 and may therefore be expressed as a percentage.

Contents
Foreword II
Introduction III
1 Scope
2 Normative References
3 Terms and Definitions
4 Principle
5 Frequency Range
6 Test Arrangement
6.1 Reverberation Room and Diffusion of Sound Field
6.2 Test Specimens
6.3 Temperature and Relative Humidity
7 Measurement of Reverberation Time
7.1 General
7.2 Interrupted Noise Method
7.3 Integrated Impulse Response Method
7.4 Evaluation of Reverberation Times Based on Decay Curves
8 Expression of Results
8.1 Method of Calculation
8.2 Precision
8.3 Presentation of Results
9 Test Report
Appendix A (Normative) Diffusivity of the Sound Field in the Reverberation Room
Appendix B (Normative) Test Specimen Mountings for Sound Absorption Tests
Appendix C (Informative) Bibliography