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GB/T 18696.2-2002 English PDF (GBT18696.2-2002)

GB/T 18696.2-2002 English PDF (GBT18696.2-2002)

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GB/T 18696.2-2002: Acoustics -- Determination of sound absorption coefficient and impedance in impedance tubes -- Part 2: Transfer function method

GB/T 18696.2-2002
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 17.140.01
A 59
eqv ISO 10534-2:1998(E)
Acoustics – Determination of Sound Absorption Coefficient
and Impedance in Impedance Tubes – Part 2: Transfer
Function Method
ISSUED ON: MARCH 26, 2002
IMPLEMENTED ON: DECEMBER 01, 2002
Issued by: General Administration of Quality Supervision, Inspection and Quarantine of the PRC
Table of Contents
Foreword ... 3
ISO Foreword ... 4
1 Scope ... 5
2 Normative References ... 6
3 Definitions and Symbols ... 6
4 Principle ... 8
5 Test Equipment ... 9
6 Preliminary Test and Measurements ... 14
7 Test Specimen Mounting ... 15
8 Test Procedure ... 16
9 Precision ... 22
10 Test Report ... 22
Annex A (Normative) Preliminary Measurements ... 24
Annex B (Normative) Procedure for the One-Microphone Technique ... 30
Annex C (Normative) Pressure-Release Termination of Test Sample ... 31
Annex D (Informative) Theoretical Background ... 33
Annex E (Informative) Error Sources ... 35
Annex F (Informative) Determination of Diffuse Sound Absorption Coefficient αst of Locally Reacting Absorbers from the Results of this Part ... 38
Annex G (Informative) Bibliography ... 39
Foreword
This Standard was formulated according to the International Organization for Standardization’s standard ISO 10534-2:1998(E) Acoustics – Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes – Part 2: Transfer-Function Method.
ISO 10534 consists of 2 parts. Part 1: Method Using Standing Wave Ratio, which corresponds to China’s national standard GBJ 88-1985. This Standard is Part 2: Transfer Function Method. This Standard is equivalent to ISO 10534-2 in the technical contents. There are two points that need to be explained in the process of equivalent adoption:
1. 1 "Acoustic impedance" is one of the key terms in this Standard. The definition of it in this international standard (see Article 2.4 of ISO 10534-2) is inconsistent with the national standard GB/T 3947-1996 "Acoustic Terms and Terminology". In order to correspond to ISO, the name of this Standard still retains the original text, but the text uses "Specific acoustic impedance". Correspondingly, formulas (19), (20), (C1) and (C3) are rewritten.
2. Microphone position is one of the important factors affecting the test accuracy. A positioning ring must be provided at the inner end of the mounting hole. Figure 1 b) of this international standard has a ring, while Figure 1 a) has no ring, which has been supplemented now. At the same time, seal the microphone mounting hole with the microphone dummy. This standard adds Clause 2 "Normative Reference". Also corrected an error in Figure A.2.
The formulation of this Standard will standardize a modern measurement technology of sound absorption coefficient and specific acoustic impedance that is more convenient, faster, less operating error, and more consistent in measurement results.
This Standard’s Annexes A, B, and C are normative.
This Standard’s Annexes D, E, F, and G are informative, only for reference. This Standard was proposed by Chinese Academy of Sciences.
This Standard shall be under the jurisdiction of National Technical Committee on Acoustics of Standardization Administration of China.
Chief drafting organizations of this Standard: Institute of Acoustics, Chinese Academy of Science; and Institute for Building Physics, China Academy of Building Research. Chief drafting staffs of this Standard: Li Xiaodong, Dai Genghua, Lin Jie, and Xu Xin. This Standard was entrusted to Foundation Subcommittee of National Technical Committee on Acoustics of Standardization Administration of China.
Acoustics – Determination of Sound Absorption Coefficient
and Impedance in Impedance Tubes – Part 2: Transfer
Function Method
1 Scope
This test method covers the use of an impedance tube, two microphone locations and a digital frequency analysis system for the determination of the sound absorption coefficient of sound absorbers for normal sound incidence. It can also be applied for the determination of the acoustical surface impedance or surface admittance of sound absorbing materials. Since the impedance ratios of a sound absorptive material are related to its physical properties, such as airflow resistance, porosity, elasticity and density, measurements described in this test method are useful in basic research and product development.
The test method is similar to the test method specified in ISO 10534-1 in that it uses an impedance tube with a sound source connected to one end and the test sample mounted in the tube at the other end. However, the measurement technique is different. In this test method, plane waves are generated in a tube by a noise source, and the decomposition of the interference field is achieved by the measurement of acoustic pressures at two fixed locations using wall- mounted microphones or an in-tube traversing microphone, and subsequent calculation of the complex acoustic transfer function, the normal incidence absorption and the impedance ratios of the acoustic material. The test method is intended to provide an alternative, and generally much faster, measurement technique than that of ISO 10534-1.
Compared with the measurement of the sound absorption in a reverberation room according to the method specified in ISO 354, there are some characteristic differences. The reverberation room method will (under ideal conditions) determine the sound absorption coefficient for diffuse sound incidence, and the method can be used for testing of materials with pronounced structures in the lateral and normal directions. However, the reverberation room method requires test specimens which are rather large, so it is not convenient for research and development work, where only small samples of the absorber are available. The impedance tube method is limited to parametric studies at normal incidence but requires samples of the test object which are of the same size as the cross-section of the impedance tube. For materials that are locally reacting, diffuse incidence sound absorption coefficients can be estimated from measurement results obtained by the impedance tube method. For transformation of the test results from the impedance tube method (normal incidence) to diffuse sound incidence, see annex F.
front surface. A sideways insertion also avoids compression of soft materials. The back plate of the sample holder shall be rigid and shall be fixed tightly to the tube since it serves as a rigid termination in many measurements. A metal plate of thickness not less than 20 mm is recommended.
For some tests, a pressure-release termination of the test object by an air volume behind it is needed. This is described in annex C.
5.8 Signal processing equipment
The signal processing system shall consist of an amplifier, and a two-channel Fast Fourier Transform (FFT) analyzing system. The system is required to measure the sound pressure at two microphone locations and to calculate the transfer function H12 between them. A generator capable of producing the required source signal (see 5.10) compatible with the analyzing system is also required.
The dynamic range of the analyzer should be greater than 65dB. The errors in the estimated transfer function H12 due to nonlinearities, resolution, instability and temperature sensitivity of the signal processing equipment shall be less than 0.2dB.
Using the one-microphone technique, the analyzing system shall be able to calculate the transfer function H12 from the generator signal and the two microphone signals measured consecutively. 5.9 Loudspeaker
A membrane loudspeaker (or a pressure chamber loudspeaker for high frequencies with a horn as a transmission element to the impedance tube) should be located at the opposite end of the tube from the test sample holder. The surface of the loudspeaker membrane shall cover at least two-thirds of the cross-sectional area of the impedance tube. The loudspeaker axis may be either coaxial with the tube, or inclined, or connected to the tube by an elbow. The loudspeaker shall be contained in an insulating box in order to avoid airborne flanking transmission to the microphones. Elastic vibration insulation shall be applied between the impedance tube and the frame of the loudspeaker as well as to the loudspeaker box (preferably between the impedance tube and the transmission element also) in order to avoid structure- borne sound excitation of the impedance tube.
5.10 Signal generator
The signal generator shall be able to generate a stationary signal with a flat spectral density within the frequency range of interest. It may generate one or more of the following: random, pseudo-random, periodic pseudo-random, or chirp excitation, as required. In the case of the one-microphone technique, a deterministic signal is recommended and a periodic pseudo-random sequence is well suited for this method, although special signal

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