GB/T 18696.2-2002 PDF in English
GB/T 18696.2-2002 (GB/T18696.2-2002, GBT 18696.2-2002, GBT18696.2-2002)
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Acoustics -- Determination of sound absorption coefficient and impedance in impedance tubes -- Part 2: Transfer function method
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Standards related to (historical): GB/T 18696.2-2002
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GB/T 18696.2-2002: PDF in English (GBT 18696.2-2002) 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
......
Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.
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