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

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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

Basic Data

Standard ID	GB/T 18696.2-2002 (GB/T18696.2-2002)
Description (Translated English)	Acoustics. Determination of sound absorption coefficient and impedance in impedance tubes. Part 2: Transfer function method
Sector / Industry	National Standard (Recommended)
Classification of Chinese Standard	A59
Classification of International Standard	17.140.01
Word Count Estimation	22,293
Date of Issue	2002-03-26
Date of Implementation	2002-12-01
Quoted Standard	GB/T 3947-1996; GBJ 88-1985; ISO 354-1985; ISO 5725-1-1994; ISO 10534-1-1996
Adopted Standard	ISO 10534-2-1998 (E), MOD
Issuing agency(ies)	General Administration of Quality Supervision, Inspection and Quarantine of the People Republic of China
Summary	This standard specifies the determination of sound-absorbing material on the surface or surface acoustic impedance acoustic admittance rate. The acoustic impedance of sound-absorbing material and its physical properties (such as flow resistance, porosity, elasticity modulus and density) related, the standard measurement method in the basic research and product development are also useful. This method is the method specified in ISO 10534-1 and similar to be used for a source at one end, the opposite end of the impedance tube test piece, but the measurement method is completely different. In this method, the tube in the plane wave generated by the noise source, the analysis of the interference field with both a position in the wall mounted microphone (or a movement in the tube may be a microphone) to make two measurements of sound pressure to achieve and then complete the complex transfer function, normal incidence sound absorption material sound absorption coefficient and impedance ratio calculations. This method is intended to provide an alternative method is more efficient than the standing wave ratio measurement method. This method will be provided with ISO 354 acoustic reverberation chamber measurement method comparison can be seen between the two are very different. (Under ideal conditions) measured reverberation chamber diffusion incidence sound absorption coefficient, can be used to test the lateral and France have significantly different structure of the material. However, the reverberation chamber method requires a larger test sample, which can only provide a small sample sound absorption materials research and development work has not to place. Impedance tube method is limited to the parameters of normal incidence, requiring the test sample with an impedance tube cross section as large. Materials for local reactions, diffusion incidence sound absorption coefficient from impedance tube method