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GB/T 18696.2 Measurement of sound absorption coefficient and acoustic impedance in impedance tubes — Transfer-function method
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Chinese
GB/T 18696. Standard Description
ISO 10534-2(GB/T 18696.2)Acoustics--Determination of sound absorption coefficient and impedance in impedance tubes Part 2: Transfer function method
ISO 10534-2 (GB/T 18696.2) Standard Introduction
ISO 10534-2 (GB/T 18696.2) specifies a method for measuring sound absorption coefficient and acoustic impedance in impedance tubes using the transfer-function method. It provides a consistent and accurate approach for determining the sound absorption characteristics of materials.Since it enables the correlation between acoustic properties and physical properties of materials (such as airflow resistivity, porosity, elastic modulus, and density), this standard plays an important role in fundamental research and product development.
ISO 10534-2 (GB/T 18696.2) Scope of Application
ISO 10534-2 (GB/T 18696.2) applies to building materials, automotive components, and other sound insulation products.
ISO 10534-2 (GB/T 18696.2) Test Method
The test specimen is installed at one end of a straight, rigid, and airtight impedance tube. Plane sound waves inside the tube are generated by a sound source using random noise, pseudo-random sequence noise, or linear frequency-modulated pulses.Sound pressure is measured at two positions close to the specimen. The acoustic transfer function between the two microphone signals is calculated and used to determine the normal incidence complex reflection coefficient, normal incidence sound absorption coefficient, and acoustic impedance ratio of the specimen.The specific test procedures are as follows:
◎ Impedance tube and equipment preparation: Use an impedance tube consisting of a specimen holder and a sound source section. The impedance tube shall be straight, with a uniform cross-sectional area and smooth, rigid walls to prevent vibration caused by acoustic excitation.
◎ Microphone installation: Two or three microphone mounting holes are arranged along the impedance tube wall. Microphones shall be installed at specified positions on the tube wall. Pressure microphones shall be used, and the connection between the microphones and mounting holes shall be properly sealed.
◎ Sound source and sound wave generation: Plane sound waves inside the tube are generated using random noise, pseudo-random sequence noise, or linear frequency-modulated pulse signals. Sound pressure is measured at two positions near the specimen to obtain the acoustic transfer function.
◎ Specimen installation: The specimen shall be installed in the specimen holder with proper sealing around the edges to prevent sound leakage. The specimen front surface shall be perpendicular to the tube axis to ensure measurement accuracy.
◎ Data collection and analysis: A digital frequency analysis system is used to calculate the cross-spectrum and auto-spectrum of sound pressure signals, and then obtain the transfer function. After measurement, the required acoustic parameters are calculated, including the normal incidence complex reflection coefficient, normal incidence sound absorption coefficient, and acoustic impedance ratio.
ISO 10534-2 (GB/T 18696.2) Basis for determination
◎ Normal incidence sound pressure reflection coefficient (r):
The complex reflection coefficient represents the proportion of incident sound waves reflected from the material surface. The absolute value of the complex reflection coefficient ranges from 0 to 1:
Absolute value = 1: Indicates that all sound waves are reflected, resulting in very poor sound absorption performance.
Absolute value = 0: Indicates that all sound waves are absorbed with no reflection, resulting in excellent sound absorption performance.
For applications requiring high sound absorption performance, such as audio studios and conference rooms, an ideal complex reflection coefficient is close to 0.
◎ Normal incidence sound absorption coefficient (α):
The sound absorption coefficient represents the ability of a material to absorb incident sound energy. Its value also ranges from 0 to 1:
Close to 1: Indicates that the material absorbs almost all sound energy, providing excellent sound absorption performance.
Close to 0: Indicates that the material absorbs little sound energy, resulting in poor sound absorption performance.
For applications requiring noise reduction or sound control, such as theaters, recording studios, or areas near noisy equipment, a higher sound absorption coefficient is generally preferred.
◎ Acoustic impedance ratio (Zs):
Acoustic impedance ratio describes the resistance of a material to sound wave propagation. It consists of a real part (Rs) and an imaginary part (Xs):
Real part (Rs): Represents the damping characteristics of the material. A higher value indicates greater energy dissipation.
Imaginary part (Xs): Represents the elastic characteristics of the material and is related to its energy storage capability.
The ideal acoustic impedance ratio depends on the specific application requirements. For example, sound barriers or sound absorbing materials may require low to moderate real impedance values to improve absorption efficiency, while sound wave guidance or reflection applications may require different impedance configurations.
More Standards Related to Impedance Tubes
ISO 10534 Acoustics — Determination of sound absorption coefficient and impedance in impedance tubes
ISO 10534-2 Measurement of sound absorption coefficient and acoustic impedance in impedance tubes (Transfer-function method)
GB/T 18696.1 Measurement of sound absorption coefficient and acoustic impedance of materials (Standing wave ratio method)
GB/T 18696.2 Measurement of sound absorption coefficient and acoustic impedance of materials (Transfer-function method)
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