ISO 10534 Acoustics - Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes

The “ISO 10534 Acoustics - Determination of sound absorption coefficient and impedance in impedance tubes” standard, developed by the International Organization for Standardization (ISO), consists of sub-sections published under the same title. Specifically,

ISO 10534-1 Part 1: Method using standing wave ratio" describes a method for determining the sound absorption coefficient, reflection factor and surface impedance or surface complex conductivity of materials and objects. The values for the normal sound event are determined by evaluating the standing wave pattern of a plane wave in a tube formed by the superposition of the incident sinusoidal plane wave with the plane wave reflected from the test object.

This method can be used to determine the sound absorption coefficient of sound absorbers for normal sound phenomena. It can also be used to determine the acoustic surface impedance or surface complex conductivity of sound absorbing materials. It is quite suitable for parameter studies and design of sound absorbers because only small samples of the absorbing material are needed.

There are some characteristic differences between this method and the measurement of sound absorption in the reverberation chamber. The impedance tube method can be used to determine the reflection factor and also the impedance or complex conductivity. Sound normally falls on the surface of the object. The reverberation chamber method (under ideal conditions) determines the sound absorption coefficient for random sound phenomena.

The impedance tube method is based on the existence of a plane incident sound wave and gives exact values under this condition (excluding measurement and installation errors). The evaluation of the sound absorption coefficient in a reverberation chamber is based on a number of simplifying and approximate assumptions regarding the sound field and the size of the absorber. For this reason, sound absorption coefficients exceeding the value of 1 are sometimes obtained.

The impedance tube method requires test object samples of the same cross-sectional area as the impedance tube. The echo chamber method requires relatively large test objects and can also be applied to test objects with distinct structures in the lateral or vertical directions. Measurements made with such objects in the impedance tube should be interpreted with caution.

This standard gives priority to numerical evaluation methods over graphical methods because of the assumption that computers capable of performing these calculations are available. Some of the quantities in the formula are complex. The arguments of trigonometric functions are in radians.

ISO 10534-2 Part 2: Two-microphone technique for normal sound absorption coefficient and normal surface impedance" describes a test method that involves the use of an impedance tube, two microphone positions and a frequency analysis system to determine the sound absorption coefficient of sound-absorbing materials for the normal sound incident. It can also be applied to determine the acoustic surface impedance or surface complex conductance of sound-absorbing materials. As an extension, it can also be used to evaluate the intrinsic properties of homogeneous acoustic materials, such as characteristic impedance, characteristic wave number, dynamic mass density and dynamic volume modulus.

This test method is similar to the test method specified in ISO 10534-1 in that an impedance tube with a sound source connected to one end and a test specimen mounted in the tube at the other end is used. However, the measurement technique is different. In this test method, plane waves are generated in a tube by a sound source and the separation of the interference field is achieved by measuring the acoustic pressures at two fixed points using wall-mounted microphones or a crosshead microphone located in the tube and then calculating the complex acoustic transfer function and quantities reported in the previous paragraph. This test method is intended to provide an alternative and generally much faster measurement technique to ISO 10534-1.

The normal incident absorption coefficients from impedance tube measurements cannot be compared with the random incident absorption coefficients measured in echo chambers according to the ISO 354 standard. The reverberation chamber method determines the sound absorption coefficient for the scattered sound incident (under ideal conditions). However, the reverberation chamber method requires relatively large test specimens. The impedance tube method is limited to studies in the normal and plane incident range and requires test specimens of the same size as the cross-sectional area of the impedance tube. For materials that respond only locally, the scattered incident sound absorption coefficients can be estimated from the measurement results obtained with the impedance tube method.

For the purposes of these standards, the following terms and definitions apply:

The sound absorption coefficient is the ratio of the sound power entering the surface of the test object (without return) to the incident sound power for a plane wave at normal incidence.
The sound pressure reflection factor for normal incidence is the complex ratio of the pressure amplitude of the reflected wave to the incident wave at the reference plane for a plane wave with a normal angle of incidence.
The reference plane is the section of the impedance tube on which the reflection factor or impedance or complex conductance is determined and is usually the surface of flat test objects.
Field impedance is the ratio of the sound pressure at a point in the sound field to the particle velocity directed towards the test object.
Impedance in the reference plane is the ratio of sound pressure to the speed of the sound particle in the reference plane.
The sound absorption coefficient at normal incidence is the ratio of the sound power distributed in the test object to the incident sound power for a plane wave at normal incidence. Here, a plane wave describes a wave whose value is constant at any instant in any plane perpendicular to the direction of propagation. The normal incidence describes the direction of the longest axis of the impedance tube.
The normal incidence sound pressure reflection coefficient is the complex ratio of the reflected wave sound pressure amplitude to the amplitude of the incident wave in the reference plane for a plane wave at normal incidence.
The reference plane is the section of the impedance tube on which the reflection factor or impedance or complex conductance is determined and is usually the surface of the test object if it is flat.
The normal incident surface impedance is the ratio of the complex sound pressure to the normal component of the complex sound particle velocity at an individual frequency defined as 0 in the reference plane. The particle velocity vector has a positive direction pointing towards the interior of the tested object.

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