Normal Incidence Absorption Coefficient Measurement in an Impedance Tube

Definition

An impedance tube, also called a Kundt tube, is a tube fitted at one end with a loudspeaker and at the other end with a sample holder in which the sample of acoustic material to be tested is installed. The measurement is done using two microphones located between the loudspeaker and the sample holder.

From a measurement of the transfer function H12 between the two microphones [ASTM E1050, ISO 10534-2], the complex reflection coefficient R, the acoustic absorption Alpha and the surface impedance Zs are calculated.

What Type of Impedance & Transmission Tube?

There is a whole range of impedance tubes in different sizes and shapes. It is therefore important to choose the right impedance tube according to specific needs.

Circular tubes are the most common. Mecanum offers three different sizes of impedance or transmission tube, each covering a given frequency range. The diameter of the tube imposes the upper limit of the useful frequency range while the inter-microphonic distance imposes the low frequency limit (phase error) as well as the high frequency (wavelength size). The impedance tubes that we offer have three microphone positions to be able to modify the microphone distance and therefore obtain a wider frequency range.

	29 mm Tube	44.45 mm Tube	100 mm Tube
Frequency Range (Hz)	50 to 6600	45 to 4300	35 to 1800
Large Microphone Distance (Hz)	50 to 1600	45 to 2000	35 to 1600
Small Microphone Distance (Hz)	175 to 6600	115 to 4800	70 to 1800

The main difficulties when measuring in a tube are related to the preparation and installation of the samples in the tube.

• If the sample is slightly smaller in diameter than the tube, acoustic leaks will be created between the sample and the walls of the tube. Depending on the type of materials, these leaks can lead to an overestimation of sound absorption.
• If the sample is slightly larger than the diameter of the tube, it will be compressed during installation, which can lead to some mechanical resonances in the material which will artificially modify the sound absorption measurement. [1][2]

The main differences between our different tubes are the following:

• The 29 mm tube covers the widest frequency range. Its small diameter makes it suitable only for measuring very homogeneous materials. As its surface-to-circumference ratio is low, the tested materials must be extremely well cut to avoid issues due to boundary conditions.
• The 44.45 mm tube offers the best compromise between the sample size and the frequency range. It allows easier measurements than with the 29 mm tube while keeping a wide frequency range.
• The 100 mm tube covers a frequency range from 35 Hz to 1800 Hz depending on the chosen microphone spacing. This type of tube is particularly well suited to the measurement of relatively inhomogeneous materials or meta materials.

References

[1] Pilon et al.: Criterion for edge-constrained foams, J. Acoust. Soc. Am., Vol. 114, No. 4, Pt. 1, October 2003.

[2] Pilon et al.: Circumferential air gaps on porous materials J. Acoust. Soc. Am., Vol. 116, No. 1, July 2004.

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