Absorption Coefficient of Acoustic Wedges: Comparison Between Impedance Tube and Reverberation Chamber Methods

SOLAMETA Panel & AS / BFW

In this article, I’d like to explain the absorption coefficient of acoustic wedges. There are actually two concepts regarding absorption coefficient: the absorption coefficient of the material itself and the overall absorption coefficient of the entire anechoic chamber.

The absorption coefficient of the material itself refers to how much sound waves incident on the material are absorbed. If a material can absorb over approximately 0.99 (0.985) of sound waves above a certain frequency, we call it the cutoff frequency for the absorption coefficient of the individual wedge.

For example, if it can absorb over 0.99 of sound waves above 250Hz, we say the absorption coefficient of this wedge is for 250Hz.

However, this discussion pertains solely to the individual wedge. Recently, more attention has been given to the absorption coefficient of the entire anechoic chamber (or semi-anechoic chamber). This refers to how effectively the collective of these wedges applied across the ceiling and walls (or floor) can absorb sound. This is expressed using the term “measurable lower limit frequency,” which indicates the lowest frequency at which sound waves above approximately 0.99 are absorbed.

For instance, if sound waves above 250 Hz can be absorbed over 0.99, we say the measurable lower limit frequency of this anechoic chamber (or semi-anechoic chamber) is 250Hz.

In practical acoustic measurements, the measurable lower limit frequency determines the range of frequencies that can be effectively measured, making it a highly practical parameter.

Today, I will introduce two methods for measuring the absorption coefficient of individual wedges: the perpendicular incidence method and the reverberation chamber method.

The perpendicular incidence method calculates based on the sound pressure before and after incidence into the acoustic tube. This method closely resembles actual usage in an anechoic chamber, thus it is commonly employed.

Example: ISO 10532-2 Acoustics — Absorption coefficient and impedance measurement — Standing wave ratio method

However, occasionally measurements are also conducted using the reverberation chamber method. In a reverberation chamber, measurements involve comparing the time it takes for the sound pressure level to attenuate by 60dB between two scenarios: with and without the absorption material. The absorption coefficient is then determined using the following formula:

α: Reverberation chamber method absorption coefficient
V: Volume of the reverberation chamber
C: Speed of sound S: Area of the absorption material
T1: Reverberation time without the test material
T2: Reverberation time with the test material

According to the formula, larger reverberation chambers (larger V) yield more accurate data. Therefore, when consulting an anechoic chamber manufacturer, it may be beneficial to confirm the method used to measure the absorption coefficient data provided for the acoustic wedges.

In practice, the absorption coefficients obtained using the perpendicular incidence method and the reverberation chamber method can differ even for the same material.

Mixing data from both methods can lead to misunderstandings, so caution is advised.

This concludes the introduction to the absorption coefficient of acoustic wedges.