The efficiency of an acoustic material is determined by measuring its absorption coefficient, denoted α. The absorption coefficient is defined as the proportion of acoustic energy that has been absorbed in the material over the total energy.

  • If all the energy has been absorbed, the absorption coefficient is equal to 1.
  • If all the incident acoustic energy bounces back into the original medium, the absorption coefficient is equal to 0.
Exemple of an Absorption Curve as Measured in an Impedance Tube

The absorption coefficient depends on the frequency of the acoustic wave, the thickness and the physical characteristics of the material. To be effective, there is a rule of thumb that states that the material should be on the order of magnitude of a quarter wavelength thick. At 100 Hz, the wavelength is 3.4 m, which means that the minimum thickness of a material must be greater than 80 cm.

The absorption curve of any material starts at zero and then rises to a maximum located at the frequency f₀.

With C₀, Sound Velocity in the Air and d the Material Thickness

This formula is valid for a material placed in front of a rigid background or a wall. This frequency can be moved down by placing an air gap between the material and the rigid backing.

Measurement Methods

There are two methods for measuring the absorption coefficient:

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