Diffuse Field Absorption Coefficient Measurement (Alpha)

Definition

The absorption measurement in a reverberation chamber or in a small cabin is also called absorption measurement in diffuse fields or under diffuse incidence. It follows the ASTM C423 or ISO 354 standards.

The measurement is performed by placing the material on the floor of a reverberant chamber or test chamber. A set of loudspeakers radiates a wide band sound into the test room or cabin to generate a diffuse field into the test chamber. A diffuse field is defined as a sound field with the same acoustic pressure at all points and without preferred direction.

The absorption coefficient is measured by comparing the reverberation time of the empty test chamber compared to the reverberation time of the test chamber with materials.

Measurements

To measure the reverberation time (also called RT60), the sound sources are suddenly stopped and the sound decay time is measured. The RT60 is defined as being the time taken by the acoustic level in the room to decrease by 60 dB.

This measurement is made in third octaves frequencies band.

The theory on which the diffuse field measurement standards are based was developed by Wallace Clement Sabine, born in 1868 and considered the father of modern architectural acoustics. This theory is based on a number of simplifying assumptions that are not always respected. This is why the absorption coefficients measured can exceed 1, which a priori has no physical meaning given the definition of the absorption coefficient.

Taking into account the finite size of the sample, in a few clicks, our NOVA simulation software can predict this phenomenon.

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