Random incidence absorption coefficient measured in reverberation room. But it might overestimate the actual absorption effects. Someone suggests to use statistical absorption coefficient calculated from surface impedance for a better result.

It depends on the specific software which you use for your simulations...

Some of these programs expect that you enter the "Sabine" absorption coefficients, coming from measurements in a reverberant room according to the ISO 354 standard. These values can be well in excess than 100%, some materials have Sabine absorption coefficients up to 140%...

Other programs instead expect the usage of normal-incidence "true" absorption coefficients, as measured inside a standing wave tube. These values are always smaller than one, and significantly smaller than the "Sabine" absorption coefficient measured for the same material.

Finally, there are programs which employ "true" random-incidence absorption coefficients, such as those measured with the sound intensity method, or employing the EN-1793-5 standard (impulsive method).

There are theoretical formulas which allow, under certain conditions, to convert among these three types of absorption coefficients. For example the formulas known as "London" and "Paris" are widely employed for converting between normal incidence and random incidence.

Or the Milington formula converts between Sabine and "true" random incidence...

My own room acoustics simulation program, named Ramsete, allows for entering either Sabine or "true random incidence" absorption coefficients, and automatically convert the Sabine coefficients to the "true" ones employing the Millington formula...

Hello, I was in charge of a diploma thesis which made some investigations in that topic. We found out, that the equivalent absorption area suggested by recommendation and standards are overestimated. We measured in situ within classrooms with children at the age of 14 to 15 to estimate absorption coefficients in different class room situations and then tried to repeat the measurements in a reverbarant room with the same results.

We measured pupils at chairs and tables and reconstructed the absorption coefficient by using the mosteller formula to calculate the surface of the persons and correct the results. In octave bands from 125 Hz to 4 kHz we obtained following values:

inclusive chair and table - 125Hz: 0.15, 250 Hz: 0.18, 500 Hz: 0.38, 1 kHz: 0.56, 2 kHz: 0.67, 4 kHz: 0.71

exclusive chair and table - 125Hz: 0.08, 250 Hz: 0.15, 500 Hz: 0.33, 1 kHz: 0.49, 2 kHz: 0.62, 4 kHz: 0.67

Please note, that absorption coefficients vary widely for additional clothing (e.g. warm jacket at winter time over the chairs) and are slightly dependent from the direction of incident.

The best is insulating wool (rock) which has absorption coefficient 0.63 at 500 Hz

Useful data, André. Would it make sense to express the absorption per child, with or without chair+table?