On entering a premises, one has often experienced an acoustic feeling of the place (e.g., it sounds lively, or alternatively, it sounds dead). Of course, it is tempting to quantify this impression. The corresponding quantity is known as reverberation time. In the olden days, it was measured by emitting a loud noise while pressing the chronometer and stopping it when it was no longer audible.

A professor at the school of architecture eventually discovered that the lecture theatre he was using was more or less dead according to the number of cushions that would be brought in by his students. This eventually led him to the notion of equivalent absorption area.

Definition

The absorption coefficient of a material is represented by the symbol a. It is defined as the ratio of absorbed energy over incident energy. One can see it cannot be physically greater than 1. However, there are some test reports that will give a value greater than 1 due to the standardized procedure (cf. Section 2.11).

The equivalent absorption area A of a material is defined as the product of its area S by its absorption coefficient a: