When a sound source operates in a closed space, the modes and times according to which the generated sound decays, after interruption of the source, also depend, in addition to room's form and dimension, on the elements in it.
The parameter that defines the behaviour of the objects present in the room in relation to such sound decay is the sound absorption coefficient.
This is defined by the reverberating room equivalent absorption area change after insertion of the subject-matter material.
To calculate a material's sound absorption coefficient, the room's reverberation time change is measured after introduction of the subject-matter element in the form of a panel, according to the formula:
S = tested element surface
V = empty reverberating room volume
c = sound speed
Te = empty room reverberation time
Ts = reverberation time of the room with the element being tested
To perform a measurement according to norm ISO 354 a series of conditions are necessary concerning sound source type, microphones type and position, and room dimensions.
The most important are listed here:
The source spectrum must be flat with a maximum difference of 6dB between bands.
The measurement procedures prescribes a series of reverberation time measurements in a certain number of positions (minimum 3), with and without the element being tested.
The minimum distances between microphones and sound sources must be observed:
1 m between each microphone and tested element.
1 m between each microphone and room walls.
2 m between each microphone and sound source.
The measurement must be carried out using 1/3 octave band filters with central frequency
100 - 125 - 160 - 200 - 250 - 315 - 400 - 500 - 630 - 800 - 1000 - 1250 - 1600 - 2000 - 2500 - 3150 (Hz)
The closest to T20 reverberation time must be used.
The minimum number of decays to consider in the average reverberation time calculation are
12 decays for 100Hz - 250Hz frequencies.
9 decays for 315Hz - 800Hz frequencies.
6 decays for 1000Hz - 5000Hz frequencies.
For a complete knowledge of the measurement requirements and procedure, please see the norm ISO 354.