Why is reverberation time significantly longer in a large empty classroom than in a fully furnished library of the same size?
The classroom has fewer sound-absorbing surfaces compared to the library
Reverberation is the persistence of sound in a closed space due to repeated reflections from walls, ceiling and floor. Reverberation time is the time taken for the sound to die away (by Sabine's definition, to fall by 60 dB) after the source stops.
Sabine's formula, \(T = \frac{0.161V}{A}\), shows that for rooms of the same volume V, the reverberation time T is inversely proportional to the total sound absorption A. A furnished library has books, shelves, curtains, upholstered furniture and other soft or porous surfaces that absorb sound, while an empty classroom has mostly hard, bare walls and floor that reflect it. So sound keeps reflecting for longer in the empty classroom.
The type of sound source does not change how long the room keeps reflecting sound. A small difference in air temperature changes the speed of sound slightly but not the room's absorption. The colour of the walls affects light, not sound absorption.
Hence, The classroom has fewer sound-absorbing surfaces compared to the library.
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