Considerations Concerning RT Measurement

In this article, our acoustics expert explains the history behind the reverberation time (RT) measurement and its importance. He then shares his preferred procedure for measuring RT.

- Richard Honeycutt

When an acoustician-or a musician-visits an unfamiliar room, (s)he often claps once or a few times to get a sense of the acoustics of the room.

An experienced listener can tell several things about the room from the sound of the clap including:

• The approximate volume of the room (large, medium, or small, but not actual cubic feet or cubic meters)

• The presence or absence of slap or flutter echoes caused by unwanted reflections

• The "liveliness" or "deadness" of the room, even (for a truly astute listener) the approximate reverberation time (RT)

Although we know that the effects of echoes and reverberation have been known since antiquity, we have no way of knowing at just what point in history people began using hand claps to assess their predominance in rooms.

We do know that Professor Wallace Clement Sabine developed the measurement and analysis of RT into a science during the closing years of the 19th century. His study of reverberation culminated in the "Sabine equation": 

where S₁ = the surface area of a particular section of the surfaces in the room, and a₁ = the acoustical absorption coefficient of that particular section.

As can be seen from this equation, Sabine's theory states that the RT of any room can be predicted from a knowledge of the room's volume and total acoustical absorption. In order for the prediction to be accurate, two conditions must be met:

(1) The sound in the room must be diffuse, meaning that at any point in the room there is equal probability of reverberant sound arriving from any direction.

(2) The acoustical absorption must be evenly distributed among all surfaces: wall, floor, and ceiling.

Given these conditions, the RT is the same at any point in the room.

RT Variation within a Room