In audio, a decay by (one time-constant) is not enough to become inaudible, unless the starting amplitude was extremely small. In architectural acoustics (which includes the design of concert halls ), a more commonly used measure of decay is `` '' (or T60), which is defined as the time to decay by dB.4.7That is, is obtained by solving the equation
Using the definition of the exponential , we find
Thus, is about seven time constants. See where is marked on Fig.4.7 compared with .