In audio, a decay by is too small to be considered a practical “decay time.” In architectural acoustics (which includes the design of concert halls), a more commonly used measure of decay is “”, which is defined as the time to decay by dB.5.5 That 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. 5.5 compared with .