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 [
4]), 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

.
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