Q equals the oscillation periods needed to reduce amplitude by a factor of 25.

Evidence

The last statement agrees with the first statement that the vibration amplitude decays to e–π = 4% of its initial value in t/T = Q periods—that is Q is the number of periods required to reduce the amplitude by a factor of 25, or down to 4% of its original value. We can write A α e–πt/(TQ) = e–t/τ, which defines the decay time τ = TQ/π. — Eric J. Heller, Why You Hear What You Hear- An…, 960

Categorization

• DEFINE: Establishes the decay-time constant tau in terms of Q and the oscillation period.