A useful approximate formula giving the decay time-constant^{9.4} (in seconds) in terms of a pole radius is
The exact relation between and is obtained by sampling an exponential decay:
Thus, setting yields
Expanding the right-hand side in a Taylor series and neglecting terms higher than first order gives
which derives . Solving for then gives Eq.(8.8). From its derivation, we see that the approximation is valid for . Thus, as long as the impulse response of a pole ``rings'' for many samples, the formula should well estimate the time-constant of decay in seconds. The time-constant estimate in samples is of course . For higher-order systems, the approximate decay time is , where is the largest pole magnitude (closest to the unit circle) in the (stable) system.