decay i t

   i - input signal
   t - decay time

Exponential decay.  This is essentially the same as Integrator
except that instead of supplying the coefficient directly, it is
caculated from a 60 dB decay time. This is the time required for
the integrator to lose 99.9 % of its value or -60dB. This is useful
for exponential decaying envelopes triggered by impulses.

Used as an envelope.

> do { n <- brown_noise_m
>    ; f <- lf_saw 0.1 0.0
>    ; i <- impulse (lin_lin f (-1.0) 1.0 2.0 5.0) 0.25
>    ; e <- decay i 0.2
>    ; audition [] (out1 (e * n)) }