Exponential *decay* occurs naturally when a quantity is decaying at a
rate which is proportional to how much is left. In nature, all *linear
resonators*, such as musical instrument strings and woodwind bores, exhibit
exponential decay in their response to a momentary excitation. As another
example, reverberant energy in a room decays exponentially after the direct
sound stops. Essentially all *undriven oscillations* decay
exponentially (provided they are linear and time-invariant). Undriven
means there is no ongoing source of driving energy. Examples of undriven
oscillations include the vibrations of a tuning fork, struck or plucked
strings, a marimba or xylophone bar, and so on. Examples of driven
oscillations include horns, woodwinds, bowed strings, and voice. Driven
oscillations must be periodic while undriven oscillations normally are not,
except in idealized cases.

Exponential *growth* occurs when a quantity is increasing at a
rate proportional to the current amount. Exponential growth is
*unstable* since nothing can grow exponentially forever without
running into some kind of limit. Note that a positive time constant
corresponds to exponential decay, while a negative time constant
corresponds to exponential growth. In signal processing, we almost
always deal exclusively with exponential decay (positive time
constants).

Exponential growth and decay are illustrated in Fig.4.8.

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