Page "Linear filter" Paragraph 16

For instance, consider a damped harmonic oscillator such as a pendulum, or a resonant L-C tank circuit.

If the pendulum has been at rest and we were to strike it with a hammer ( the " impulse "), setting it in motion, it would swing back and forth (" resonate "), say, with an amplitude of 10 cm.

But after 10 minutes, say, it would still be swinging but the amplitude would have decreased to 5 cm, half of its original amplitude.

After another 10 minutes its amplitude would be only 2. 5 cm, then 1. 25 cm, etc.

However it would never come to a complete rest, and we therefore call that response to the impulse ( striking it with a hammer ) " infinite " in duration.