[sdiy] Wanted: non mathematical description of the function of RC-filters

[Byron G. Jacquot]

A couple of intuitive models from the physical world that might be worthwhile to explore:

The sieve:  Consider that you have a pile of mixed gravel, with particles from maybe 2mm up to 50mm.  With a screen, you can separate particles of a size the fit through the holes in the screen.  Depending on whether you take the particles that pass through the screen, or the ones that won't pass, you've effectively got a "large pass" or a "small pass" filter.  With a pair of screens of slightly different hole diameter (say 9mm and 11mm holes), you could achieve a size filtering that either passed only ~10mm particles, or everything except ~10mm particles (roughly equivalent to bandpass or notch filtering).

The shocks and springs in a car:  The mass of the car pushes the tires to the ground through the flexible medium of the spring.  When terrain varies gradually (say, climbing and descending a hill), the spring translates the tire position to the car body more or less literally.  But when the terrain varies quite rapidly (such as a heavily rutted "washboard" dirt road), the tire will track the surface details, but spring absorbs the motion, and the mass of the car body doesn't track those small movements.  It's a mechanical lowpass filter.

Do either of these help?

-Byron Jacquot