Tweakers' Asylum

Posted by Al Sekela (A) on April 23, 2007 at 15:37:32

In Reply to: Re: It depends on the details of your AC wiring. posted by michaelv20850 on April 22, 2007 at 20:01:02:

An organ pipe takes white noise at its input and creates a tone, with a specific pitch and harmonics that give it character. The fundamental tone is related to the length of the pipe. The harmonics are integer multiples of the fundamental. Changing the length of the pipe changes the fundamental pitch, and all the harmonics.

The noise at the input is soft. You can hear it in live performances if you are close to the pipes. However, the sound level of the tone can be quite loud. The pipe takes a soft input and makes it loud through the physical phenomenon of resonance. The sound inside the pipe is reflected from the ends and builds up to a high level.

The same thing happens in electrical cables where the terminations cause reflection. In radio engineering, the concept is called "impedance mismatch." The cable has an impedance to a traveling wave, determined by the geometry and properties of the cable conductors. If the wave sees a different impedance at its end, part of the energy in the wave is reflected. Reflections between the ends allow standing waves to build up, just as in an organ pipe. Because the speed of propagation on the cable is so much faster than the speed of sound in a pipe, the standing waves will have frequencies in the VHF and UHF bands.

In house wiring, the wires go from the circuit breaker panel to a junction box, then from there to another junction box, and so on. Each segment between junction boxes acts like a resonant cable, because the geometries of the fixtures to which the wires are attached cause impedance mismatch. Any given outlet may have a lot of resonant energy, or a little, present. This depends on the particular lengths of the segments.

This is why it is difficult to predict which outlets will be useful for damping. We just have to try them and listen for improvement.