Resonance, standing waves, & Eigentones

Hollow objects or rooms have amplifying qualities for certain frequencies.

In rectangular rooms the frequency amplified is that with a wavelength twice that of the distance between the parallel surfaces and is called the room's Eigentone.
When a frequency matching this Eigentone is sounded in the room a standing wave occurs where the sound wave and its reflection are in phase with each other and reinforce each other.

The popularity of singing in the shower is partly explained by this phenomena. The surfaces of the shower enclosure are typically reflective and spaced at a distance which tends to provide resonance at vocal frequencies.

Also known as “Q”, the resonance of a filter is the measure of the amount of feedback from the filter’s output to its input. Filters with high “Q” will amplify the signal at the cut-off frequency and thereby narrow the filter’s effective bandwidth.

A good example of a filter with a high Q is a wah wah pedal for the guitar: the familiar “wah” is the sound of a swept low pass filter with high Q.

Some filters permit the Q to increase to the point of feedback, at which point the filter becomes a sine oscillator.