Resonance
When audio 'howl-around' occurs, what starts off as a general low ambient noise-level,
quickly escalates into the familiar ear-piercing howl. The reason will be familiar to
anyone who sings in their bath! In a word - resonance. All rooms resonate at their own
particular frequency and it is this frequency that is picked up, emphasized and fed
round the howl-around loop. Very quickly, it is only that sound that is heard.
A similar process occurs inside radio frequency oscillators but, instead of the room determining the resonant frequency, certain electronic components are used - in particular, capacitors and inductors (coils).
The frequency at which circuits resonate depends on the values of the inductor and capacitor. By choosing specific values, any resonant frequency may be selected.