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.