About two years ago I became interested in 'throat singing', an unusual form of singing, traditionally perfomed in Tuva, Tibet, Mongolia and several other places. Throat singing differs from conventional singing in that the singer controls the overtones in the voice rather than the fundamental, which is done by varying the shape of the mouth and position of the tongue. The voice box itself is only ever producing a single sound, a low pitch drone. The cavity of the mouth then acts as a bandpass filter, which is selectively tuned to different frequencies emanating from the voice box. In order for this to work effectively the voice box must produce a rich spectal profile, i.e. have a significant component of overtones, otherwise there won't be anything to tune into. This is achieved by droning with a very constricted throat, which makes a gruff, and therefore spectrally rich, tone.
Naturally, I was curious to try it for myself, so I tracked down some instructions and started practising. Although experienced throat singers can produce incredible harmonics using just their mouth, I have found that the effects can be enhanced enormously by practising in the right places. In particular, bathrooms, concrete stairwells, underground carparks and caves have particularly good resonance, and without much practise an amateur like myself can soon have an entire room ringing and buzzing using just their voice. It's a very exhilarating experience. Even within a particular environment, like a bathroom, different locations can have completely different resonant characteristics. For example, I've found that under the shower I get particularly good resosance when I stand directly over the drain pipe, facing down. At the right pitches this sets up a standing wave in the pipe, much like inside a flute or other woodwind instrument. Also very effective is facing into the corner of the room.
The moral of the story is that if you're ever walking down the stairs and the whole thing starts sounding like someone rang a churchbell, don't be too concerned, it's probalbly just some innocent throat singer wanna-be, like myself, trying to squeeze in a bit of practise.