Lots of acoustic experiences... As an avid caver in my teens I found many chambers with harmonic qualities needing the slightest vocal push. Like gently pulling a bell rope, fine tuning and timing bring amazing results. In some places you can sing one note and while it is still ringing, introduce another a fraction off of the first - you may hear a 'pulse beat'. Bowing the rim of a Tibetan singing bowl and throat singing harmonics near it also can produce hammering pulse beats. Any nearby cat will enjoy it less than you.