This humming sound is so delicate that it cannot stand up to the slightest noise intrusion. Even the listener must remain absolutely quiet—no talking, no foot or arm movements, and quiet, slow breaths with open mouth. I believe an open mouth improves hearing for two reasons: it straightens the auditory canal and it enables the mouth to serve as a resonant chamber, amplifying faint sounds to more audible levels. This is exactly what children do instinctively when the lights go out: the jaw drops and faint sounds are more easily heard.

Gordon Hempton, John Grossmann, One Square Inch of Silence-…, loc. 191