See the articles HEARING, ATTENTION, &c.

WHISPERING PLACES depend upon this principle. If the vibrations of the tremulous body are propagated through a long tube, they will be continually reverberated from the sides of the tube into its axis and by that means prevented from spreading, till they get out of it; whereby they will be exceedingly increased, and the sound rendered much louder than it would otherwise be.

Hence it is that sound is conveyed from one side of a whispering gallery to the opposite one, without being perceived by those who stand in the middle.