Listening Vessels consists of two large parabolic reflectors set at least 50 feet apart which act as mirrors to reflect sound from one to the other. Two people sit opposite each other at approximately the focal point for each reflector, so that the sound coming from each reflector is focused at this point, allowing each visitor to clearly hear the other’s voice, even at a very low decibel in spite of the distance separating the vessels.
Source: The Exploratorium