When I toured Ireland, I saw several 11th century churches with “whispering arches.” Apparently these were used as confessionals during time of contagious diseases. The arches were 7-8 feet across, but if one person stood on one side facing the stone and whispered, the other on the opposite side could hear the words clearly.
On the topic of political intrigue, the Capitol rotunda has something similar due to the elliptical shape of the room. If you stand at one point and talk, some guy at the other locus can hear, although people closer to the talker might not.