Whisper into one side of this ornate doorway and the architrave carries the sound to the other side.
This ornate Gothic doorway dating to the fifteenth century is apparently a popular spot for wedding proposals. Folklore has it that the doorway once had a very unusual use: Lepers would stand at one side of the doorway and whisper their sins into the half pipe in the architrave. The priest would stand at the other side of the arch, far enough away to avoid infection, listening to the confession emerging from the architrave.
Like all whispering arches, sound skims around the inside of the curve because of the geometry – see below for a simple model of sound as a snooker ball bouncing around a circular table. Being a half-pipe probably helps keep the sound close the arch.
This sound effect was discovered when “a workman dropped a hammer on one side of the arch and a painter on the other side, nearly 40 feet away heard him.”
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The whispering arch is a treat, talk towards the wall and someone 20 yards away can plainly hear you. pretty cool.
Aquariuz, Trip Advisor
The whispering arch is the structure to the left in the old postcard, there’s a plaque marking the spot. This whispering arch dates back to when the building was a large railway station in the nineteenth century. When someone talks into the wall, you can get sound that stays close to the edge of the arched ceiling. A classic way of illustrating this is to consider sound as a snooker ball bouncing around the edge of a circular table.
In the lobby of the St. Louis Union Station Hotel. Head for the stained Tiffany glass window featuring three robed women figures.
Bridge arches can have great echoes, this one is meant to repeat a human voice up to 15 times
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Built in the 1870s, this large arched bridge spans the Charles River. There are steps down to a specially built listening platform so you can hear the sound effect. In September 1948, Arthur Taber Jones wrote to the Journal of the Acoustical Society of America, detailing a small study. ‘A handlcapp [sic] is returned in a series of about a dozen echoes of decreasing loudness, and at a rate of about four echoes per second.’ 
The question Jones posed in his article was whether the sound was skimming around the inside of the curved arch, like a whispering gallery or propagating horizontally just above the water. The video below, created with a modern acoustic prediction model shows how sound moves under the bridge. No wonder Jones struggled to work out what was going on, because it seems that the sound both skims around the arch and bounces back and forth horizontally just above the water.
While Echo Bridge is unique in having been a subject of scientific study, I’ve found that other arched railway and canal bridges have the same sound effect once you know the general shape to look out for.