Echo Bridge, Massachusetts
Built in the 1870s, this forty-metre wide arch spans the Charles River. There are steps down to a specially built platform so visitors can test out 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.
Location and logistics
A Walking Tour of Hemlock Gorge (including a visit to the echo platform). The bridge spans the Charles River between Needham and Newton.
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