Echo Bridge

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.’ [1]

By Daderot – Own work, CC0

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.

Location

A Walking Tour of Hemlock Gorge (including a visit to the echo platform). The bridge spans the Charles River between Needham and Newton.

Sources

Thanks to Jonathan Sheaffer for modelling the sound propagation.
[1] A. T. Jones, “The echoes at Echo Bridge,” J. Acoust. Soc. Am. 20, 706-707 (1948).

Greenwich Foot Tunnel

Startling distortions of footsteps and voices in this incredibly atmospheric foot tunnel.

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Is it just me, or is it virtually impossible to resist the urge to shout and whoop when you’re in a tunnel? I’ve noticed that some tunnels work better than others, and this one under the Thames near Greenwich, London is remarkably effective. A study by artist Peter Cusack found this soundscape to be one of London’s favourite sounds [1]. The sound qualities mentioned in the study included the amazingly long reverberance and echoes as well as the acoustic distortions to familiar sounds such as footsteps and voices.

The hard tiled surfaces allow the sound to rattle back and forth in the tunnel for a long time before dying away. What’s more, sound takes ages to go up and down the length of the tunnel, which leads to amazingly long echoes.

Logistics

The south end of the tunnel is at Cutty Sark Gardens, Greenwich, London. The north end is at Island Gardens. It’s open 24 hours a day, but has a hundred steps at either end.

Credits

  1. http://www.favouritelondonsounds.org/search/list.php
  2. Sound Matthias Kispert (c) some rights reserved