MA – Sound Tourism


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The Mapparium is a giant hollow globe of the world, with the seas and continents vividly drawn on stained glass. It was built in 1935 following a suggestion by architect Chester Lindsay Churchill. It took eight
months to paint and bake all 608 glass panels, which are mounted on a spherical bronze frame.

You traverse a walkway cutting through the center of the Earth linking up two opposite points on the equator. Three hundred lightbulbs illuminate the globe from the outside. Looking at the world from the inside out is an odd experience, but what also strikes visitors are the strange acoustics, which were an accidental by-product of the geometry.

One of the effects you’ll hear is false localisation:

“Suppose you are on the Mapparium bridge facing South America. There is a source of noise to your right, but you discover that you hear the noise coming from your left!”
William Hartmann, Michigan State University

This happens because the reflections from the globe are focussed and very loud. Your brain thinks the sound is coming from the reflection direction and not direct from the source. This focussing also allows you to do things like whisper in your own ear!

As you approach the exact center of the Mapparium sphere you suddenly become aware of strong reflections of your own voice . . . If you sway to the left, you hear yourself in your right ear. If you sway to the right, you hear yourself in your left ear.
William Hartmann, Michigan State University

Location

Marky Baker Eddy Library in Washington. They have regular tours. At the end of my tour, they allowed us to linger a little to play with the acoustics.

Credits

Photo: Smart Destinations


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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]

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).