The acoustics of eavesdropping: How curved ceilings and walls redirect and amplify sound.

Where to Hear Yourself Whisper Into Your Own Ear

Where to Hear Yourself Whisper Into Your Own Ear

Arts, entertainment, and more.
June 6 2014 3:34 PM

The Acoustics of Eavesdropping

Where not to have a private conversation.

(Continued from Page 1)

But while it was great fun for the tourists, it was a poor place for the House of Representatives to hold debates. As Maine’s Lewiston Daily Sun put it in 1893:

The orator who was not cautious enough to remain in one spot during the delivery of his address found the acoustics of the hall taking strange liberties with his elocution, transforming his crescendo sentences into comical squeals, or causing his pianissimo phrases, his stage whispers, to shriek and wail as he moved to and fro from one echo point to another.

A gas explosion and fire in 1898 elsewhere in the building led to replacement of the wooden dome by a fireproof construction. Real plaster coffers were installed in place of the trompe l’oeil—changing from a smooth surface to one covered in lumps and bumps—making the effect of the focus weaker and less remarkable.


While domes are fun, a completely spherical room is even better because reflections are amplified even more.

The Mapparium in Boston is a 30-foot sphere and was built in 1935 following a suggestion by architect Chester Lindsay Churchill. It is a giant hollow globe of the world, with the seas and continents vividly drawn on stained glass. Visitors traverse a walkway cutting through the center of the Earth linking up two opposite points on the equator. Three hundred light bulbs 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 byproduct of the geometry.

The Mapparium in Boston

Courtesy of Smart Destinations/Flickr

William Hartmann, from Michigan State University, and colleagues documented the various illusions that can be heard. Usually, moving farther away from a listener makes a talker’s voice quieter, but that’s not always the case in a spherical room. Imagine, Hartmann writes, “you are on the Mapparium bridge 7 feet to the left of dead center. Your friend is exactly at the center and is talking to you. His voice seems rather quiet. Now your friend walks away from you, and his voice gets louder and louder until he is about [7 feet] to the right of dead center.”


Courtesy of Trevor Cox

The sketches at left show what is happening (to make it easier to see, this drawing uses a circle rather than a complete sphere). When the talker speaks from the center (top diagram), all the reflections are focused back, so the talker appears to be surprisingly quiet to the listener left of center. If the talker moves to the right, then the place where the reflections focus moves closer to the listener. The sound will be loudest when the talker and listener are arranged symmetrically about the center (bottom diagram). The unusually strong focus makes it possible to experience the strange sensation of whispering into your own ear. Or, as Hartmann puts it: “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.”

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Whispering walls do not focus sound as happens with elliptical ceilings and domes; the listener and speaker are too far from the focal point of the arc. Instead, the sound hugs the inside of the concrete wall and is transported with surprising loudness to the other side.

The largest real whispering wall I know of is 460 feet long—it is the concrete dam that withstands the Barossa Reservoir in South Australia. For some reason the dam was built to be a precise arc. This vast, gray slab of concrete has turned into an unlikely tourist attraction, with visitors chatting with each other from opposite ends of the dam.

Whispering arches behave in a similar way, and they also show up in the most unlikely of places. On the lower level of Grand Central Terminal in New York City, outside the famous Oyster Bar & Restaurant, sweeping tiled archways, designed by Rafael Guastavino and his son in 1913, support the ceiling. If you whisper into one side of the arch, the sound follows the curve of the tiled ceiling before coming back down the other side. For the best effect, the whisperer and listener need to get close to the stone, like naughty children standing in opposite corners of a classroom.

The Grand Terminal Oyster Bar arch in Grand Central Station in New York.

Courtesy of Nick Gray/Flickr Creative Commons

The sound effect has inspired literature and films; the author Katherine Marsh uses the whispering arches as the starting point in her children’s books The Night Tourist and The Twilight Prisoner, describing the arches as “one of the coolest places in New York.”

The delight in these places comes from hearing a voice carry an unexpected distance, and this effect is more dramatic if the sound is a quiet whisper to begin with. Mathematical analysis by Lord Rayleigh, the Nobel Prize–winning physicist and author of the 19th-century acoustic bible The Theory of Sound, suggests another reason for whispering: High frequencies, like the sibilant tones in whispers, hug the walls closer than the lower-frequency sounds of normal speech.

Excerpted from The Sound Book: The Science of the Sonic Wonders of the World by Trevor Cox. Copyright (C) 2014 by Trevor Cox. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.

The Sound Book: The Science of the Sonic Wonders of the World

Trevor Cox is professor of acoustic engineering at the University of Salford and president of the Institute of Acoustics.