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

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.

Excerpted from The Sound Book: The Science of the Sonic Wonders of the World by Trevor Cox, out now from W.W. Norton & Company.

In 1824, naval officer Edward Boid described how a curve can dramatically amplify sound, and not always for the best. He wrote, “In the Cathedral of Girgenti, in Sicily, the slightest whisper is borne with perfect distinctness from the great western door to the cornice behind the high altar—a distance of two hundred and fifty feet.” Unfortunately, the confessional was badly sited: “Secrets never intended for the public ear thus became known, to the dismay of the confessors, and the scandal of the people ... ​till at length, one listener having had his curiosity somewhat over-gratified by hearing his wife’s avowal of her own infidelity, this tell-tale peculiarity became generally known, and the confessional was removed.”


For centuries, people have known that curved surfaces amplify sounds and allow covert listening. Athanasius Kircher, who wrote extensively on echoes, gave a good explanation in the 17th century. His publications also document some fantastical devices, including giant ear trumpets built into the walls of royal chambers for eavesdropping. Probably his most famous—or infamous—device is the Katzenklavier (literally, “cat piano”). It has a normal piano keyboard in front of a line of cages, each of which has a cat trapped inside. Every time a piano key is pressed, a nail is driven into the tail of one unfortunate feline, which naturally screeches. With the right set of cats, ones that shriek at different frequencies, a sadistic musician could play a tune on the instrument.


The sound would have been excruciating, but then it was designed to shock psychiatric patients into changing their behavior, rather than being a genuine instrument for playing Monteverdi or Purcell. Fortunately, it is unlikely that it was ever built.

At this point you might be doubting the sanity and rationality of Kircher. Yet he drew diagrams that illustrated a good scientific understanding of how an elliptical ceiling can enhance communication between two people.

Simplified tracing of a plate from Athanasius Kircher’s Phonurgia Nova, 1673.

Courtesy of Trevor Cox

The lines in the diagram show the paths that sound “rays” take when going from the speaker to the listener. These ray paths can be worked out using a ruler and protractor. Alternatively, by treating the room as a weird-shaped pool table, the paths can be worked out by following the line a cue ball would take (ignoring gravity). If the cue ball is placed at the speaker’s mouth and fired toward the ceiling, it will always go to the listener. So all the sound going upward is focused at the listener, allowing even quiet whispers to be heard across a large room.

The problem with Kircher’s design is that the listener and speaker have to stand in particular places—the foci of the ceiling ellipse. The design is not very useful if one person wants to talk to an audience of listeners scattered around the room.

A few years ago I presented two science shows at the Royal Albert Hall in London to thousands of children. Though better known as a music venue, the hall is actually dedicated to the promotion of art and science, and it was built on land purchased with the profits of the Great Exhibition of 1851. Fortunately, the acoustics have been significantly improved since the hall opened 130 years ago. Indeed, the Prince of Wales struggled with his opening speech. According to the Times of London in 1871:

The address was slowly and distinctly read by his royal Highness, but the reading was somewhat marred by an echo which seemed to be suddenly awoke from the organ or picture gallery, and repeated the words with a mocking emphasis which at another time would have been amusing.

The hall’s ubiquitous curved surfaces are probably what caused the mocking echoes. From above, the floor plan appears as an ellipse, and the whole structure is topped with a large dome. The curved surfaces focus sound like Kircher’s elliptical ceiling. But how such reflections are perceived depends on the size of the room. In the vast Royal Albert Hall, the curves cause disastrous echoes. Sound appears to come from several places in the room and not just the stage. In a small room the focused sound arrives quickly; in a larger room the reflections are delayed.

The way the brain combines sounds is important, because otherwise we would rapidly become overwhelmed by the vast number of reflections that accompany us. As I type this sentence, the rattle of the keyboard is being reflected off the desk, the computer monitor, my phone, the ceiling, and so on. Yet my hearing is not overwhelmed by all these different reflections; the sound still appears to be coming straight from the keyboard as it should be.

The same thing happens in Kircher’s small room. The reflections from the elliptical ceiling arrive quite quickly, and unless the reflections are very loud, the brain does not hear them as separate from sound traveling directly between talker and listener. By contrast, the Royal Albert Hall is so vast that the focused reflections arrive much later, creating “mocking” echoes.

* * *

The renovation of the U.S. Capitol in the 19th century in Washington ruined a fine ceiling echo from a famous whispering dome. The Capitol’s dome used to be an almost perfect hemisphere centered at the head height of visitors, and although the ceiling appeared coffered with indented squares, it was actually smooth, with trompe l’oeil painting creating the illusion of structure and texture. Before 1901, this domed space was a great draw for tourists. According to the New York Times in 1894:

The whispering gallery still holds the palm among the show places of the great marble structure. Once in a while an old resident of Washington is initiated into the mysteries of the echoes and other acoustic phenomena which abound in this old-time chamber, and he feels a little ashamed of his tardiness in seeking this remarkable entertainment.
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