By Jack Sharkey, February 26, 2014

Generally, when I listen to music I listen without a subwoofer and just my two front speakers using the Pure Direct setting on my receiver. When I sit in my preferred listening spot in this 2.0 (two speakers, no sub) configuration, the bass guitar sometimes sounds like it's coming from behind my right shoulder. What I am experiencing is a standing wave at certain frequencies that is "piling up" in the space where I imagine the sound is coming from. This is due to a sympathetic resonant frequency (that increases the SPL of the sound wave). If my room dimensions were even a few inches different, or if I moved any of the furniture in the room even a little, this standing wave would disappear (or in reality, move elsewhere). 

I talked about standing waves and how they can mess up an otherwise awesome home theater system last summer but there's another type of standing wave phenomena that occurs simply as a result of architectural design.

 

Whispering Gallery Waves 

The term Whispering Gallery Wave was coined by John William Strutt (Lord Rayleigh) in 1878 after he dispelled a common misconception that you could hear whispers from across the room in any part of the dome at St. Paul's Cathedral in London. Strutt, an English physicist and pioneer in the field of interaural audio dynamics, found that sound waves traveled along the edge of the smooth surface of the arch of the dome without dissipating as they would in air, hence arriving at a listener's ear placed near the smooth edge of the arch on the opposite side of the dome with much the same volume as when they left the speaker's mouth. 

The proper definition of a Whispering Gallery Wave:

Whispering gallery waves are specific resonances of a wave field (e.g. sound waves, electromagnetic waves) inside a given resonator (a cavity) with smooth edges. They correspond to waves circling around the cavity, supported by continuous total internal reflection off the cavity surface, that meet the resonance condition (after one round trip they return to the same point with the same phase (modulo 2pi) and hence interfere constructively with themselves, forming standing waves). These resonances depend greatly on the geometry of the resonator cavity. - Matjaž Gomilšek, University of Ljubljana

Basically what this means to those of us without advanced degrees in physics is that a sound wave emanating near a smooth, arched surface will dissipate much slower along the surface of the arch than it will in free space. Couple that with the fact that the waves circle the arch and theoretically return to the source in the same phase, thereby causing a resonant standing wave, and you can propose to your girlfriend as she stands alone against a wall in Grand Central Terminal (in front of the Oyster House) by whispering from the other side of the gallery* (depending on a number of factors, including the density and smoothness of the building material and the geometric form of the arch, as well as the frequency at the source and the resonant frequency of the diameter of the arch, but you get the idea). 

In free space, sound dissipates as a proportion to the square of the distance from the source, while along the narrow layer next to a smooth, arched surface, such as in a whispering gallery, the sound intensity dissipates proportionally to the distance from the source, not the square of it (therefore it dissipates much slower). That's why a whispering gallery wave can be heard at very near the original volume as its source, even across a distance as large as the dome at St. Paul's Cathedral. 

In this World War II era photograph, schoolboys test out the Whispering Gallery at St. Paul's Cathedral in London.

Most of us non- or marginally-scientific types aren't necessarily interested in the equations to describe a whispering gallery wave, but that doesn't mean we can't visit places where they exist and have conversations with each other while facing the walls on opposite sides of the room. In fact, the next time you're visiting somewhere and you see people talking into the wall remember: They are either a) nuts, or b) having fun with whispering gallery waves.

Here's some cool places to check out and have a whispered conversation with someone in spite of the noise of your surroundings:

Grand Central Terminal in New York, in the gallery in front of the Oyster Bar restaurant:

Courtesy NYC MTA.

Statuary Hall in the United States Capitol Building:

 

Other Whispering Galleries you can easily visit on your travels:

  • Chicago's Museum of Science and Industry
  • San Francisco City Hall
  • Salt Lake Tabernacle in Salt Lake City
  • The rotundas of the Texas and Missouri state capitols
  • Royal British Columbia Museum in Victoria, BC, Canada 

Check in your area to see if there is an example of the physics of sound waves near you. 

 

* - It actually happens all the time.

Jack Sharkey for KEF America