The orchestra crescendos as a woman of ample proportions strides to the front of the stage, blonde braids trailing from under a horned helmet. Her gilded bosom heaves as she inhales, opens her lipsticked maw and lets loose an earthshaking high note. Champagne flutes shatter, monocles crack and the chandelier explodes as the power of her voice wreaks havoc on the concert hall. The scene is in countless cartoons and comedies, but is this parody based on reality? Can an opera singer really shatter glass?

Physics suggests that a voice should be able to break glass. Every piece of glass has a natural resonant frequency—the speed at which it will vibrate if bumped or otherwise disturbed by some stimulus, such as a sound wave—as does every other material on Earth. Glass wine goblets are especially resonant because of their hollow tubular shape, which is why they make a pleasant ringing sound when clinked. If a person sings the same tone as that ringing note—a high C in legend but in reality the matching pitch could be any note—the sound of her voice will vibrate the air molecules around the glass at its resonant frequency, causing the glass to start vibrating as well. And if she sings loudly enough, the glass will vibrate itself to smithereens.

"It's possible, but you have to be both good and lucky," says Jeffrey Kysar, a mechanical engineer at Columbia University who studies the different ways in which materials can fracture and fail. "Even if you could excite the cup, that doesn't guarantee it would break. Fracture depends on the size of the initial defects." So in order for a diva to successfully demolish a wine glass, she would have to fortuitously choose one with microscopic defects that are big enough to buckle under pressure.

Invisible cracks and chinks cover every material's surface but their size and location can vary wildly, according to Kysar. Wine glasses that look identical to the naked eye could have radically different fracture strengths, enabling some to withstand much higher levels of volume than others.

Volume is a key player in the glass shattering game, because the loudness of a sound is directly related to the extent it displaces air molecules. In essence, the sound passes from molecule to molecule until it hits the glass. As Brunhilde sings louder, she is, in effect, pushing air at the glass harder. The effect is much like pushing a kid on a swing—the harder each shove, the sooner the kid will go over the top. But a strong shove has little effect unless it is timed so it matches the natural oscillation of the swing—just as a hopeful glass breaker must sing a note that matches the glass's resonant frequency.

The physics involved in the art of vocal destruction seem straightforward enough. But although stories of powerful singers shattering wine goblets, vases and eyeglasses abound, real instances of this feat are suspiciously missing from the historical record. The famous tenor Enrico Caruso was said to have had the ability, but after he died his wife denied these rumors. What gives?

It turns out that most pieces of glass, including most wine glasses, are the equivalent of a kid on a swing who weighs hundreds of pounds. Push away, but that baby probably won't get anywhere close to the top.

Only the finest leaded crystal is dainty and resonant enough to break at volumes that some people can produce without amplification—upward of 100 decibels. A famous commercial from the 1970s showed Ella Fitzgerald shattering a wine glass with ease through Memorex speakers, and the trick has been repeated many times with amplification. The principle of directing sound at a brittle object is used, for example, to break up kidney stones—except doctors don't bother to find the resonant frequency, preferring just to blast the stone with lots of sound energy (and if a singer were as loud as, say, an explosion, she wouldn't have to find the resonant frequency to break a glass, either). Yet, it seems that until a couple of years ago there was no proof that any person had ever broken glass with his or her voice alone.

4 Comments

1. 1. tomlee59 08:55 PM 10/14/08

   Minor nit: Memorex does not make speakers. The Ella Fitzgerald advert showed her breaking a glass while singing live, and then breaking another glass when a recording of her voice on Memorex *tape* is played back. This was the famous "Is it live, or is it Memorex?" ad. So, assuming that no trickery was used in the first part of the ad, then the Mythbusters episode does not constitute the first time such a breakage was filmed.

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2. 2. bobpritchard 10:07 PM 4/23/11

   Back around 1978 I was a member of the UBC University Singers, an elite choir many of whom went on to have successful careers in choral and opera performance. For one of our rehearsals our conductor, Prof. James Fankhauser, placed curved reflectors behind us to help refine tuning. About midway through the rehearsal we crescendoed to a cadential chord on which we had been doing some very precise tuning work. At the exact moment that Prof. Fankhauser cut us off there was a loud "ping" from the ceiling and a jagged 2" thick section of one of the heavy glass lenses covering the hall lights dropped, shattering on the edge of the reflectors.
   
   I was always intrigued by this: if we hadn't had the reflectors in place one of our basses would have suffered a severe head injury, given the size of the lens fragment. However, if we hadn't had the reflectors in place we probably wouldn't have had the tuning, resonance and amplitude to cause the fracture (if, indeed, it was caused by our voices). The distance from the stage to the ceiling is around fifty feet, but one of the singers was Ben Heppner, who went on to top international status as an opera singer, and this was a full choir, not a solo voice, so there was a lot of sound coming off the stage. As well, Professor Fankhauser often insisted on using just intonation in tuning chords, which could contribute to reinforcing a resonant frequency.
   
   While I'm aware that the heating of the lens and manufacturing defects will likely have contributed to the event, I have been associated with UBC Music since the 1970's and have never heard of a similar occurrence for any of the 50 odd lights in the Recital Hall.
   
   Prof. Bob Pritchard

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3. 3. Operadical 04:26 PM 5/23/11

   Can the high C of a trained soprano quiver glass into dissolution? I don't know about the high C, but the high Ebs in the new song OpeRADical sung by Markella Hatziano probably could.

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4. 4. JAllan 04:29 PM 2/21/13

   The Memorex ad showed Ms. Fitzgerald breaking the glass live WITH amplification, and the recording (presumably at the same volume setting) breaking the glass also.
   
   The Mythbusters replayed that classic ad on their show, and noted that amplification was used; the microphone in front of the singer and the speaker in front of the glass were both visible. As they said on the show, they believed they were the first to do so with the glass directly in front of the singer's mouth WITHOUT amplification.
   
   It's just too bad, in my personal opinion, that they used a punk rock singer instead of an opera singer; maybe Pavarotti refused or wanted a fee beyond their budget.

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