Loud and long are important for glassware destruction. The volume of a sound is related to the amplitude of the sound wave and the extent that it displaces air. The louder the tone, the harder you're pushing air at the glass. Sustaining the note allows the vibrations to build up enough to cause the glass to fracture. Plain old luck also factors in, since the size and location of microscopic defects in glasses vary and some glasses can withstand more tonal punishment than others. In , the guys on MythBusters brought us the first recorded and confirmed proof that an unamplified voice can break glass.
Rock singer and vocal coach Jamie Vendera had the right frequency Hz and, after 20 attempts, worked up the volume decibels needed to needed to shatter a glass. This is why they make a melodic ringing sound when they are clinked. It follows that if someone sings the same tone as the ringing note, then the sound of their voice will vibrate the air molecules around the glass at the resonant frequency.
Traditionally, the note is depicted as being a high C. In reality, it can be any note that causes the glass to start vibrating. If that note is sung loudly enough, then the glass will vibrate and can suddenly shatter into pieces. During a study carried out at Columbia University in the United States, mechanical engineer Jeffrey Kysar pointed out there was no guarantee the glass would break.
He suggested that for an opera singer to be able to break a glass, it would have to have microscopic defects in the first place. These would then buckle under pressure. In fact, every glass has invisible cracks and minuscule defects on the surface.
Even wine glasses that look the same to the naked eye can differ dramatically. Their varying fracture strengths mean some can withstand a higher level of volume than others. The volume of the sound relates directly to the amount of air molecules it displaces. 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. Fracture depends on the size of the initial defects. 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.
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