How a Singer’s Voice Can Shatter Wine Glasses
When struck with a spoon, a wine glass will produce a high-pitched tone, which is its natural resonance. Every object possesses a natural resonance that causes it to vibrate. When the fork causes the glass to vibrate, the natural resonance sings out. Theoretically, a human voice could shatter glass by matching her voice with the high-pitched natural resonance tone of the glass, if the vibrations produced are powerful enough to break it.
Physical Structure of Vocal Cords
The sound of music begins with the vocal cords, or vocal folds, which are located within the larynx, also known as the voice box. See Fig. 1 below. Vocal cords/folds are two skin-covered flaps of muscle that open and close the passage to the lungs and oscillate with a wave motion as air rushes out during exhalation. As they oscillate or vibrate, it produces sound. The faster the vibrations, the higher the pitch of the sound it creates.
The human voice can be modified in many ways and can produce a spectrum of sounds – whispering, speaking, orating, shouting – as well as the different sounds that are possible in a variety of forms of vocal music, such as rock singing, gospel singing, and opera singing.
The Physics Behind Vocal Cords - The Bernoulli Effect
The Bernoulli effect, officially known as Bernoulli’s Principle, was founded by Daniel Bernoulli, a Swiss mathematician who stated in his published book, Hydrodynamica in 1738: “As the velocity of a fluid increases, the pressure exerted by that fluid decreases”. When air travels through a tube, the speed of that air increases, creating a lower pressure in the tube. A vacuum (an area of no pressure) is created, where anything in its surrounding will be sucked into the vacuum. In our vocal system, the tube is known as our trachea (our windpipe), where the vocal cords are sucked inwards. In other words, when we are ready to make a sound, our body responds by inhaling. As we exhale, air travels up from the lungs through our windpipe. Inside the throat, the air encounters the vocal cords, which can close or open to allow for breathing. As the air reaches the cords, the stream of air creates pressure. As this pressure builds up, the vocal cords are blown apart, causing the air traveling through to speed up. This acceleration of air in turn causes the pressure to drop at the glottis. Because of the low air pressure, the vocal cords are sucked together and closed again. The cords stay together only until the pressure builds up again to blow the vocal cords back apart. This cycle continues. The ongoing opening and closing of the vocal cords, which occur hundreds of times per second, is what is called vibration. The vibration of our vocal cords is what chops up the air into sound waves. Fig. 2 to the right visually shows how sound is produced.
The Amplitude of Sound and Resonant Frequencies
Sound that is produced at the right frequency and at a proper amplitude can be glass shattering. A loud voice is required to produce enough energy to break the glass. According to Jaime Vendera, the singer needs to produce a high intensity sound of at least 105 decibels, compared to approximately 50 decibels, in normal conversation. If one is exposed to 120 decibels or more, the effects can be deafening. Constant exposure to anything above 85 decibels can result in hearing loss. A highly trained opera singer can sustain note of up to 100 decibels. But this alone is not enough to break glass. The laws of physics show that every material on Earth has a natural resonant frequency. A resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object . This frequency is the speed at which it will vibrate if struck. A child’s playground swing is an example of a pendulum, a resonant system with only one resonant frequency, its natural resonant frequency. With a tiny push on the swing each time it comes back, one can continue to build up the amplitude of the swing. But the frequency does not change. Trying to increase the frequency of the swing will just shake the swing but will eventually return to its natural frequency. Wine glasses are especially resonant because of their hollow, tabular shape . If a wine glass is flicked with finger, a ringing sound can be heard as the glass vibrates. The flicking causes waves of air pressure to radiate from the glass, to which the human ears and brain interpret as sound. The sound gradually becomes quieter and dies out as the amplitude of the vibrations decreases as energy is being carried away by the sound waves. The resonant frequency of a glass could be any particular musical note. The resonant frequency of a piece of glass depends on the glass shape, size, material used and how it was made.
The human voice is a series of air pressure waves, with the pitch related to the frequency of the waves, and the volume related to the amplitude of the waves. If a singer sings the same musical note that matches the resonant frequency of the glass, the sound waves will vibrate the air particles around the glass at its resonant frequency and this will make the glass begin to vibrate too. Imagine pushing the swing after one big push, the swing slows down, but continues oscillating for a while with a given frequency. Randomly timed pushes are unlikely to get the swing moving very much but if the efforts are carefully timed and administered regularly in each cycle, the efforts can add up and the amplitude (height) will increase a bit each time. If the singer sings loudly enough, the glass will try to move in its vibration farther and faster than the material in the glass is able to move, and the glass will break under the strain. Though given the right frequency, the glass will keep storing the vibrational energy and it is also subject to various damping mechanisms through which energy is dissipated. Therefore, the amplitude of sound has to be adequate to cause the net vibrations in the glass to increase and break it. Fig. 3 below shows the vibration of a wine glass.
In summary, for a wine glass to shatter, a singer needs to sing the same musical note that matches the resonant frequency of the glass, setting it into vibration. In addition, the volume of sound has to be high to make the vibration hard enough that the glass shatters under the strain. Both the amplitude of sound and the resonant frequency have to work together to break a wine glass.
2. “The Bernoulli Effect: the Physics Behind Your Vocal Folds”, Vibrant Voice Technique, August 21, 2015
3. Glottis is the opening between the vocal cords; it opens during breathing and closes during swallowing and sound production
4. F. Dharawat, “How Do Opera Singers Break Glass Just by Singing?” Embibe, September 28, 2015
5. Marriam-Webster Dictionary
K. Billah and R. Scanlan, "Resonance, Tacoma Narrows Bridge Failure and Undergraduate Physics Textbooks," Am. J. Phys. 59, 118 (1991).
A. P. French, "In Vino Veritas: A Study of Wineglass Acoustics," Am. J. Phys. 51, 688 (1983).
K. Schrock, “Fact or Fiction?: An Opera Singer’s Piercing Voice Can Shatter Glass”, Scientific American, August 23, 2007.
A. Scodary, “Shattering a Wine Glass with Sound”. December 9, 2007