Breaking a glass using your voice is not as easy in reality as on film. It takes a strong singer, a suitable glass and the right circumstances.

Text: Nicklas Hägen


THE SCENE is classic: a singer holds a high tone for a long time and somewhere in the room a glass breaks, often a crystal wine glass or in a cut-glass chandelier. But is this actually possible to do outside the world of film, and in that case, how does it happen? We asked Torbjörn Björkman, lecturer in physics at Åbo Akademi University to tell us what is required.

“Yes, it is possible to break a glass using sound. What it takes is to hit the resonance frequency of the glass. But I dare not promise that it’s possible to do it the way it is done in films,” says Björkman.

Each material has its natural frequency, a frequency at which the material tends to oscillate. For a glass, it is the tone the glass makes when you tap it with your finger, for example. The mechanical tap causes oscillations that make a sound at a certain frequenc­y.

The effect is the same when you touch a guitar string: a basic frequency arises as well as several overtones, which are all natural frequencies.

“If you sing the same tone as the string makes, the string starts to oscillate, resonance is created. You can drive the oscillation of the string with your voice. The link between them is weak, but sufficient. What happens is that your voice makes the air oscillate and the air particles reach the string in waves, which in turn make it also oscillate,” Björkman explains.

“Cars, too, have their natural frequencies, which make them vibrate when they reach a certain rotational speed. This mainly pertains to older cars. In modern cars, this has been prevented, so if it occurs, there is a fault in the construction.”

Brittleness of glass crucial

What is needed to break a glass using your voice is a strong frequency, which matches the natural frequency of the glass, and which continues for long enough with sufficient intensity.

“The sound wave pumps in energy, seen in the form of vibrations in the glass. If you can sustain the tone for long enough, the oscillations become larger and larger. Glass is a brittle material and will eventually break. Other materials, such as plastic, also vibrate, but since it is flexible rather than brittle, it will not break.”

However, what makes Björkman hesitant as to whether it is actually possible to break a glass with one’s voice as on film, are the circumstances required. It’s partly a case of the pitch being right and the glass being of the right thickness; but also the effect of the sound waves weakens as they travel through the room.

“The singer must have a fairly strong voice. But more crucial is how fragile the glass is and what inherent tensions there are. These make it brittle. You must be lucky with your glass,” says Björkman.

“In films, it is usually high tones that break glasses. But those are more likely to break prisms in cut-glass chandeliers, which have a very high natural frequency since their circumference is smaller than that of drinking glasses. I have no trustworthy statistics on how many glasses are broken by singing, but it should be possible, even if it is not as common an occurrence as in films.”


Breaking glass

Some examples from popular culture of breaking glass by using voice:
•  Victor/Victoria (1982): Victoria Grant, played by Julie Andrews, breaks a wine glass by singing.
•  Who Framed Roger Rabbit (1988): The animation rabbit is given so strong a drink that it makes him sound like a steam whistle and the sound breaks all glasses in the room.
•  Shrek the Third (2007): Prince Charming breaks glasses at the opera with his squeaky voice.
•  Last Action Hero (1993): Frank McRae’s character, Lieutenant Dekker, tells Jack Slater (Arnold Schwarzenegger) off so that the glass pane of the door breaks.
•  Batman (1966, ”The Bat’s Kow Tow”, sea­son 2, episode 30): In Adam West’s unforgettable portrayal of Batman, he and Robin (Burt Ward) flee from Catwoman’s glass echo chamber by finding the right tone to break the glass. The tone? F sharp above high C, which, if the editor’s interpretation is correct, should be at 740 Hz for men.