# Other frequencies and breaking glass

I know an individual can use the natural frequency of glass to break it, but is it possible to break glass with a frequency other than the natural frequency? Will the glass still vibrate if the frequency around it is not its natural frequency?

• You should be able to do it with one of the higher eigenfrequencies, it will take more power, though. May 30, 2015 at 4:47

## 2 Answers

Reductio ad absurdum - place the glass in a beam of sound of sufficient intensity and it will shatter if it does not melt first. An ultrasonic siren is a good place to start.

An object like a glass has multiple harmonics. Exciting any of these will cause resonance. During resonance you can build up the amplitude of response more than when you use an off-resonance frequency.

It is actually quite hard to real a glass by exciting its fundamental frequency - and even harder to do so with harmonics. The problem with the higher harmonics is that the side from which the sound arrives will have multiple nodes/antinodes, so your pressure wave will have a hard time exciting a pure wave (it would be pushing in the right direction in one place and the wrong direction in another- because the phase requires from the exciting wave would change with position).

The following diagram might illustrate what I mean:

On the left is a glass resonating at the fundamental frequency: on the right, it is oscillating at a higher harmonic. You can see the green state (antinode towards the sound source) and its opposite, the blue state (antinode away from the sound). But you can see that a short distance along the wave front, the phase needed to excite the resonance is "wrong" - so it will be hard to excite these higher resonances with sound. You can, however, excite them with a more local force (e.g. by gently tapping the glass)

• Why do you say that it is harder with higher pitches? Deconstructive wave interference can occur at any frequency. What do you mean in the second paragraph? The waves coming back off of the glass? Jun 2, 2015 at 1:07