If you've spent even just a few Saturday mornings watching television cartoons, then you're probably familiar with the image of an opera singer whose loud, high notes causes glass to shatter. Is such a thing really possible? In physics, this phenomenon would be considered an extreme case of resonance or sympatheic vibration.
The tuning fork is a useful instrument for investigating sound vibrations because it vibrates at only one frequency, in contrast to most musical instruments that produce several different frequencies simultaneously. If the stem of a tuning fork is set on a table top, the tone becomes louder because the fork forces the table top to vibrate. Because the table top has a larger vibrating area, the sound is more intense. This principle of forced vibrations is applied in most musical instruments by using a part of the instrument, such as the piano sounding board, to intensify the sound.
Now imagine two matched tuning forks with the same frequency mounted on sounding boxes, side by side. The vibrating column of air set up by one tuning fork will cause the other tuning fork to vibrate weakly. This action is resonance, or sympathetic vibration, at work. Resonance occurs when the natural vibration rates of two objects are the same or when one has a natural vibration rate that is a multiple of the other.
So how does the opera singer force the glass to crack with just the vibration of her voice? First, the singer must be able to sustain a note perfectly in tune and project it loudly (doing so is much more difficult that it sounds). The reason this works is because every glass flexes in sympathy with every note sounded near it. If the resonant frequency of the singer's voice is loud and steady enough, it could cause the glass to break once the vibrations of the glass build up past its elasticity limit, causing it to fail.