Tuning forks, used to tune musical instruments have two long arms connected to a stem; the vibration of the arms (or "tines") produces a particular pitch defined by the frequency of the sound it produces. In John Shore's original tuning fork, invented in 1711, the note A vibrated at 423.5 Hz, but modern A tuning forks vibrate at 440 Hz.
The most common way to make a tuning fork of a certain frequency resonate is to hold the stem and strike the tines against a hard object. Any hard object is suitable for hitting a tuning fork; you can use a rubber hammer, your knee or any other surface that won't damage -- or be damaged by -- the fork. This percussive strike starts the two main tines vibrating, which then produces the note that the tuning fork is designed to create. Good tuning forks can resonate for up to two minutes.
The basic physics behind tuning forks explains why the vibration of a fork produces a certain pitch. When you strike the tuning fork, the vibration of the tines disturbs the air molecules around it, squashing them closer together on its outward journey. As the two tines bend inwards, the surrounding air molecules are expanded. The frequency of these expansions and compressions determines the frequency of the sound wave produced and therefore determines the note produced by the tuning fork. The patches of this compressed air vibrate your ear drum, and your brain translates this to a sound. The pitch is dependent on the size of the tuning fork.
Tuning forks are made from a complex alloy (as opposed to simple steel) to ensure that they resonate as well as possible. Different companies use different materials to make tuning forks; for example, a mixture of blue steel and chrome. The size of the tuning fork determines the note, so buy whichever tuning fork is most suitable for your instrument; A is the most common tuning fork, but an E tuning fork can be useful for guitar players as it vibrates at the same pitch as the thickest string on the guitar.
If you produce a sound at the same frequency of a tuning fork in close proximity to the fork, you can make it vibrate remotely. This reaction operates under the same principles responsible for the old joke about an opera singer smashing a glass with her voice. One loud, sustained note at the correct pitch will make a glass -- or tuning fork -- vibrate at its resonant frequency. The compressed air molecules vibrate the fork in the same way as they do your eardrums. Save your voice, however, as it takes a powerful set of vocal chords to vibrate a tuning fork, and you're likely to lose your voice before smashing a crystal glass.
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