Tuning fork vibrations displacing water, illustration. A tuning fork is a piece of metal designed to vibrate and produce a musical note when struck. The frequency of the vibration, the number of times it vibrates every second, is measured in Hertz. Tuning forks help to illustrate the nature of sound as simply the vibration of air at a frequency that is audible to the human ear. Tuning forks are used to help tune musical instruments.