A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs tines formed from a U-shaped bar of elastic metal usually steel. It resonates at a specific constant pitch when set vibrating by striking it against a surface or with an object, and emits a pure musical tone once the high overtones fade out.
A tuning fork's pitch depends on the length and mass of the two prongs. They are traditional sources of standard pitch for tuning musical instruments. The tuning fork was invented in by British musician John Shore , sergeant trumpeter and lutenist to the royal court. A tuning fork is a fork-shaped acoustic resonator used in many applications to produce a fixed tone.
The main reason for using the fork shape is that, unlike many other types of resonators, it produces a very pure tone , with most of the vibrational energy at the fundamental frequency. When the tuning fork is struck, little of the energy goes into the overtone modes; they also die out correspondingly faster, leaving a pure sine wave at the fundamental frequency. It is easier to tune other instruments with this pure tone.
Another reason for using the fork shape is that it can then be held at the base without damping the oscillation. That is because its principal mode of vibration is symmetric, with the two prongs always moving in opposite directions, so that at the base where the two prongs meet there is a node point of no vibratory motion which can therefore be handled without removing energy from the oscillation damping.
However, there is still a tiny motion induced in the handle in its longitudinal direction thus at right angles to the oscillation of the prongs which can be made audible using any sort of sound board. Thus by pressing the tuning fork's base against a sound board such as a wooden box, table top, or bridge of a musical instrument, this small motion, but which is at a high acoustic pressure thus a very high acoustic impedance , is partly converted into audible sound in air which involves a much greater motion particle velocity at a relatively low pressure thus low acoustic impedance.
