Tuning fork

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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 after waiting a moment to allow some high overtones to die out. The pitch that a particular tuning fork generates depends on the length of the two prongs. Its main use is as a standard of pitch to tune other musical instruments.

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Description

The tuning fork was invented in 1711 by British musician John Shore, Sergeant Trumpeter to the court, who had parts specifically written for him by both George Frideric Handel and Henry Purcell.

The main reason for using the fork shape is that it produces a very pure tone, with most of the vibrational energy at the fundamental frequency, and little at the overtones (harmonics), as is not the case with other resonators. The reason for this is that the frequency of the first overtone is about 52/22 = 25/4 = 6 1/4 times the fundamental (about 2 1/2 octaves above it).[1] By comparison, the first overtone of a vibrating string is only one octave above the fundamental. So when the fork is struck, little of the energy goes into the overtone modes; they also die out correspondingly faster, leaving the fundamental. It is easier to tune other instruments with this pure tone.

Another reason for using the fork shape is that, when it vibrates in its principal mode, the handle vibrates up and down as the prongs move apart and together. There is a node (point of no vibration) at the base of each prong. The handle motion is small, allowing the fork to be held by the handle without damping the vibration, but it allows the handle to transmit the vibration to a resonator (like the hollow rectangular box often used), which amplifies the sound of the fork.[2] Without the resonator (which may be as simple as a table top to which the handle is pressed), the sound is very faint. The reason for this is that the sound waves produced by each fork prong are 180° out of phase with the other, so at a distance from the fork they interfere and largely cancel each other out. If a sound absorbing sheet is slid in between the prongs of a vibrating fork, reducing the waves reaching the ear from one prong, the volume heard will actually increase, due to a reduction of this cancellation.

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