Tuning fork

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Tuning fork on resonance box, by Max Kohl, Chemnitz, Germany

A tuning fork is an acoustic resonator in the form of a two-pronged fork with the 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.

1 Explanation
2 Calculation of frequency
3 Uses
4 See also
5 References
6 External links

[edit] Explanation

Currently, the most common tuning fork used by musicians sounds the note of A (440 Hz, international "concert pitch"), which has long been used as a standard tuning note by orchestras, it being the pitch of the violin's second string played open, the first string of the viola played open, and an octave above the first string of the cello, again played open. However, they are also commercially made to vibrate at frequencies corresponding to all musical pitches within the central octave of the piano, and other pitches.

Tuning fork by John Walker showing note (E) and frequency in hertz (659)

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

The reason for using the fork shape is that, when vibrating, there is a node in the vibration pattern at the bend of the 'U' where the handle is attached, so the handle doesn't vibrate. This allows it to be held there without damping the vibration.

When struck, it gives out a very faint note which is barely audible unless held close to the ear. For this reason, it is sometimes struck and then pressed down on a solid surface such as a desk which acts as a sounding board and greatly amplifies the note.

Well-known manufacturers of tuning forks include Ragg and John Walker, both of Sheffield, England.

[edit] Calculation of frequency

The frequency of a tuning fork depends on its dimensions and the material from which is made: ^[1]

$f \propto \frac{1}{l^2} \sqrt{\frac{AE}{\rho}}$ , and where the tines are cylindrical,^[2] $f = \frac{R}{\pi l^2} \sqrt{\frac{E}{\rho}}$

Where:

f is the frequency the fork vibrates at
A is the cross-sectional area of the tuning fork
l is the length of the fork's tines
E is the Young's modulus of the material the fork is made from
ρ is the density of the material the fork is made from
R is the radius of the tines

[edit] Uses

They are commonly used to tune musical instruments, although electronic tuners also exist, and some musicians have perfect pitch. Tuning forks can be tuned by removing material off the tines (filing the ends of the tines to raise it or filing inside the base of the tines to lower it) or by sliding weights attached to the prongs. Once tuned, a tuning fork's frequency varies only with changes in the elastic modulus of the material; for precise work, a tuning fork should be kept in a thermostatically controlled enclosure. Large forks are often made to be driven electrically, like an electric bell or buzzer, and can vibrate for an indefinite time.

[edit] In musical instruments

A number of keyboard musical instruments using constructions similar to tuning forks have been made, the most popular of them being the Rhodes piano, which has hammers hitting constructions working on the same principle as tuning forks.

[edit] In electromechanical watches

Electromechanical watches developed by Max Hetzel for Bulova used a 360 Hertz tuning fork with a battery to make a mechanical watch keep time with great accuracy. The production of the Bulova Accutron, as it was called, ceased in 1977.

A tiny quartz tuning fork is used in crystal oscillators, the most notable use of which are quartz digital watches. The piezoelectric properties of quartz crystals cause a quartz tuning fork to generate a pulsed electrical current as it resonates, which is used by the computer chip in the watch to keep track of the passage of time. In today's watches, they generally resonate at $215 = 32,768$ Hz. (See quartz clock.)

[edit] Medical uses

Main article: Hearing test

Tuning forks, usually C-512, are used by medical practitioners to assess a patient's hearing. Lower-pitched ones (usually C-128) are also used to check vibration sense as part of the examination of the peripheral nervous system.

Tuning forks also play a role in several alternative medicine modalities, such as sonopuncture and polarity therapy.

[edit] Radar gun calibration

A radar gun, typically used for measuring the speed of cars or balls in sports, is usually calibrated with tuning forks. Instead of the frequency, these forks have the calibration speed and radar band (e.g. X-Band or K-Band) for which they are calibrated.