How Does a Tuning-fork Work?
A tuning-fork vibrates to give a musical note of definite pitch or frequency and is marked on the fork by a letter or symbol referring to its position in the musical scale. The instrument has two hard steel prongs and a handle or a stem. It is sounded by giving one of the prongs a light tap on a wooden surface, and then holding the handle firmly in contact with a wooden board or table. Tuning-forks are used by piano-tuners and musicians. Most forks give the same musical note as Middle C on a piano and are usually marked 256. The number gives the frequency and means that it vibrates at a rate of 256 times a second.
A tuning fork is an acoustic resonator and 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 have died out. The pitch that a particular tuning fork generates depends on the length and mass of the two prongs. It is frequently used as a standard of pitch to tune musical instruments. The tuning fork was invented in 1711 by British musician John Shore, Sergeant Trumpeter and Lutenist to the court.
A tuning fork serves as a useful illustration of how a vibrating object can produce sound. The fork consists of a handle and two tines. When the tuning fork is hit with a rubber hammer, the tines begin to vibrate. The back and forth vibration of the tines produce disturbances of surrounding air molecules. As a tine stretches outward from its usual position, it compresses surrounding air molecules into a small region of space; this creates a high pressure region next to the tine. As the tine then moves inward from its usual position, air surrounding the tine expands; this produces a low pressure region next to the tine.
The high pressure regions are known as compressions and the low pressure regions are known as rarefactions. As the tines continue to vibrate, an alternating pattern of high and low pressure regions are created. These regions are transported through the surrounding air, carrying the sound signal from one location to another.
In solids, sound can exist as either a longitudinal or a transverse wave. But in mediums which are fluid (e.g., gases and liquids), sound waves can only be longitudinal. In a longitudinal wave, particles of the medium vibrate back and forth in a direction which is parallel (and anti-parallel) to the direction of energy transport. In this sense, a sound wave (like any wave) is a phenomenon which transports energy from one location to another without transporting matter.