Friday, July 16, 2010

Some Physics of the Acoustic Guitar

The stringsThe pitch of a vibrating string depends on four things.
The mass of the string: more massive strings vibrate more slowly. On steel string guitars, the strings get thicker from high to low. On classical guitars, the size change is complicated by a change in density: the low density nylon strings get thicker from the E to B to G; then the higher density wire-wound nylon strings get thicker from D to A to E.
The frequency can also be changed by changing the tension in the string using the tuning pegs: tighter gives higher pitch. This is what what you do when you tune up.
The frequency also depends on the length of the string that is free to vibrate. In playing, you change this by holding the string firmly against the fingerboard with a finger of the left hand. Shortening the string (stopping it on a higher fret) gives higher pitch.
Finally there is the mode of vibration, which is a whole interesting topic on its own. For more about strings and harmonics, see Strings and standing waves.
The strings themselves make hardly any noise: they are thin and slip easily through the air without making much of disturbance - and a sound wave is a disturbance of the air. An electric guitar played without an amplifier makes little noise, and an acoustic guitar would be much quieter without the vibrations of its bridge and body. In an acoustic guitar, the vibration of the string is transferred via the bridge and saddle to the top plate body of the guitar.


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