Lectures and Writings

How Bowing a String CreatesThe Vibrations We Hear as Sound

By Gregg T. Alf

Friction between the hair of the bow and string is a primary factor. This friction is created not, as is often supposed, by barbs or scales on the hair - the hair is actually quite smooth - but by rosin. Tiny particles of rosin cling to the hair due to an ion exchange between them. Rosin has the property of becoming sticky when heated above room temperature. One might be surprised to learn that, as a result of friction, the temperature at the point of contact between string and hair can increase by 25 to 30 F in a matter of milliseconds.

     A special property of warm rosin is its high coefficient of friction when static, but low coefficient while moving. This means that rosined bow-hair grips the string firmly until the string breaks free and starts to slide, at which point it glides fairly freely. It is this property of rosin which makes it useful on a dancer's shoes and a gymnast's hands.

     When drawing the bow, the rosined hair pulls the string to one side, until string-tension causes it to break free and begin sliding back toward its original position. This motion sends a 'shock-wave' down the string. The wave bounces off the top-nut (or the player's finger) and heads back toward the bow, which is again displacing the string. The returning wave dislodges the string from the bow hair and the cycle begins again.

     The frequency at which this happens determines the pitch of the note produced. When a player shortens the effective string-length with his or her finger, the distance the wave needs to travel goes down. Thus more round-trips happen each second, and the pitch goes up.

     Because of their small surface area, making music with the strings alone would be as effective as (to use a famous analogy) fanning oneself with a toothpick. The vibrating strings need the body of the instrument to convert their vibrations into something more audible, and this is where the violinmaker comes into the picture.

1. Norman Pickering, "The Bowed String", Amereon, Ltd. 1991.

 

 

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