Questions: String Tension

by Thomas Knatt

Originally published in American Lutherie #95, 2008



John C. from the Internet asks:

String tension will deform an instrument in an elastic manner so that when the tension is removed, the instrument will return to its original shape. But over time the tension will also deform the instrument permanently, often requiring repair work to make it playable again. Is this permanent deformation proportionally related to the amount of time the instrument is under tension? If the instrument is tuned to pitch only when it is played, will the time it takes to deform to the point of unplayability be lengthened in proportion to the amount of time the instrument is not under string tension?


Thomas Knatt from Groton, Massachusetts responds:

The short answer is yes, detuning every time would probably lengthen the life of the instrument. But....

Let’s do a thought experiment. Suppose we significantly loosen one side of a drum or banjo head. The drum would sound duller when struck on the side with the low tension. The sound of the banjo, when played, would change as well, although I won’t predict exactly how. Carleen Hutchins says that you don’t want a lumpy system, because it doesn’t behave well. I have done a glitter test on a well-tuned kettle drum, and the glitter jumped a foot off the head at one frequency. If I went one cycle above or below the tuned frequency, it only jumped an inch. I went ±2 cycles and it barely moved. That is a good example of low damping. A system becomes lumpy by adding lumps (weight) or changing the stiffness in sections.