Science Buddies: "Ask an Expert"

Why does resin make the violin sound better

Hi,

Why use rosin on a violin string? The answer is all tied into how a violin makes sound. Here are excerpts from websites discussing the issue:

“ Bow hair is not just any hair--it's hair from a horse's tail. Bowmakers use horse tail hair because it has tiny ridges, or barbs, on the surface, and because it's really durable. When you rosin your violin bow, the rosin attaches to the tops of these barbs; and it is these tiny barbs passing over the string that makes the sound. You need just enough rosin to make the tops of these ridges a little sticky, so that each one grabs the string, just a little, as it is drawn over the string. This is what makes the string vibrate.
With use, and over time, the little ridges (barbs) will wear down and you'll be left with just the flat surface of the hair. When this happens, the bow hair should be replaced.”

“Rosin which is made from tree sap, helps create friction between the bow and the strings on the violin. Rosin is put on the horsehair on the bow. The bow will then grip the strings as it is being pulled across and create vibrations, which are amplified by the body of the violin. Then, you hear the sound produced by the violin. Without the rosin, the horsehair just slides over the strings, and most likely, you wouldn't hear anything when the violin is played.”

“Sound on a violin is generally produced by a bow. The bow is made of flexible wood with a hank of horse hair connecting either end. The type of wood and the hair can change the sound of the instrument as well. The hair is coated with rosin, a pitchy resin that makes the string sticky. Sound is made when the violinist creates enough friction between the hair and the string by means of pressure placed on the bow to pull the string along the direction of the bow's travel. The force pulling the string back will eventually become greater than the force from the bow. When this happens, the string quickly recoils back toward its natural position. This causes vibrations to travel up and down the violin string. This can be observed by slowly drawing a bow across the string. By carefully drawing the bow across the string, one can also produce a continuous note. Displacing the string further will cause higher amplitude vibrations—more sound. Describing the process mathematically, the force the bow exerts on the string can be approximated by the relationship

Where μ is the coefficient of friction for a given material, and n is the force of the bow pressing down on the string. The force the string exerts as it is stretched can be described using hooke's law,

where k is a coefficient measuring the stiffness of the spring (or in our case, the stretchiness of the string) and x is the distance the string has been pulled by the bow. Sound is generated at the precise instant when the restoring force of the spring overcomes the frictional force of the bow. If at that point, the forces are the same:

Changing the amount of rosin on the bow affects how sticky it is, which determines μ. During the course of a recital, this would stay essentially constant. Because the string is stretched a very small amount compared to its total length, k is also constant. This leaves the relationship

The displacement x, or how far the string gets pulled determines how loud the sound is. A violinist can change the volume of play by adjusting the normal force of the bow. Pressing harder on the string increases the frictional force, which causes the string to displace more, which results in a higher-amplitude sound wave.”

:Thanks for asking the question. It is kind of hard to find a simple clear answer to this question using a google search. I hope this answers your question.

Best regards,

Barrett Tomlinson

It's rosin, not resin.