This is quite interesting stuff! A bowed string is nowhere near a sine wave -- it's approximately a sawtooth wave! The reason for this is that the string sticks to the bow (traveling relatively slowly) until the tension of the string overwhelms the coefficient of static friction, where it quickly slips (the jump discontinuity) across the bow until the relative velocity of the string drops enough to stick again.

https://www.physicscentral.com/explore/action/fiddle.cfm