The bow and the string, how they communicate
What is the role of the bow in sound production, that is a question for which we have little scientific information. Every string player would agree that the bow is just as important as the instrument. But still, in all of the sound competitions where Strads are compared to new instruments, nobody says a word about the bow. It is widely known that a bow can suit one instrument, but not another one. And yet, there is still no word about the bow. This is curious.
One reason might be is that a bow is something very personal. It doesn’t make sound by itself. A bow comes to life in the hand of a musician. It is difficult to judge, because we don’t want to judge the musician, but the bow.
The string player can give his intuitive impression, but that is far away from a scientific judgement.
First of all, it is the expectation of the musician that clouds his impression. If he knows that a certain bow is very expensive, far above his budget or not for sale at all, he feels free to love it deeply. Even years later he will tell you that this bow was amazing. A bow that costs a little less than he wants to spend is not sexy at all. So the real quality of a bow is impossible to judge. But, we can have a closer look at what a bow actually does. Two things are certain: it is in the hand of a musician and it lies on the string of his instrument.
Without movement there is no sound. When the musician starts to play, the bow hair sticks to the string until the pulling force is big enough to let the string snap back. That is what the books say and, this is called the Helmholtz motion.
I am not convinced that the so-called Helmholtz motion explains it properly. I think it is more complicated than a slip-stick movement. The string makes a very complicated movement, not only does it go up and down and sideways, but it also has a torsional movement that looks very irregular if you see a string in movement filmed by a digital camera (see link above).
The movement of the string is the sound itself, which gets amplified by the instrument. The bow is in continuous contact with the string. It is not just slip and stick. The adherence can become stronger or less in a gradual way. It depends on the input of the musician and on the property of his material.
The role of rosin is underestimated. Without rosin there is no sound. Rosin and horsehair have been used for a thousand years. One of the characteristics is that rosin changes its adherence with temperature. Friction between the hair and the string heats up the rosin, thus changing its stickiness. At 20 C it is not very sticky, at 30 C or 40 C it gets stickier and then with the increase of temperature the stickiness decreases, at 80 C it will boil and therefore will become fluid and less sticky. The temperature of rosin can change in milliseconds. The adherence of the bow changes constantly depending on the friction, which is controlled by the player through the pressure he uses and the speed of the bow.
So you see the confusion. Everything is in motion and influenced by the musician and the properties of the bow and the instrument. And even these properties are in continuous change. That is why science is helpless; as soon as you separate one aspect from the others you come to insignificant conclusions. It takes a musician’s whole life to get it under control. It is amazing how sensitive a string player is about his bow. If I take a tenth of a gram away from the stick just behind the head, they will tell me that this is a completely different bow. Also, for me as a bowmaker, it is impossible to have everything under control. I‘ve been trying for 30 years and it’s still full of surprises and mysteries for me.