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All violin bows I have seen are designed to hold a width of at least 1cm of hair. However, in my string training, I was always taught to rotate the bow so that only the outer-most hairs contact the strings. So what is all the other hair for? I imagine the answer is either:

  1. The extra hair serves only to support the tension of the bow. Or,
  2. My training is incorrect, and there are times when it is correct to press the bow flat against the strings so they are engaged by the full width of bow hair.

(To elaborate on my training: In my youth I had a decade of classical violin lessons. Revisiting violin technique now I realize that I was always trained to rotate the bow so that only the outer-most hairs (i.e., those opposite my thumb on the bow hand) make contact with the strings. I was taught to modulate bow speed and bow distance from the bridge in order to control volume. The only time it was correct to attack, "pronate," or otherwise press the bow against the strings with more force so that more hair contacted them, was to get the bow to bounce or skip; and in such a case the purpose was to effect a rebound, not to engage the string with more hair. Indeed: intentionally pulling the bow with all hairs flattened across a string seems to only produce a more muddy, scratchy tone.)

  • As you get more accomplished, you'll find that combinations of bow speed, bow pressure, amount of hair in contact, and position relative to the bridge all affect the sound quality. – Carl Witthoft Jan 29 at 13:55
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Even if you angle the bow quite strongly, the entire breadth of hair does touch the string as soon as you put some pressure on the bow (the outermost hairs will get stretched and give way, i.e. the bow sinks deeper and the other hairs touch the string too. Only at very low pressure, i.e. very low dynamic level, is it really only the outermost hairs that touch the string. That's probably the idea behind this whole angling thing: to allow pianissimo to be even gentler, while still having all those other hairs for louder passages.

  • Also, sometimes the bow isn't angled and sometimes you reverse the angle, meaning you are primarily loading the hairs on the other side of the bow. – Todd Wilcox Jan 28 at 20:07
  • Yes, some pressure puts more of the hair in contact, but even when I put the maximum pressure I would use for a sustained note on my bows I see less than half of the hairs engaging the string, and because of the angle the "contact patch" is even narrower. (Granted, I prefer very stiff bows.) Importantly: You seem to be refuting one of the conclusions I drew from (limited, unrecorded) experimentation on my own instrument – in particular, do you assert that drawing more hair across the string at the same pressure (and speed, and rosin) produces louder sound? Not just scratchier sound? – feetwet Jan 28 at 20:18
  • @feetwet More hairs at the same pressure&speed does not seem to make it louder, that's true; but more hairs allow you to put more pressure in before the vibration stalls. So, in effect, more hairs do allow you to play louder, because more forceful. – leftaroundabout Jan 28 at 20:36
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    @feetwet I think it's pretty simple: to excite and sustain string oscillation, a hair of the bow must have enough pressure so when the string is moving in direction of the bowing, it will stay “stuck” on the rosin, yet not so much pressure that it prevents the string from slipping the same distance back with the lower dynamic friction coefficient. If you double the number of hairs, each hair takes up only have as much pressure, i.e. you can double the total force on the bow and still each of the hairs is in its “non-stalling range”. – leftaroundabout Jan 28 at 22:05
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    As a bowmaker, I'll just add that the reason for the rosin is that it grabs and pulls well when the string is nearly at a brief standstill at the maxima of its swing, but as soon as the tension becomes too much and the bow lets go, the string quickly moves fast enough to liquify the rosin and reduce its friction to practically nothing, until the next cycle. This repeated 440 times a second on the A string. – Scott Wallace Jan 29 at 11:56

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