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Sometimes we can tell; but often we don't know if your bow needs a rehair
or not, only you can answer that.
We can tell if the hair is obviously very old and oxidised and we can also
tell if a lot of the hair is missing or broken, but we can't readily tell if all
the hair is in place and it looks reasonably fresh.
So, how should you know? Well, the hair is covered with microscopic
'barbs' or scales which grip the rosin and, as you play, the friction
against the strings slowly wears these barbs down and sometimes tears
them off. Over a long period of time (conscientiously practising) so many
of these barbs are damaged that the hair no longer grips the rosin and you
find you have to keep putting in more effort for diminishing results.
Your bow is now losing the essential friction with the string to easily
produce the sound you want. You try to apply more and more rosin but it
just falls off onto the front of the instrument without improving the
sound (at which point you suddenly remember why you have that clean dry duster ...!).
Now you know you need a rehair!
And how long between rehairs?
Again, we can't say exactly because of different playing pressures and
techniques and how much playing is actually being done on a daily basis,
but for a professional or advanced student, perhaps from three to six
months would be reasonable and for beginner and intermediate students six to twelve months.
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