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Anyone heard of "bifurcation" effects in violin st

 
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jethro
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Joined: 07 Apr 2007
Posts: 178

PostPosted: Wed Nov 28, 2007 3:29 am    Post subject: Anyone heard of "bifurcation" effects in violin st Reply with quote

Once I listened to a talk on the topic of bifurcation. It was a physics
seminar- a masters thesis defence I think.
The Idea as I remember it is that things which can oscillate follow a simple
linear "spring-like" perscription for their movement. As the driving force
increases past a certain point which depends on the propertys of the material - the law which governs the resulting motion shifts from linear
to some more complicated and messy nonlinear equation. For simple
mechanical systems this bifurcation point might be where the material
has difficulty returning to it's neutral position or maybe where internal damage starts happening. (?) We have all sometime bent a spring just
a little too far and it didn't return back to it's resting point. When
the driving amplitude reaches the the magnitude at which the bifurcation
happens, the frequency of oscillation splits into two different (new) frequencys and the driving frequency greatly diminishes. It is like the
new frequencys are robbing the energy away from the old one so that
as input energy is added it goes into the 2 new ones at the cost of the
old driving frequency.

The reason I ask about this is that I thing this was happening to my G
string when I would get my bridge too thin on the faces. All the strings would show it but usualy G would be first.

Anyone ???????????????????

Tim
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Dave Chandler
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Joined: 31 Oct 2007
Posts: 691
Location: Mt Mitchell in North Carolina

PostPosted: Wed Nov 28, 2007 11:09 am    Post subject: Bifurcation Reply with quote

Tim:
I know that as a player, you can "overmodulate" if you will, attempt to play louder than the strings can respond to (and usually the G), and the sound deteriorates. Is this what you refer to? I've watched my G string under bowing pressure create two different and visible vibrating areas.
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jethro
Member


Joined: 07 Apr 2007
Posts: 178

PostPosted: Fri Nov 30, 2007 2:59 am    Post subject: bifurcation Reply with quote

Dave: I am only a beginner at playing. But Yes It could be.......
What I see when it happens (usualy using poor, too thin bridges)-
do a long slow open G starting easy (piano) and go the whole bow length
and dig in and speed up as you go to gradualy increase the sound power.
While doing this watch the G string. When you start you can kind of see
the blurred outline of the standing waves -nodes and antinodes. As you
attempt to increase the output -it does increase -but then at a point you
see the standing wave envelope kind of "pop" or like "jump" into a seperate and different set of standing waves. You may hear the tone
get a little less "deep" or "rich" . But more than that you can FEEL the
string get sloppy or mushy after the standing wave shift. You can keep the sound going and bow as hard as you want but NO MORE volume comes
out as long as the string is in this 2-nd mode condition. once you stop and start over again the string resets and the process repeats. Once you
get used to seeing and feeling it , it seems easier to spot. I have had bridges that where in this bad mode from the point the bow touched the
string. For the good but could be better ones, the mode breaks as you
play louder. On the best bridges - you can never play loud or hard enough to get the mode to "break" ! I think this is depending on
NOT thinning the bridge too much in the wronge places ( and maybe
correspondingly having the bridge's frequency being too low )
I have an article I downloaded about resonant frequencys of bridges
and what was optimal. It found that 2700 hz was the sweet spot that
you didn't want to go lower than. I can pass you the title the next time
I post.

I have played many new instruments which showed this problem at very
low (piano) playing levles. It seems like the strings allways feel mushy
lateraly under the lateral push and pull of the bowing action. Some
good instruments I have tried the strings feel wonderfully firm and rigid.
Like the difference between toned muscle and FLAB ! Violas seem to
do this even more than violins.

Probably more chatter than you wanted to hear.... it was a brian dump !
More pebbles than boulders ......

Tim
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