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Pitch-bending Issues: What Causes Them?


monian
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16 hours ago, monian said:

Hi all,
What causes pitch bending issues in orchestral string instruments? I've noticed that in some circumstances that when you bow hard, the pitch starts abnormally high and bends down to normal. This is evident to a degree when you pluck hard. Any ideas?

The more a string is deflected, the higher the tension, which all other things being equal, would result in a rise in pitch. Aside from that, I can't offer much more without seeing your instrument, and observing how you play it.

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My experience is a little different.  If I start a down bow with too much pressure, and far from the bridge, the note begins about a semitone flat.  The problem goes away if I use less pressure and more speed, and stay closer to the bridge.

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Steel string cores are far stiffer (longitudinally) than gut or synthetic, therefore tension will go up faster with the same amount of deflection, and therefore going sharp is much more obvious when playing (or plucking) loud on steel strings.  But the basic physics is as David mentioned, deflection increases the tension on the string.

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14 hours ago, J-G said:

My experience is a little different.  If I start a down bow with too much pressure, and far from the bridge, the note begins about a semitone flat.  The problem goes away if I use less pressure and more speed, and stay closer to the bridge.

I've often wondered about this!

It only seems to happen on the G - I think this is because there isn't enough clearance on the D and A to press hard enough to make it happen.

Perhaps pressing hard with the bow somehow pulls the top of the bridge forward temporarily ....

 

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3 minutes ago, martin swan said:

I've often wondered about this!

It only seems to happen on the G - I think this is because there isn't enough clearance on the D and A to press hard enough to make it happen.

Perhaps pressing hard with the bow somehow pulls the top of the bridge forward temporarily ....

 

Interesting! Some time ago this discussion was in another thread and I said that a string sometimes goes flat when bowed hard. I was corrected by "physics" that this is not the case but I think it can bend both flat and sharp. Good explenation

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51 minutes ago, martin swan said:

I've often wondered about this!

It only seems to happen on the G - I think this is because there isn't enough clearance on the D and A to press hard enough to make it happen.

Perhaps pressing hard with the bow somehow pulls the top of the bridge forward temporarily ....

 

I doubt that the tension or bridge position is involved, but has to do with the stick/slip movement of the string and bow.  My hypothesis: if you have more pressure than speed, the "slip" event will get delayed just a tiny bit, flattening the note.

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1 minute ago, martin swan said:

Don, can you make that idiot-proof? Sounds interesting.

I'm not sure what you mean by idiot-proof in this instance.  Nothing is idiot-proof, as we see in our political news here every day.  

However, as an expansion of my idea:  the cyclical movement if a string looks like a moving kink in the string; when the kink gets to the bow hair, the kink initiates a slip, then the string re-attaches to the bow hair shortly thereafter.  If the bow pressure is high and/or speed low, the friction of the hair on the string will fight the kink for just a microsecond until it finally breaks loose and slips.  That would lower the frequency.

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4 hours ago, Marty Kasprzyk said:

Does the pitch flattening occur  for open string notes as well as for fingered notes?

Yes.

Don, thanks for the explanation - it's still a bit beyond me but I appreciate your trying. I get the slip/stick concept, but how does the sticking cause the pitch to lower?

I've been having fun practising the technique, and have managed to get a sustained flattening on all strings, though the E is more difficult and tends to just howl.

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23 hours ago, monian said:

Hi all,
What causes pitch bending issues in orchestral string instruments? I've noticed that in some circumstances that when you bow hard, the pitch starts abnormally high and bends down to normal. This is evident to a degree when you pluck hard. Any ideas?

When you say "bow hard" do you mean large downward bow force or do you mean fast bow speed    or both?

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Interesting some of the comments made regarding steel versus gut or nylon strings. I've tried many different gut and synthetic core strings and found them all to be wishy washy for pitch, highly dependent on bow pressure compared to steel core strings for correct intonation. I've often wondered if the pro classical musicians use this somehow to their advantage, but I still go back to Thomastic Super Flexible or similar steel rope core strings for my own use.

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I use both steel and synthetic at times, and prefer the sound of the synthetic but the easier bowing of the steel (Helicore is steel, by the way, with some synthetic added).  And for cross tuning, steel gets there and back with stability that you can't get with stretchy creepy synthetic.

I think at least some of the bow pressure sensitivity of synthetic/gut is related to tension; steel strings tend to be at the top of the tension charts.  Fiddling tends to be high on the bow pressure and low on the bow speed, which I don't think low-tension strings like very much.  It just takes a different technique to play the different strings, but the pitch uncertainty of steel strings played at high volume I could see as a serious problem for a soloist violinist.

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You have two opposing effects going on while drawing a bow across the strings: dampening and pumping up string displacement.

 

Dampening: When the string slips back after being displaced by the bow, some of the energy of the string is absorbed by the friction of the hairs on the string. Think of brake pads squeezing the rotors on your brake. Without getting into a lot of messy math, a vibrating string that is being dampened by this type of action has its natural frequency of vibration reduced (w ~= sqrt(wo^2 - d^2), where w0 is the free vibration frequency and d is some dampening factor related to bow pressure).

So putting pressure on the bow will tend to reduce the frequency of vibration during the slip phase and some of the recovery phase until the string is caught by the bow hairs and pulled until it slips again.

Pumping: Dragging the bow across the string will pump energy into the string that is proportional to the pressure on the bow times the speed of the bow. At some bow speed point, the energy being lost to the bow hair friction during string slip is off set by the increase in energy being pumped into the string with the faster bow speed.

An ability to juggle bow pressure and speed as a function of sounding point to change string frequency in a dynamic way is what separates people who saw on the strings, like me, from people who enthrall the strings, like <insert any well known violinist here>.

 

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There have been two pretty different topics discussed here. The „pitch flattening“ effect has been thoroughly explained by Ctanzio. It is a bowing technique imperfection and is almost completely independent of the string quality. No any string can cope with such bowing mistake.

But the original question describes a different phenomenon. When you bow fast (especially open strings, mostly G), the pitch may increase, sometimes even a semitone. It is a string imperfection and you can observe it mostly on junky metal core strings. It is the lack of longitudinal elasticity as David Burgess mentioned in his first post. The elastic core of the gut or synthetic string increases its tension (whilst being deflected) in much less extend as the typical metal core string does. This is why gut and synthetic core strings can mostly avoid this type of behavior.

You can imagine a pendulum for example. The frequency depends on the pendulum length only. The amplitude does not change the frequency. But try to install a spring (or rubber block) at the stop points of the pendulum. It will accelerate the pendulum and increase its frequency. The string is being accelerated by its tension increase two times per oscillation the same way.

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I didn't realize that Helicore was kind of steel strings. So I guess steel strings are legal then, even in Finland ;), because ~15-20 years ago every other violin had Helicore here. This was the time they entered the market here. Nowadays I don't see them that much. They are odd, thin and rope like. On some instruments they work well and gives a sensitive and sorrowful voice in upper positions.

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I have forgotten to mention that the pitch flattening effect occurs when you bow slower that the current string speed on the particular contact point. You certainly know that although the oscillation frequency remains always the same (whilst changing the bowing contact point), the speed of the string increases if you bow farther form the bridge. If you bow slower, you brake the string not only during the "sliding" , but also during the "sticking" period. 

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