I decided I should try to set down a pretty detailed account of my thinking that led me to the tuning system I use, that I have been describing (similar in many ways to David Love's, but with a few differences in detail). IOW, the theory behind the practice, the thoughts and experiences that have led me to develop a particular technique and approach to tuning that is rather different from what most people do. It's a long post.
First, I don't believe there is any issue with leaving the pin itself in an unstable state. Tuning pin steel is such that as soon as you remove the twisting and flexing forces from the pin, it will return to its rest condition, essentially instantaneously. To test this notion in real life, take a piano with tight pins and VERY low string bearing friction. Just barely touch the pin, and pitch changes, that sort of piano. Flex the pin in either direction, then let go of the hammer. Pitch moves away, then returns immediately when the hammer is removed, either direction. Twist the pin (not enough to move it in the bock) in either direction, and let go. Same thing happens. Try to leave an instability in the pin: do your utmost to make it so that the string will remain temporarily sharp or flat of its stable state after you remove the hammer. Can you do it? I don't think so. If you can, I believe it is due to there being a wee bit of bearing friction.
If there is bearing friction, a little or a lot, it is quite possible to leave pitch above or below the stable point, at will. The same experiment on a piano with significant bearing friction will prove that this is so. Thus, the problem we face when trying to achieve tuning stability is centered in string friction, not in pin twist or flex.
To separate things out, look at tuning from the perspective of a screwstringer: In a screwstringer you just turn the screw, there is no flex or twist of the tuning pin involved. (BTW, Levitan's C hammer emulates this. So does a T hammer). Turn it clockwise and pitch rises, counterclockwise and pitch falls. There IS bearing friction, and the instructions inside the Mason & Hamlin tell you always to approach from below. If you overshoot and go sharp, turn it back to flat and come back up. Why? Because experience shows that coming at pitch from above will leave more unequal tension between string segments than coming from below, or at least an inequality that will more probably lead to instability, creeping over the bearing points later. If you tune one, you will notice that the pitch lags behind the screw: turn is a slight bit and nothing happens, turn a little more and pitch starts to move.
In fact, if there is a good bit of bearing friction, coming up from below straight to pitch, you might find that the piano will tend to go sharp a bit after being played a while, as the tension equalizes over the friction points. But it will be less noticeable than if you were tuning downwards in pitch, because the magnitude will be less. (A "trick" is to back off the pin just a bit after you reach pitch, no more of a turn than it took before the pitch started moving when you first started on that string - but it is guesswork). One of the problems of the screwstringer design is that you have no way to compensate for the string friction other than playing the note hard - usually good enough, but not always, and in the little experience I have tuning one, it seems impossible to get a really clean refined tuning. The steel pin in wood block design allows you to compensate for string friction by using pin flex and twist, and thereby get a more refined tuning - if you are skilled at it.
Focusing in on string bearing friction and the difference between raising pitch and lowering it: When raising pitch, you add tension to the string segment between bearing and the pin, and you can add more through the twist of the pin and by flex. The differential you can create is pretty large. Tension from the high tension segment pulls across the bearing from where the tension is lower. If you are lowering pitch, the high tension segment is the speaking length, so the speaking length is doing the pulling, and you have no way to increase its tension besides hard blows on the key. You are creating a lower tension in the bearing to pin segment, and the higher tension of the speaking length is all there is to pull the string over the bearing (unless you press on the speaking length with a dowel or something - which is a helpful thing to do in some circumstances, but not very predictable as to eventual results). To summarize, "it is easier to pull up than to push down."
You can test this on a piano with moderate to high bearing friction. "Push" the pitch flat as much as you can by pin flex and twist without moving the pin in the block. Try to get the string to go back to pitch by playing the note loudly. Then try by flexing and twisting the pin (not enough to rotate it). Now do the same on the sharp side. In my experience, I can usually pull up from flat quite a bit more easily than I can push down from sharp. Often I can't push flat from the sharp side at all, even with hard blows and pin flex and twist, but that doesn't mean the pitch won't go flat over time, back to the original stable position.
The main driving factor in pianos going out of tune due to unstable condition of the string is the blows of the hammer on the string, which momentarily raise the tension in the speaking length (when the string goes into its widest excursion) - hence that action of hammer blows will tend to pull lower tension wire from the non-speaking length, resulting in lowering of pitch. It won't "push" tension into a higher tension speaking length. It may facilitate pitch going a bit sharp (by getting the string moving) if the tension is unequal enough in that direction, but far less than it will facilitate pitch going flat when the opposite is true. And we rarely see a real problem unison due to a string going sharp - maybe a cent or two in most cases. Flat, 5-10¢ or even more can be common. That is my experience, at any rate. Bottom line: it is better to approach from below, as you have more opportunity of control, especially if there is significant bearing friction to overcome, and you will leave the string in a more stable position.
As I see it, the fundamental problem of tuning is to get the pin into the precisely correct rotational alignment, the alignment that corresponds to the string being at the correct pitch in a stable state. There is a small window of leeway if there is any string bearing friction at all, as there will be some degree of inequality of string segments that won't cross the bearing point readily, but that window is pretty small. The task is first and foremost to be able to move the pin in the block by a tiny enough amount to get it to that precise alignment, and to be able to do that with a variety of pin block situations (tightness, stickiness, sponginess, etc.). But then there is the puzzle of knowing when the pin is in the correct window of alignment: the only reference we have to go on is string pitch.
So now we come to the real heart of the issue: how to find out when the pin is in its precise rotational alignment to achieve a stable target pitch. In the normal course of intuitive tuning practice, we don't control things very much. There is twist of the pin, there is flex of the pin, there is bearing friction, and everything is somewhat chaotic in how it adds up. We may put the hammer on at whatever alignment to the pin is most comfortable to the body, resulting in a flex we don't really know about or account for. Or we put it where some authority told us to,, not really being aware of why and how that impacts the results. Pitch will usually go past target (most often multiple times in both directions), and most commonly that will mean going above target and then being "settled back." We manipulate the hammer and pin, guessing how much to pull up and push back (or vice versa), and when it seems we may probably have reached the target pretty closely, and we are tired of wrestling, we call it good enough. I am describing how I learned to tune. Eventually I began to take some aspects into account, but not in any systematic way. I more or less evolved a certain set of tricks, including the pull up/settle back "pin setting" theory. I ALWAYS went beyond pitch and settled back. And it was always guesswork whether the pin really was in the spot it should have been. And I had stability issues, not so bad, but definitely a problem when you come to the high expectations I work under most of the time.
Putting all these factors together, my solution is a simple one: to tune in such a way that the change of pitch of the string changes simultaneously with the turn of the pin in the block (as close as I can possibly accomplish that), and always to approach pitch from below. I've already described how I do that, so I won't repeat it here. The point is that I know where the target pin alignment is at all times (it's in the sharp direction unless I overshoot, in which case I go back below target), I know when I have reached it, and when I have reached it the string is also in a stable state, because I haven't created instability by my actions. When I reach target pitch, I am there. The amount of added tension in the non-speaking segment was just enough to pull the string over bearing, overcoming friction there, so when I release the hammer, the added tension will disappear, and the tension of that segment will be equal to the tension of the speaking length. Nice theory - AND it works (within the parameters of human frailty interacting with the physical world).
Re-reading this, I see it is pretty dense stuff, and rather rambling. I have been wrestling with how to describe all this for a long time, and I end up chasing my tail and becoming frustrated. I think I will call it the best I can do for now. I hope I have succeeded in explaining some aspects in a way that makes sense, and might be helpful in some way to others.
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Fred Sturm
University of New Mexico
fssturm@unm.edu http://fredsturm.net "When I smell a flower, I don't think about how it was cultivated. I like to listen to music the same way." -Federico Mompou
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