On 29-mei-05, at 21:13, V T wrote: > Hello Stephane, > > The problem of what exactly happens at impact in the > string portion between the hammer and the agraffe/capo > is complex, so I only speculate here: > > I don't know if it is the peak tension after impact, > or the absorbed power that actually breaks the string. > In a static measurement, we stretch a string very > slowly and record the tension at which it breaks. In > a dynamic case as we encounter in the piano, the > stretching of the string is fast and the force is more > like a pulse of limited duration. Is it the total > energy transfer that matters here, or the peak > tension? I don't know the answer to that, so the > following is very much open for discussion. > > When the hammer hits the string, two pulses propagate > along the string. One goes in the direction of the > bridge, the other in the opposite direction, to the > agraffe/capo. The pulse that goes to the agraffe is > the shorter one of the two, but the peak tension is > higher. So this pulse would be more likely to break > the string if it's the peak force that matters more > (regardless of the duration of that force). You would > be right in his case, the string would break from the > first impact. > > If it's the power transferred to the weak spot of the > string that matters, I would guess (again, no math > done here), that the pulse that formed on the section > of string between the hammer strike point and the > bridge will deliver more of it. What returns after > reflection from the bridge may still be a substantial, > relatively long pulse. > > Also, I was thinking about the observation that when > you bend a piece of wire back and forth to work harden > it, it will break when you try to "undo" the previous > bend by bending it in the opposite direction. The > reflection from the bridge causes a bending of the > wire at the agraffe in the direction that is opposite > to the bend of the wire as strung. > > Regarding the second point (regulation for maximum > power transfer), my thoughts are as follows. There > are two issues at work here: > > First, we certainly want an action that doesn't > convert the player's energy into heat before it even > reaches the strings. That means, we want to minimize > friction where it doesn't help playing control. We > don't want key sticks that flex too much. We don't > want a capstan/wippen heel contacts that have more > sliding motion than necessary. Also, excessive motion > of the knuckle on the repetition lever/jack is > suspect. (Except for Ron Overs' action, I don't know > any that have minimized the amount of knuckle dragging > - I am curious, why?). We also don't want balance > rail felt punchings that are too thick and action > center felt that is too compliant. The hammer shanks > have to flex "just so" to aid the best energy transfer > from hammer to string, etc... > > The second issue has to do with the efficient transfer > of energy from the hammer to the string. This is a > separate issue from everything that happened to get > the hammer up to the string. Here, we have a > difficult problem; we want as much energy transfer as > possible to make the piano loud, but that may not give > a good tone. I think the hammer dwell time on low > notes is not sufficiently long to absorb the first > reflection from the bridge. Somewhere in the mid/high > tenor, the dwell time is sufficiently long in > comparison with the pulse travel time that the > reflected wave coming from the bridge is damped > somewhat by the hammer. The shape, mass, hardness > (and who knows what else) of the hammer will play a > role here. > > I think, a good regulation will take care of all the > factors that maximize energy transfer to the hammer, > but some energy will be intentionally discarded in the > hammers in order to find the best compromise between > loudness and tone quality. > > Best regards, > > Vladan > > ================================== > > Hello Vladan. > > I would guess that the string is most >> likely to break when the first reflection from the >> bridge returns and hits the agraffe or capo. > > What makes you think it is not the first hammer-string > contact that is the > more offensive to the string ? How can the first > reflection be more > dangerous ? Before the first reflection, the first > impact wave from the > hammer strike point also reaches the capo, and with > more amplitude than the > reflection wave from the bridge, doesn't it ? > > The >> magnitude of the reflection is a function of the >> impedance match between the string and soundboard, > so >> it would seem possible that a piano with a high >> reflection coefficient at the bridge is harder on > the >> strings. > > Ok. but still, I think the first shock is the more > demanding on the string. > > > As Andre mentioned, regulation and voicing >> would also matter because the hammer dwell time >> (string contact time) would have an effect on the >> damping of the reflected wave too. > > Mmmm. But a well regulated action with well voiced > hammers (at least the > way André understands it) is meant to get the most > power out of the string, > thus again the most demanding, no ? Ok if the hammer > blocks against the > string, but this doesn't happen that much, does it ? > In every other case, > less well regulated action means less power > transmitted to the string. > > But maybe you mean this is not about power ? > > > Best regards. > > Stéphane Collin. > > > > __________________________________ > Do you Yahoo!? > Yahoo! Small Business - Try our new Resources site > http://smallbusiness.yahoo.com/resources/ > _______________________________________________ > pianotech list info: https://www.moypiano.com/resources/#archives > > friendly greetings from André Oorebeek www.concertpianoservice.nl "Where music is no harm can be"
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