aftertouch / hammer striking distance

[A440A at aol.com]

Stéphane writes:

<<  I fine tune the let off this way : I lift the dampers and depress very 

slowly the key until I feel that the jack hits it's let off button, and the 

key stops in this place.  Then, I add the absolute minimal pressure on the 

key in order to go through the let off (this is the pianists absolute 

pianissimo blow).  If I hear no sound from the string, I regulate the letoff 

higher.  Eventually, the absolute pianissimo will produce a sound from the 

string, but at this level, the hammer will wobble a tiny bit on a normal 

piano blow.  So, from this point, I take my regulating tool in one hand and 

play the note with my other hand, with the dampers still lift, and playing 

piano, and I turn the let off button until I get an absolutely clear touch. 

This is very obvious, as until then you feel a kind of double shock in the 

bottom of the key, corresponding to the jack to knuckle wobbling and the 

bottom of dip. >>

Greetings, 
   I am going to assume Stephane, by saying "wobble" is meaning what I call 
"bobble", ie. the double striking of the hammer and string. 
   I have regulated pianos this close, but the let-off is not sufficient to 
keep the hammer from being hit by the string when the string is moving at its 
full excursion distance. Something that happens under fast repetion at forceful 
levels. 
     I set let-off in the piano by lifting the damper via sostenuto and 
striking the note a full FF and quickly raising the hammer back through let-off.  I 
increase let-off distance until it is impossible for the moving string to 
contact the hammer while the knuckle is still on top of the jack. Otherwise, I 
have found that under some forceful, fast repetition, the hammer is sent 
downward by the moving string so fast that its tail gets "under" the backcheck and 
the key must be allowed to come all the way up before the jack can reset.  If 
the action tends to have the hammers high off the rest felts, this problem is 
exacerbated. 
    Obviously,setting let-off to miss the string while the string is moving 
makes the let-off distance greater in the bass, and greater in longer pianos.  
The strings simply move in a larger zone, requiring more clearance to avoid 
capsizing the action in all playing circumstances.  I then set drop to the same 
height,since it is also tasked with keeping the hammer out of the excursion 
zone.  I don't want any more drop than absolutely necessary, since it brings the 
spring into play at a very crucial point in the stroke,(right before release) 
and I find ppp control is adversely affected by the added resistance.  
   Above C5, the excursion zone is quite small, and I use a different 
application of touch to determine let-off.  If I play the note very slowly, yet with 
a steady movement, I can decrease the let-off until I feel it hitting the 
string before escapement.  Continuing to play the note, as softly and steadily as 
possible, I increase the let-off until contact cannot be felt.  This MAY be 
optimum, but often, the elasticity of the let-off punching, knuckle, and 
whatever else is in the action path, causes a need to increase the let-off more to 
avoid the micro-blocking that can happen under fast repetition.  This is not to 
be confused with another phenomenon that appears.  
   Observe in "Five Lectures for the piano", 

http://www.speech.kth.se/music/5_lectures/askenflt/motions.html#keymotion

that the key motion is not constant as we depress it, but rather, that there 
is a change in velocity on the way down.  It is necessary to reduce this 
"plateau" in the downward velocity as much as possible when checking let-off by 
feel in the upper regions of the piano.  The behavior of the key/hammer 
relationship depends on how much resilience there is in the system, i.e., if you 
depress a key with your knuckle, using a straight arm directly above the key, you 
will hear the hammer double strike the string, almost regardless of the amount 
of let-off you have. 
   At the convention last year, I demonstrated this to a senior technician 
and he described it as "blocking."  I took issue with that, instead preferring 
to regard it as double striking due to the knuckle leaving the top of the jack 
BEFORE escapement, then bouncing off the string and hitting the jack again as 
escapement was underway.  How can the knuckle leave the jack before 
escapement?  
     I believe that the elasticity in the system, (key flex, balance punching 
compression, capstan pad compression, pinning elasticity, flex in the whippen 
and hammer rails, compression of the knuckle and flex of the shank), allows a 
transient wave to go from keytop to hammer and back again before the hammer 
is actually beginning to move.  If there is nothing to absorb this energy 
wave,(a task usually performed by the pad on the pianist's finger), the wave 
reflects back to the hammer with enough energy to launch it off the jack before 
escapement.  So, in essence, it is not blocking when it bobbles, but rather, 
jumping the gun and springing off the jack, hitting the string, rebounding to the 
jack before the jack escapes and then being sent once again to the string as 
escapement occurs.  
     Try playing a note on a grand piano with the middle finger knuckle and a 
straight wrist and arm directly above.  It doesn't even have to be a strong 
note, just a steady movement downward. I find that pianissimo and mf will both 
produce this double striking. Increasing let-off to any reasonable distance 
will not stop it.  



Ed Foote RPT 
http://www.uk-piano.org/edfoote/index.html
www.uk-piano.org/edfoote/well_tempered_piano.html