My question was really more about whether the Rebliztian suggestion that blow distance--for the modern concert grand--should sit ideally between 1 3/4" and 1 7/8" is still considered pretty accurate, these days. You hear reports of new grands having blow distances from the factory of 4.4 CM or less, which seems awful small!
Many factories set blow distance some shallow knowing full well it is going to settle (increase) over time. There may also be some who specify a shorter blow distance for some mechanical or action ratio reason.
An example might be a piano with particularly heavy hammers that has been balanced with a relatively (numerically) low overall action ratio. By setting a slightly short blow distance they could still use a reasonable key stroke (i.e., not too deep) while still providing an adequate amount of key aftertouch.
Also, I suspect just from common sense, certainly not from any professional experience, that one would aim towards a greater blow distance (towards the 1 7/8) as long as let-off and after touch are preserved. Seems to me that one gets in this way both power and subtlety--if everything else is OK.
Now my piano is in a small room; so here's what I think might be an exception: if I push the blow distance to 1 7/8", this grand gets quite loud. Fine--great--in a concert hall; but not in a small room. So I'm playing with something in the range of 1 3/4".
Generally small variations in hammer stroke—by themselves—do not have a great effect on the acoustical power produced by a piano. Assuming, of course, that the rest of the action parameters are regulated accordingly. If you really are hearing a significant variation in acoustical power—as opposed to a perceived difference—I’d be looking somewhere else to find out why.
Subtlety is achieved by making sure all
of the various regulating parameters are precisely and correctly adjusted to balance with the chosen hammer blow distance.
There is nothing magical about this; it’s really just a matter of several different lever ratios working in synergy with each other. Unless you are prepared to modify the overall action ratio—Not advised unless something is screwed up in your action and you really, really
know what you are doing!—then the appropriate hammer blow distance ends up being a compromise between the physical location of the hammer relative to the bottom of the pinblock and the rest rail (or cushions) at the back of the wippens and the desired amount of key travel and aftertouch.
In most pianos there is enough room to move the hammers a little closer to, or further away from the strings than is specified by the manufacturer. It is OK to experiment with this as long as you remember to compensate for the changes in hammer blow distance with suitable adjustments in key travel and aftertouch.
I suspect that if you fully re-regulate the action after changing the hammer blow distance you won’t find all that much difference in overall acoustical power. You will notice a change in how the action feels and this is often translated into a perception change in how the piano sounds.
A real discovery for me was the fact (as I said above) that the string height actually varies from section to section, and even within sections!! Sure, the wound strings will vary in thickness: that's obvious. But I had no idea that I couldn't just set the trichord hammer line smooth from top to bottom (although that's the way it came from the factory.)
Yes, the string height, as measured from the keybed, can vary considerably from section-to-section as well as within a section. This is a function of how “flat” the casting patterns and molds are, how the string frame twists and warps as it cools and how it is machined and drilled. In theory modern string frames should be somewhat “flatter” than their predecessors because the machinery doing the cutting and drilling is now often NC (numerical) or CNC (computer/numerical) controlled. Many string frames are now cast with a little extra iron in critical areas and the milling/drilling machine simply hog their way through cutting off the excess and leaving the V-bars and agrafe drilling seats relatively flat. But, obviously, this is not always the case.
In terms of a piano’s performance a string frame would have to be wildly out of square before there would be any noticeable degradation of tone.