(Long)entrainment, impedance, and falling arches

[Brent.Fischer@ASU.Edu]

Dear Ed and list:

    Thanks for your insights. Your perspective gives me a new understanding
about what I have been doing with Steinway soundboards. By maximizing
the compression, which is different from one board to the next, I see that I
have tried to optimize the potential output of each instrument. Your're
right, there is an art in the procedure of getting the load matched to the
structure since compressing the board and planing the bridges with 2.5
degrees of front
to back bearing does demand some measure of thoughtful consistency.

    The way to maximize the energy displaced by the board is to compress
the board in the piano when setting downbearing by wedging the board under
the middle plate strut until it is almost flat, simulating the loaded
condition. Each board reacts differently, so if I were to use a predetermined
equation, I think in many cases the end result would be less than the ideal
amount of compression. What Steinway does then is a balanced relationship
between downbearing and the strength of the arch.

  I dry soundboard panels to around 4.5% MC before pressing with flat
sugar pine ribs into 54' radius cauls, then allow them to take on crown to
10 mm. measured along the longest rib before clamping into the rim for
downbearing adjustments.  Once clamped into the rim the crown is
automatically reduced by half and then once loaded it will result in
about a 2 to 3 mm. measurement of crown showing. This formula has been
fairly consistent. The bevel on the inner rim is also an important
consideration
which I think averages around a couple of degrees.

  Yours thoughts about the coupling effect of case parts is well taken.
In a similiar question brought up a few threads ago, concerning rim
stress, Mike Mohr has told me that the rim on their instruments has never
been intentionally spread to produce this effect. If anything he says it
is just a natural by-product of the construction to have the sustain effect
of the rim as described earlier. When the beam and keybed structure is
comprised of spruce that is meant to provide residual resonance, it is
difficult to scientifically measure the contribution of each. As a test,
it is interesting to pluck the keyframe return spring and listen to the
tonal quality and sustain that it produces. As spruce keybed rails age
they should contibute more volume and sustain when this test is done.

 Well, thanks again for taking time last Saturday morning to contribute
to the forum for what I think is great and valuable information. Brent






====
Brent Fischer, Piano Technician
School of Music, Arizona State University
Tempe, Arizona 85287-0405
602/965-6760 FAX 602/965-2659 Msg 602/965-3371
http://www.asu.edu/cfa/music/