Sound Board Crown

[A440A@aol.com]

Greetings all,

In response to my posting concerning downbearing pressure, i.e.,
>       Amount?  Seems like all pianos have their own optimum about of
bearing
>pressure.  It must be enough to tighten the whole structure, but not so much
>that the sound board has been forced to it's elastic limit.

Tom writes;

>I just HAVE to be a little nit-picker here, and point out the distinction
>between bearing and crown...

Sloppy editing on my part,  I intended to write; "Seems like all pianos have
their own optimum amount of bearing pressure, and it must be compatible with
the crown."

Tom continues;
>Crown ...<snip>
>Bearing ....<snip
>These two measurements are independant of each other.  A piano can have
>lots of crown and no bearing, or lots of bearing and no crown, or lots of
>crown and back bearing - but no front bearing. etc...

     I  think of the crown and downbearing as two components of an acoustical
containment and production system. In this sense,  they cannot be viewed as
separate.
     As repair people, we often have to decide how much board loading we are
going to create in a piano.  How much pressure will a board best respond to ?
  It is not that difficult to flatten the crown with excessive pressure,
Especially on an old soundboard that has settled down .
    If I have a soundboard with 1/8" of crown, I will set my beginning
downbearing at 1/2 of that.   With nosebolts turned to zero plate flex,  and
with strings under tension,  I am then able to see if half of the crown has
gone,( the usual result).  If so, and I have a responsive sounding structure,
I give thanks and leave it alone.  If there is no change on the crown, as
though the board is "stiff",    I can usually find some additional
downbearing pressure with the nosebolts to increase the sustain, and often
the power.
      I haven't cracked a plate yet, but I keep a magnetic base dial
indicator on the plate to know exactly how far I am bending these things.
Virtually any plate can go up or down .030" , and that seems to be enough for
most bearing considerations.

    Certain models of the smaller Chickering grands have flexible inner rims.
 The method of setting downbearing by measurement of crown  should  be
altered  for these , as for some reason, the bass bridge seems to need a lot
more bearing than the rest of the piano, ( which is backwards to most of what
I think I have observed!)
    For these, the sound seems to favor less pressure on the treble bridge,
so that the bass can use the board's flexibility.

Regards,
Ed Foote
Precision Piano Works
Nashville, Tn.