chopping bridges

[A440A@aol.com]

Greetings,

Inre bridge wedging, Ted writes;

>The theory was to cut down to the soundboard
vertically under the struts, jack up the board and glue wedges in the saw
kerfs. This is intended to increase the curvature of the  of the
soundboard. I never did it as every instinct rebelled against the idea, and
later I encountered three pianos that had had this type or repair.Each one
sounded very dead.

       I have to agree with Ted about the destruction caused by sawing the
bridges.
You may notice this treatment on key frames,  I believe the technique is
called a "Dutchman" cut.  It does provide a restorative force in the
direction mentioned, but there are greater implications on acoustical
transmission.
    We understand the great difference in speed and efficiency of sound
transmission  between with the grain vs across the grain.   I believe this is
related to what happens to mechanical energy when it is transferred from one
medium to another, and until that energy is moving our TMs,  it is
mechanical.
        As you go across the grain,  the differing characteristics of the
wood layers, ( what were "rings" in the tree), cause this transmission loss
every time it goes from soft "summer wood" to hard "winter wood", ( though I
accept Cliff Geers explanation that it is more like "spring" wood vs
everything else).
     Sound traveling with the grain doesn't suffer these losses, as the
energy travels with the grain without encountering varying densities .
  Thus, the bridge is the major component in transferring the sound to the
board as a whole,  ( with a lot of help from the ribs!!).  Observe the effort
put into the continual bridges that make the treble and bass bridge as one
unit.  This is not an easy manufacturing process, but the results are worth
it when you have a large board to energize.
       Cutting the bridge, and inserting glue and more wood, effectively
damps this transmission of mechanical energy.

Richard asks;   I wonder what holds the soundboard up?

    I have always considered there to be a circle of compression/tension set
up by the downbearing force attempting to spread the crown, which is resisted
by the containment of the case, which is assisted by the connection to the
plate, which is compressed by the tension of the strings, i.e., the sound
board is held up by its inability to spread out.
       Thus we have the Centripedal tension regulator of the Mason and
Hamlins.  The "perma-crowns of some older Wurlitzers, etc.
      Simply propping up the soundboard does not increase its response to
vibration, though it may address rattling strings due to negative bearing.

      I know there are some out there that have a much more scientific
explanation,  but I am not holding my breath that our Steinway correspondants
are going to tell all the secrets that make their piano soundboards so
responsive.

Regards,
Ed Foote