[pianotech] Increasing bridge height

[Delwin D Fandrich]

As with most things having to do with piano design there is no perfect bridge
height. 

The bridge is an integral part of the overall soundboard assembly and, as such,
changing the height-or any other physical parameter-of the bridge will change
the acoustic performance of that assembly. 

In my original comment on this topic I said I prefer working with bridges that
are a minimum of 30 mm in height at C-88. As the original question concerned a
Steinway Model A-2 I believe this to be an appropriate answer. But now the
question seems to have become, "if a 30 mm bridge height is good won't 40 or 50
mm be even better?" 

In very broad terms-leaving out the issue of string height relative to the
action-changes in bridge height will affect both the transverse stiffness of the
bridge and its mass. Assuming its width and construction material remains the
same, a taller bridge will be both stiffer and more massive. As to whether or
not this is a good thing depends on a number of soundboard variables such as the
relative thickness of the soundboard panel, the number and location of the ribs,
the height and width of those ribs, soundboard assembly crown, etc. It will also
depend on the overall tension of the string scale. This is definitely not a case
of "one-size-fits-all" and a bridge height that works well in one piano design
won't work at all well in another.

Generally speaking a piano with a relatively low tension string scale (such as
the Steinway Model A-2 in question) will work well with a bridge having a
nominal width of about 32 mm and a height in the 30 - 34 mm range. (Starting
with that 30 mm height at C-88 they will usually get taller on down into the
tenor.) Trying to fit a bridge much taller than this into one of these pianos
will probably lead to action geometry (and/or hammer bore) problems. Everything
else being equal it will also overload the string scale and give the piano a
somewhat strained sound; overall sustain would be good but the attach would be
weak and energy at the fundamental would be some lacking. 

A piano with a relatively higher tension scale and an under-designed rib scale
can accommodate the greater mass of a taller bridge, though the same acoustic
results can probably be obtained through properly sizing an appropriate rib
scale. 

A soundboard design using fewer ribs can also usefully use the added stiffness
of a taller bridge though a better approach would be to design and install a rib
scale having a reasonable number of appropriately placed and sized ribs.

ddf