[pianotech] Increasing bridge height

[Will Truitt]

Hi Del:

Thank you for your further comments.  I would like to ask you the same
question I earlier posted to Ron Nossaman:  If you added say 5 mm. to the
top treble to get it up to 30 mm total height, would you be adding that same
5 mm all along the treble bridge?  Would you increase the thickness of the
pinblock by the same amount and therefore raise the string height an
equivalent amount?

Will 

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Delwin D Fandrich
Sent: Tuesday, March 24, 2009 1:53 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Increasing bridge height


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