Piano Rims (rambling post)

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip L Ford" <fordpiano@lycos.com>
To: <pianotech@ptg.org>
Sent: January 04, 2002 3:55 PM
Subject: Re: Piano Rims (rambling post)


> >Del
> >
> Thanks, Del.  I think that what was confusing me is that we're talking
about
> two situations.  One is the situation where you want (controlled) energy
> transfer and the other situation is where you are trying to avoid energy
> transfer.  In the case of the soundboard, where you want energy transfer
> from the string to take place, you don't want stiffness in the bass or
mass
> in the treble (correct?).  But in the case of the of the termination at
the other
> end of the string or the case of the attachment of the soundboard to the
rim
> you don't want energy transfer to take place so you do want stiffness in
the
> bass and mass in the treble.  Sound right now?
>
> Phil F


>
Well, in the soundboard assembly especially, you want a balance of stiffness
and mass. Obviously, this is a real-world system so there is always going to
be some of each. At issue is coming up with an acceptable--a
workable--balance.

For example, if you have a soundboard system with a really narrow (about
22 - 24 mm) and short (that is, not tall--about 24 - 25 mm) bridge with a
laminated body of alternating layers of mahogany and spruce and the piano
has a sharp, percussive tone coupled with a relatively short sustain through
the treble, stiffer ribs aren't going to help much. Neither will it help all
that much--some, surely, but not enough--to make the soundboard thicker.
(The piano in question had a conventional rib system and a soundboard that
was 9 mm thick in the treble tapering to about 8 mm at the bass end.) The
problem was the relatively low impedance of the system to the high frequency
vibrating energy in the strings and that was due primarily to the
excessively low mass of the bridge. Just adding more stiffness wouldn't have
solved the problem. Giving the piano the mass of a maple (beech would also
have worked) bridge that was 32 -35 mm wide and about 30 mm tall (yes, the
plate had to come up a bit) on an 8 mm thick soundboard eliminated the
overly percussive attack sound and picked up the sustain nicely. (The ribs
were redesigned but were left in their original locations.)

Conversely, if you have a bass section with the bridge mounted right back
there close to the rim and you find you aren't hearing much in the way of
bass fundamental in the tone envelope it isn't going to do much good to
drill holes in the bridge body even though that will reduce mass. Here the
problem is the relative immobility of the bridge body due to the high
stiffness of the soundboard panel itself--follow the grain line from the
bass bridge to the inner rim--and to the relative shortness of the string
backscale. This system has very high impedance at low frequencies. Several
possible solutions would be to thin the soundboard panel (as is common
practice), to cut it free completely (as we are now generally doing) and to
increase the length of the string backscale (convert to vertical hitches).
With these procedures you're reducing the stiffness component of the
impedance relationship.

Of course, it is easy to go too far with this and alter either the stiffness
or the mass component of the impedance relationship too much. In the case of
the above described bass section, for example, if you find yourself with too
much bridge mobility--i.e., too little low frequency impedance--sustain will
suffer and the tone envelope will take on a decidedly percussive sound. The
fundamental and lower partials will die out too quickly. Adding mass to this
system will not help--at least not any reasonable amount of mass. You would
then have to figure out how to add back some of the stiffness you took away.

Yes, your comments about the rim are right...at least as I see it.

Del