Untapered Soundboard Ribs

[Delwin D Fandrich]

----- Original Message -----
From: "Farrell" <mfarrel2@tampabay.rr.com>
To: <pianotech@ptg.org>
Sent: July 18, 2002 3:48 AM
Subject: Untapered Soundboard Ribs


> The Mehlin grand I worked on the other day had untapered ribs on its
soundboard. Ribs on other pianos are somewhat abruptly tapered near the
ends. Is that so the panel will tend to move as one unit and flex around the
perimeter - somewhat like a speaker cone might move with its firm cone and
soft surround material that connects the cone to the basket (I kinda don't
think this is really a good analogy). It would seem to me that untapered
ribs are quite a departure from "the proper way to build a soundboard".

You do raise some interesting questions. It is my belief that ribs in the
'modern' piano are end-tapered the way they are to accommodate the practice
of compression-crowning. In this process the rib actually resists the
formation of crown--i.e., the rib wants to remain straight but is forced
into a curve by the stress interface created by the expanding soundboard
panel. The rib must be kept to a reasonably consistent height and thickness
if the crown radius is going to be anywhere close to uniform.

Even as most manufacturers began converting from compression-crowning to
rib-crowning techniques (or some combination of the two) the traditions of
rib sizing and feathering carried over.

The problem, of course, comes at the parameter where the tall ribs will
restrict the motion of the vibrating panel. More or less--considering the
range of motion of the typical soundboard, however, there really isn't a lot
of bending going on around the parameter of the soundboard. So, in real
life, that untapered (un-feathered) rib may not matter all that much.
Indeed, it may well help through the killer octave region.


>
> If soundboards vibrate their various frequencies in small random
(seemingly) areas of the board...

Don't confuse reality with desirability. Just because the typical soundboard
does break up into a myriad of miscellaneous resonances doesn't me this is
the ideal. Or that this is desirable.


> ...why do we taper ribs like we do (the abrupt taper near the end). Why
not an even taper along the entire rib? Or no taper?

Why not indeed. Have you ever seen the ribs on one of our soundboards? It's
been some years since we've feathered anything in the conventional manner.

Del