Aprons and holey bridges

[Delwin D Fandrich]

----- Original Message -----
From: "John Delacour" <JD@Pianomaker.co.uk>
To: <pianotech@ptg.org>
Sent: February 27, 2002 2:47 PM
Subject: Aprons and holey bridges


> The
> cutting out and dowelling at the tenor of a Steinway O could hardly
> be intended merely to lighten the bridge in that section.  I've not
> had time to study exactly what is aimed at here, but maybe you can
> explain it.

I'm not sure I can. It is my impression they were trying to alter the
coupling efficiency of the bridge to make up for rather poor scaling through
the low tenor. These pianos (including the S, M, and L) are all better off
with a transition bridge and bi-chords running well up into the tenor. My
redesigns of these scales uses six. And we replace the bridges with solid
bodies—no cut-outs.


>
> I've read a few anti-apron posts of your and Ron N.'s and would like
> to hear precisely what the argument against the apron is.  I have
> seen so many pianos that behave excellently with a suspended bridge
> and yet from the very beginning of my career it has always seemed to
> me that a direct bridge was preferable, maybe even in spite of the
> evidence!

Of course, decent pianos have been built using bridge cantilevers. That is
not to say they wouldn't be better without them. The problem, of course, is
that once the piano is designed and built it is pretty hard to tell what it
would have been like some other way. We regularly replace cantilevered bass
bridges with solid bridges when the plate design allows. That is, when we
can significantly shorten the speaking length of the bass strings
(simultaneously increasing the backscale length) and mount the bridge some
further in on the soundboard surface. In some cases we will undercut the end
of the bridge as a compromise between a solid bridge and a cantilever. Not
ideal, but less bad.

The bridge cantilever was introduced to allow bass strings of the longest
possible speaking length on pianos of a given length while allowing for the
attachment of the bridge to the soundboard further in where everything is
somewhat more flexible than right next to the inner rim.

I dislike bridge cantilevers for a number of reasons:

1)  The idea of placing strings having the longest possible speaking length
on a piano of given size is itself flawed. It ignores the necessity of a
reasonable backscale length.

2)  Bass bridge cantilevers reduce the efficiency of energy transfer between
the strings and the soundboard assembly.

3)  They absorb—and dissipate as heat—low frequency energy from the strings.

4)  They cause physical distortion in the soundboard. This is especially so
with compression-crowned soundboard assemblies. (I published an article with
a drawing describing and illustrating this some time back in the Journal but
you can probably visualize what happens just by looking at the system.)

5)  There are better ways of accomplishing the same thing if it is required.
One method is to simply undercut the tail end of the bass bridge.

Del