Bridge pressure bar

[Delwin D. Fandrich]

----- Original Message -----
From: "Alan Forsyth" <alanforsyth@fortune4.fsnet.co.uk>
To: "Pianotech" <pianotech@ptg.org>
Sent: Saturday, April 12, 2003 9:04 PM
Subject: Bridge pressure bar


>
> Greg wrote
>
>        >  " In my view the only thing that you can assure with this
pressure
> bar is string termination. At some point , though, you will simply crush
the
> string into the bridge body and wind up with poor termination. "
>
> No! The string does not come into contact with the wood at all. There are
> bridge inserts on either side of the pressure bar, same as on a guitar, or
> similar to the rods inserted at the top bridge. The string is threaded
over
> the front insert, then under the bar and over the rear insert.
>
>          > "I still don't understand what this has to do with downbearing
> unless you are somehow adjusting the height of the strings with this bar,
> although I don't see how. Is the bridge body somehow height adjustable? "
>
> You can adjust the pressure of the bar same as on a normal top bridge.
> It does not affect string height.

After thinking about this a bit I rather wondered if this was not the
case--the earlier attached image was too small for this to be clear. While
the system surely does ensure positive string contact (of a sort) to the top
of the bridge, it would have no means of adjusting string bearing (or
"downbearing). The overall string height will remain the same no matter what
you do to the pressure bar. As will the overall string deflection angle
across the bridge.


>
>           >"The second point you have is about a soundboard being able to
> be made perfectly flat as long as sufficient downbearing exists. The two
are
> interrelated and depend upon each other for good piano tone."
>           >"If you had a piano with poor downbearing but plenty of crown
the
> tone would suffer. In like manner if you had a piano with plenty of
> downbearing but poor crown you would, again, have poor tone."

>
> Correct me if I'm wrong but crown in a soundboard is a method of resisting
> the pressure of the strings i.e an arched object is much stronger than a
> flat beam.

I don't like the term "stronger" here. The soundboard system will not be
stronger but it will be stiffer with some degree of string bearing. The
whole idea of loading the soundboard with string bearing is to make it
stiffer (or "springier").


>
> ... String pressure is used as a means of ensuring good contact with
> the bridge so the string vibrations can be transmitted to the soundboard.

Well, not really. The coupling of energy from the string(s) to the bridge
takes place primarily at, and because of, the string offset between the two
bridge pins. Remove the bridge pins (or the pressure bar and the termination
pieces in the case of the piano under discussion) and no matter how much
soundboard crown and/or string bearing there is energy coupling between the
strings and soundboard will be poor.


> As
> far as I know it is this string pressure that is known as downbearing. If
> you use downbearing as a method of bridge contact then you need crown in
the
> soundboard to resist or counteract the pressure (otherwise the soundboard
> would simply go concave and negate the downbearing). If you used a
different
> method of string contact with the bridge,(i.e. not use string pressure)
then
> you would no longer need a crown in the soundboard.

Hmmm. There seem to be several misconceptions operating here. As mentioned
earlier, string bearing operating against a soundboard with or without
crown--i.e., as long as there is string deflection and this string
deflection is applying some downforce against the bridges--is a mechanism
for making the soundboard assembly stiffer, more springy, than it would
otherwise be. This is still true even if the soundboard is made in such a
way that it bellies back away from the string plane. That is, having
"negative" crown. Several manufacturers have successfully built pianos this
way.

If you use some other method of maintaining efficient string coupling to the
top of the bridge the same principle still applies. The soundboard system
will be more flexible if it does not have a string plane bearing against it.
It will be stiffer if it does. There are two different principles at work
here. They are only loosely related.

Del