downbearing

[rhohf@eagle.idcnet.com]

As it happens, I am preparing a class called "Stress and Resonance" for the
Central East seminar in Milwaukee, and have been reviewing material on various
topics including downbearing.  I have been enjoying Mr. Hartman's articles
(they are timely to my preparation) which present some interesting ideas.  For
those who have not seen them already, I would highly recommend "Five Lectures
on the Acoustics of the Piano" edited by Anders Askenfelt.  My copy came from
Steinway several years ago, and it is probably still available there since
this is not the sort of thing which sells out quickly.

In his introduction, Askenfelt presents a clear  description of impedance,
matching and mismatching, and the effect on volume and sustain, with (in my
opinion) due respect for the elegance of the resonance system of the piano.
When an expert on acoustics lays out the constraints that physical reality
imposes upon the piano, I think it is a good idea to accept what he says as
true.  T he principles of impedance seem to say, "You can have either sustain
or volume, but you can't have both."  However, making pianos with both sustain
and volume has historically been a fundamental goal, which calls for a closer
look at the idea of impedance.

In my experience, applying physical principles to simple and controlled cases,
and then applying the same principles to the (extremely) complex cases which
exist in pianos, is never a direct progression.  There always seems to be a
twist which causes unexpected results.  Consider a piano with a total
string-on-bridge downbearing of 800 lbs. (this is a number one hears tossed
about frequently).  The principles of impedance seem to dictate that
increasing this downbearing to 850 lbs will increase volume and decrease
sustain, and decreasing downbearing to 750 lbs. will have a converse effect.
This cause and effect relationship is probably true, and would be difficult to
verify.  But it does not rule out the possibility of >increasing both volume
and sustain< without changing the total 800lb load on the soundboard.

This effect is possible because the total 800 lb. load can be distributed
across the length of the bridges in many different ways, with some load
distributions much better than others.  I would like to venture the opinion
that the distribution of the downbearing has a greater effect on the overall
response of an instrument that the total load.  This is a big topic and I
would be very interested on other opinions.

A word on total load.  Mr. Hartman mentions setting total load in the range of
1000-1500 lbs.  Can you describe how you determine this? (If this will be
covered in a future article, I can wait).  I once heard Andre Bolduc state in
a class that downbearing in a piano totals 10,000 LBS(!).  This was not a slip
of the tongue because someone questioned the number and he insisted.  I
believe this was in Toronto, so may be outdated.  In my own work, I feel even
1000 lbs is too much and set the total load (evenly distributed
load/bridge-length) to less than 750lbs.

Bob Hohf