Downbearing----Sustain?

[HartmanCJ@aol.com]

Michael,

Thank you for your encouraging words. The frequencies and soundboard mode
shapes in part one are from my own crude experiments in the shop using a tone
generator and speaker. Salt was distributed on the soundboard surface and the
first seven or eight modes where found. I have done this on a number of
soundboards. The frequencies and shapes as shown in the article are, more or
less, a compilation of several tests. The exact frequencies and shapes are
not essential to my argument only that the surface divides into smaller areas
with higher frequencies.
The frequency shift in the soundboard modes is based on simple logic. If the
stiffness is increased and the mass remains the same then the frequency will
increase.

Frequency equals the square root of the stiffness divided by the mass.

This is similar to the frequencies of a wire shifting higher as the tension
is increased.

Your idea about the bridges playing a part is well noted and I do agree that
the shapes of the modes could be altered.  I dont think that in a modern
soundboard, with crown and a relatively thick panels, the bridge will effect
the modes in this manner to a great degree. If the soundboard panel where
much thinner, as on antique pianos, I think the bridge will tend to divide
the surface in the way you described.  I base this conviction on direct
experience with soundboard building not on direct scientific proof.

I applaud your experimental efforts. The differing results may be due to the
amount of pressure applied in relation to the size, shape and thickness of
the soundboard. You didnt say how large a soundboard you have attached to
your model. It may take a large amount of bearing pressure to effect the
sustain. The total amount of bearing force on a typical piano is from 1000
lb. to 1500 lb. On your model, with only one string,  your soundboard would
have to be very thin and small. Remember that my argument isnt that the
bearing angel changes the sustain but that bearing force will alter the
impedance of the soundboard and bridges and that will effect the sustain.
I appreciate your interest but you may wish to read more of the article
before commenting on the lack of references. For the sake of simplicity I
decided not to include technical references as points were made, rather
waiting until the end of part three to make a list of text references. Also
my article is not intended to be proof of the theories or to be a scientific
paper. After all, I am a piano rebuilder not a rocket scientist. What I had
in mind was a review of the basic information available and how it may apply
to piano theory and to put this information into a form that is assessable to
the average technician with an interest in piano design and rebuilding.
Please make further comments after reading part three and four. Your input is
appreciated.

John Hartman RPT
Hope to see some of you at the North Carolina Conference, November 2-5.