Negative crown

[Ron Nossaman]

>What would happen if you recapped bridges to gain downbearing on an old
>piano with a soundboard with no crown left, or even with an oilcanned
>soundboard?  Would the soundboard totally collapse?  

*The soundboard has already 'collapsed' if you see negative crown. 



>Del Fandrich says that
>a soundboard does not act like an arch, so crown does not help support
>bearing load, right?  

*Del's not the only one who says that, and as for crown not supporting
bearing load... let's see. This is very roughly the way it works, as I
understand it. String bearing load could be supported just fine by flat
ribs, so the soundboard assembly is negatively crowned under load. Trouble
is, load isn't the only factor of concern here. Acoustically, we are
concerned with the impedance match between the strings and the soundboard
assembly. Impedance is a combination of mass and stiffness. Crowned
soundboards stiffen under load, so the assembly can be built lighter and
still support a given string load. Since compressing a crowned board
stiffens it, it also raises it's mechanical impedance to more closely
correspond to what the strings produce. Over time, the panel compresses, and
the ribs deform. This is a cumulative compression set, and is irreversible.
By the time the board has flattened, or below, it is not stiff enough to
maintain crown under string load, and the impedance is too low for the
strings. Raising bridge height, or lowering the plate, to increase string
bearing on a board that has collapsed still doesn't do anything to increase
that lowered impedance, so it really doesn't cure the acoustical problems,
even when you can measure positive bearing at the bridge..       

>Would the tone improve?  Wasn't there a piano made in
>the past with negative crown?  I realize that one should replace the
>soundboard in these circumstances, but some pianos aren't worth the
>expense.
>
>Kevin Aldrich
>

It would possibly sound a little better just from the new strings, but it
isn't a fix. Since the piano isn't worth a new board, I'd think it would be
in the customer's best interest not to waste money on attempted fixes of the
dead soundboard by adding bearing. I'd rather see them save their money
toward the purchase of a piano that IS worth the money. That's my call. 

Del has published a good series of articles in the PTJournal explaining this
in much better detail. It's worth looking them up and reading. 

Regards, 
 Ron