Negative crown

[Doug Richards]

Ron,
I have a question for you.  You said that when the crowned soundboard is
loaded by the strings, the impedance goes up.  I would agree with that.  The
question is from the string perspective.  The piano has strings with
frequencies ranging from ~50 Hz to ~4kHz and the I imagine the soundboard
has 30-50 natural modeshapes in that range.  So my question is: with such a
broad range of soundboard and string frequencies, what is the soundboard
impedance "matching"?

I don't really have an answer to this either...


doug richards
San Jose, CA


> -----Original Message-----
> From:	Ron Nossaman [SMTP:nossaman@SOUTHWIND.NET]
> Sent:	Tuesday, December 29, 1998 9:05 AM
> To:	pianotech@ptg.org
> Subject:	Re: Negative crown
> 
> >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