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
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