It's a difficult and complex subject because it ties into so many other aspects of soundboard performance and design. There were a series of articles written by Nick Gravagne in the PTJ back between 1987 and 1990 (at least) under the heading "Good Vibrations". One of the articles is called "How Much Crown Should There Be (April 1987). There are some other related articles written at that time that are worth reviewing. If you have the archives on CD it's very easy to find these.
The short answer is that failure can be determined by the sound. Rarely do I need to measure to determine that the soundboard is not performing well. Soundboards are all about controlling the rate of energy flow: impedance. Properly functioning soundboards control the energy flow in such a way that attack is modulated and sustain is maximized, or a balance between the two is achieved. Soundboards operate along a continuum in terms of attack and sustain. Lower impedance, higher levels of attack and shorter sustain; higher impedance, lower levels of attack and higher levels of sustain. There is an area along this continuum that we operate successfully in. We can push it to one side or the other but if you move outside of what we expect we won't probably like it. Thus if the board is too stiff (rare unless designed that way) you will have diminished power at the attack and probably fairly long sustain. But as the board weakens and becomes compromised in terms of stiffness, just the opposite occurs.
When the soundboard fails the tone will be percussive, lifeless, distorted and with shortened sustain. C4 - C6 is most telling. Test it especially at the upper dynamic levels. Many poor soundboards can get by at pianissimo but begin to fall apart above mf. The bass often doesn't suffer as much and sometimes not at all. Not surprising since the bass end of the piano is designed to be less stiff, more woofer like. When the piano sounds like that and I'm embarking on a restringing job or assessing the piano for one, the customer will know that a soundboard will likely be needed. New strings and hammers will not salvage a failed soundboard. I will take measurements to confirm but at a certain point it's probably not necessary. I suppose there are some rare occasions where the bearing is set so poorly that the board will react in a similar manner but I haven't been able to figure out what that is exactly. Gravagne's article gives a very way of quantifying all this and determining if there is adequate crown to be able to set enough bearing and stress the board enough to both raise the impedance level and introduce enough potential energy into the system to make it lively. Worth reading if you have not. However, I don't think Nick would suggest that when the sound of the board is telling you that it's gone that there is some way to reset the plate height in order to bring it back to life. Those cases are reserved for boards that are modestly compromised but still usable.
It's important to note, again, that not all areas of the board will necessarily sound bad when the board has failed. Don't be fooled into thinking that the board is fine because the bass sounds good.
It may be interesting to note that since the trend in soundboards over time is to get louder (at attack), have shorter sustain, with the attack/sustain relationship in the tonal envelope becoming more separated. So when replacing hammers on old boards, they will almost always benefit from something lighter and softer. You will need less energy to drive the board at the attack and the lighter weight hammer will filter less string energy (especially in the treble) and you'll at least get the best out of what sustain you have. The worst thing you can do on an old board is move to a heavier and harder hammer. That will simply exaggerate the attack and shorten the sustain byu filtering more high partial energy. Because the board is more reactive to energy input--energy transfers more quickly--you don't need as much to drive it. The lighter and softer hammer will likely produce just as much power as the previous heavier and harder hammer did when the soundboard was stiffer but with a somewhat smoother and longer lasting tonal envelope. Heavy and soft is also bad. You'll simply get a dull and lifeless "whump".
Digression, sorry.
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David Love RPT
www.davidlovepianos.com davidlovepianos@comcast.net 415 407 8320
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Original Message:
Sent: 06-12-2015 11:02
From: Richard West
Subject: soundboard assessment
Thanks, David, for that post related to soundboard cracks. Lots of good information. I've changed the subject line to a related topic, since people on this list seem to want more clarity in posting.
I've always wondered what defines soundboard failure. It's good that so many competent people are replacing soundboards these days, but as a person who has never done that and probably never will, it's been hard for me to know the factors that determine whether a soundboard is shot. I've heard from Steinway that a board loses its full tonal capabilities after just 7 years. That's partly why they tell their dealers with rental D's to sell the instrument after 7 years. That may be a lot of bull, but maybe compression soundboards are, indeed, prone to early failure.
Could you (and others) give me your top ten list of signs that the soundboard is shot. Loss of crown seems to come up a lot. Cracks, too, can be indicators of problems, or, as you say, soundboard stress.
Better yet how do I measure the gradient between a great soundboard and a mediocre one? If I have a way of measuring soundboard effectiveness, I might recommend more soundboard replacements to customers. More business for the rebuilders.
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Richard West
Oro Valley AZ
520-395-0916
440richard@gmail.com
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