> Time for another query from the list of what's been buggin' me. I would > like to know how to evaluate a soundboard's condition beyond cracks. > I'm aware of one method of measuring the crown after searching the > archives. A string stretched from rim to rim between ribs, and a flashlight to cast a shadow you can see will tell you what the crown is. Take this measurement at every rib you can reach, not just once at the longest rib. > But I don't know what constitutes a GOOD or a BAD > measurement, or WHY, exactly. Ideally, you'd want positive bearing and positive crown, but if board is stiff enough to sound good with positive bearing and little or no crown in the killer octave, you often won't hear a problem. Crown was originally intended to provide extra stiffness without extra mass and provide an opposing spring to the string downbearing on the bridge. Listen to the piano. Soundboard produced tone problems (from insufficient stiffness) tend to show up in the octave 5-6 (killer octave) as short percussive attack, often with short sustain. Since the killer octave is where you will find most of your problems, consider it first. If it's a field service situation, don't bother taking any measurements unless your, or your customer's ears tell you there is a problem. Then take measurements to try and diagnose it. Evaluating a soundboard for someone else or for restringing, you need to be more thorough and cautious. A marginal board will sound better, often much better in high humidity seasons than it will in low humidity seasons. A piano with near zero bearing and no crown in the KO that sounds reasonable in humid weather, can easily sound really bad in the heating season. If the sound is poor where the board is collapsed and bearing is minimal to negative, you aren't going to fix it as a field repair. > Is there a formula I need to figure the radius of the crown based on the > length of that (7" removed) tangent? What are "good" numbers? "Bad" > numbers? No magic formula or checklist. It's essentially a balancing act between compression of soundboard crown and string bearing. You will need a fairly clear idea of how soundboards work to make an evaluation you can feel confident with. >What happens to sound and performance with too little crown? Depends on the string bearing and soundboard assembly stiffness. There might be no obvious performance penalty at all with a dead flat board. > Is there such a thing as too much crown? Yes, when the lid won't close. > Did I miss anything? Libraries worth. There are tons of detailed explanation of this stuff in the archives. Ron N
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