A fine and easy way to measure crown is to simply stretch a heavy thread along the back of the soundboard panel between ribs (parallel to the ribs). Hold the thread against the panel at the opposite edges and observe any gap between the thread and the panel near the middle - that will by your crown (or lack thereof). If you can pick the ends of the thread up off the panel near the panel edges and the thread still touches in the center, then you have reverse crown. It is most useful to measure crown between/adjacent to each rib. Holding a light at an angle to the panel will help you see any gap between the thread and the panel. Different belly designs and construction methods will target differing amounts of crown. It will vary quite a bit, even within one design. Unless you designed and built the piano yourself, it's pretty hard to identify any "bad" numbers. Most pianos will have less than a quarter-inch of crown when strung. The amount of crown, or lack thereof, must also be considered along with string scale tension and downbearing (front and rear). Whether a piano is "good" or "bad" with a given amount of crown can also depend on soundboard panel thickness, stiffness, grain density, grain angle, rib stiffness, rib crown, etc., etc. Some pianos were designed to have negative crown. Bottom line for most pianos is to simply measure the crown in a number of areas on the soundboard - especially in the "killer" octave area. Measure downbearing in the corresponding areas. Also listen to the piano for power, dynamics and evenness across the scale. If the piano sounds bad, has negative or no crown, and little or no downbearing - you got a problem - the piano belly is shot. If the panel has an "S"-shaped cross section, it is likely toast. That piano will likely have bearing on one side of the bridge and negative bearing on the other - or something approaching that. But many other combinations of these conditions are not necessarily bad. I have a Boston GP-178 that has a soundboard panel that is as flat as a pancake - no crown (at least in the top half of the string scale), but it has nice even moderate downbearing throughout, and the tone is as great or better than any Japanese piano I have ever heard. Because it has good bearing, you know that if you unstrung the piano, the soundboard would bend up and you could then observe crown. Your question really requires a book to explain everything (Del, where's that book on piano design?!). There are many parts to the piano belly whose function and interactions need to be understood in order to fully understand the implications of any crown measurement. Alone, a crown measurement most often doesn't tell you much. I will often look closely at crown and downbearing, etc. when diagnosing a belly problem, I also work very closely with all that when setting up a new piano belly. However, I have found that when inspecting an low value piano for someone - like for a prepurchase inspection on a 1948 Gulbranson spinet, or somesuch - there is usually no reason to even look at the soundboard crown and/or downbearing. Who cares? What difference does it make? If the piano sounds good (or as good as can be expected), the piano sounds good, and that is good enough. If the piano has a bad killer octave, who cares what is causing it - the piano sounds bad. A detailed analysis of belly components often won't get you much in return - I usually just check that the parts are there and are not falling off or cracked in half in a situation like that. Have you read the pianotech archives? There is quite a bit there about soundboard function. Do you have the PTG Journals on CD? Del Fandrich has several very good articles on soundboard function. Hope this helps a bit anyway. Terry Farrell ----- Original Message ----- > 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. > But I don't know what constitutes a GOOD or a BAD measurement, or WHY, > exactly. > > Here's the method I read about: You take a straight edge as long as the > back of the piano (I'm assuming they meant an upright), attach to EXACTLY > 7" blocks at a right angle to each end, then measure from the bottom of > the straight edge to the soundboard at the mid point. Subtract the > middle measurement from 7" and that's the amount of crown. > > I'd love to hear about other ways to measure. And I want to know if this > is also how one measures crown on a grand piano. Is the grand measurement > taken paralell to the keyboard? Is there another measurement taken > perpendicular to the grand's keyboard? Or vertically on an upright? > > 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? What happens to sound and performance with too little crown? Is > there such a thing as too much crown? > > Did I miss anything? > > THANKS in advance, > John Dorr
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