Soundboard Crown measurement and evaluation

[John Dorr]

Good Morning List,

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