evaluating sdbd. crown & bridge downbearings in a new piano

[Ron Nossaman]

At 12:57 PM 9/20/99 EDT, you wrote:
>I'd appreciate feedback from experienced techs regarding the measuring and
>evaluating of soundboard crown and bridge downbearings as well as the 
>importance
>of these readings along with the impact of factory hammer voicing on the 
>quality
>of tonal production on a new high quality instrument.
>
>Yesterday I examined, while a client played, 6-7 new & "slu" $52K  pianos in 
>NYC.
>I measured crown with a thread adjacent to the longest rib & bridge bearings 
>with
>a Lowell bubble gauge. Of the 2 best sounding (treble sustain, singing 
>quality and
>sonority) one had very "positive" measurements while the other a nearly flat 
>board
>with a slight rise toward the treble side.  The "positive" one had obviously 
>been 
>voiced more brightly while the second one projected beautifully with less 
>brilliance.
>
>I'm also curious as to the long range effects of what appear to be "pressure 
>ridges" -
>the client even noticed these and asked me about them.
>
>Bruce Greig, RPT
>


Hi Bruce,
First off, what will measuring the crown at the longest rib tell you about
the clarity and ring time in the treble? It will only tell you the crown
under load at the longest rib. You really need a more complete picture of
the crown/bearing relationship in as many parts of the scale as you can
reach to mearure. In the case of a compression crowned soundboard, the board
generally goes flat in the killer octave (5-6) first. That's where you will
probably hear the difference first, and that is one of the first areas you
should be checking for crown and bearing. Also, what were the bearing
readings in various parts of the scale in each piano? The cause and effect
relationships between crown/bearing, and the sound quality produced have a
lot to do with how the crown is formed during assembly, and a bunch of other
factors, including hammer voicing. 

A compression ridge is a compression failure. The panel has tried to expand
beyond what the ribs and string bearing load will allow and the softer early
wood growth rings crush first. Compression ridges during humid periods are
where cracks will likely eventually appear during dry periods. Compression
ridges are pretty inevitable where there are seasonal humidity swings, but
they are much more severe in compression crowned, rather than rib crowned,
soundboards. 

 Ron N