Killer Octave Question

[Ron Nossaman]

>Greetings,
>    The killer octave is usually supported by the section of belly rail that
>is farthest from buttressing.  The normal connection of the plate to the
>belly rail at the plate's "horn" provides resistance to outward movement, and
>the rails connection to the case at the treble end, likewise.  However, the
>ribs that must support crown under the killer octave bear against the
>bellyrail at its greatest unsupported span and it just might be one reason
>that this section of the piano loses its bearing first.  (The rib under the
>C5 on a Steinway O meets the bellyrail approx. halfway between the plate horn
>and the treble end of the rail).
>    Many other reasons could go into the equation, too.
>
>Ed Foote RPT


Soundboard crown still isn't supported like a buttressed arch, Ed, and rim 
or belly rail spreading the few thousandth's difference between the length 
dimension measured straight from rib end to rib end, and that measured from 
rib end to rib end over the crown won't make the crown go flat. It just 
won't. Flexibility in the belly rail and rim will certainly damp the 
soundboard and affect tone production adversely, but the crown is formed 
and supported by rib and panel, independent of the rim. What drives the 
soundboard flat in the killer octave first, I think, is load and leverage. 
The affected area of the scale is in the part of the bridge where the bend 
is greatest. If the bridge was straight, it could act as a rib and help 
support string bearing load in that area. But it's bent, so as it's loaded, 
it rotates. This is why crowning the long bridge is of little benefit - if 
any. It doesn't help support crown. The extreme treble is sitting on a part 
of the board that just isn't going to deflect all that much no matter what 
kind of load is put on it. It's essentially sitting on a brick, which acts 
as a fulcrum. Bearing load between the killer octave and the "brick" forms 
a third class lever that transfers a portion of the leverage down scale to 
the killer octave region, which already has it's own bearing load to 
contend with. As a consequence, that area is getting a whole lot more load 
than we are aware of, and since the bridge is curved there and doesn't 
supply straight line support  commensurate with the support the fulcrum is 
supplying up scale of the load, the bridge rotates down, bushing the crown 
down and concave. It will even push crown down far enough to leave both 
concave crown and negative bearing in the killer octave - wherever, 
precisely, it should fall in a given piano. This is naturally much worse in 
compression crowned boards than in rib crowned.

Ron N