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Hello Don,
>Very interesting demonstration - thanks for posting it!
>
>Can you tell us the approximate sample rib / soundboard crown radius?
Crown radius of rib no. 6 is 15.9 metres or 52.2 feet. The crown
radius of this sound board changes from bass to treble. So each rib
has its own jig for machining the gradually reducing radius from bass
to treble. The smallest radius, on the highest rib, is 13 meters or
42.7 feet. On our 225 we use a smaller 11.5 metre radius in the
treble, but our 225 rib profiling jigs are about 20 mm too short for
forming the ribs for a D, so I used the profiling set I built for the
280 piano, which are a little large in radius I think. I'm
reconsidering the 280 sound board layout yet again, even before I've
built the first instrument.
> Or if you could provide the unloaded measurement of the difference
>from the bottom of the soundboard strip (at center) and the cord
>line to give an idea of how much crown was in the sample.
Crown height at the middle, relative to ends was 5.5 mm. This rib is
loaded in a D with 17 Kg at 0.75 degrees downbearing. Using this test
load caused the rib (with both ends firmly supported) to sink 1.75 mm
under load at the middle. To derive the unstrung down bearing
settings, we calculate the down bearing in mm required to achieve the
downbearing angle (for the load we have chosen), then add the amount
the soundboard drops under that load. This figure should result in an
at-pitch down bearing which is within 0.5 mm if we have done our
homework correctly.
>What style rib was it - crowned on top?
Yes. It was an I-rib, which has the crown machined onto the top of
the center web, so the top flange of the I-rib is glued onto the
center web to form the crown radius. The bottom flange is flat in the
unloaded condition. So an I-rib gets deeper towards the centre (33.5
mm overall depth in this case), where it needs to be stronger since
the centre of a rib is where the maximum stress in extreme fibre is
encountered.
> Was there any "compression" style crowning in the combination?
No, it was fully rib crowned. But it wouldn't have mattered wether it
was rib, compression crowned or a combination of both. All styles
will pull in under the down bearing load in the same way. I've got
compression crowned models which do the same thing.
The myth of rib expansion, I believe, has come about through the use
of those neat little piano-exhibition demonstration models which
represent a small crowned sound board model with no ribs. Without
ribs a sound board panel will push out. But when the ribs are added,
substantially more overall offset of the sound board assembly is
positioned below the line between the end support points. Therefore,
a ribbed board will always pull in under load (unless it had huge
crown and very shallow ribs). Nevertheless, rim rigidity still
remains an important factor regarding tonal sustaining qualities, but
it works in the reverse way that most of us have assumed (to answer
your post 'Thump', when you mentioned the outstanding sustaining
qualities of M&H, and its attribution to the Gerz system - M&H also
have substantial rims which are a great help, and the rods certainly
will help to improve rigidity to some degree, as Phil Ford mentioned,
but not in quite the same way as advertised). Furthermore, I can now
safely conclude that Ron N. and Del were correct when they claimed,
on several occasions, that the rim has little influence over the
maintenance of sound board crown.
Sorry for not answering your post Phil. You raised some excellent
points which I hope to reply to soon.
Its interesting and surprising stuff.
Ron O.
--
OVERS PIANOS - SYDNEY
Grand Piano Manufacturers
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