[pianotech] soundboard grain angle vs "faux"stiffness

[John Delacour]

At 01:23 -0400 15/7/10, Dale Erwin wrote:

>... Did the attached pictures help answer any questions.

Hello Dale,

Looking at your pictures of the board, I'm curious to know what 
decided you to design the high treble like that and what engineering 
or acoustic principles led you to the splayed rib pattern.

The way I understand things a spruce board is adequately stiff along 
the grain and the main purpose of the ribs is to bring the stiffness 
of the board across the grain to a similar degree of stiffness in 
order to achieve something like an orthotropic plate.

1.
Your longer ribs perform this function but the further you get 
towards the treble the more the ribs are reinforcing the stiffness of 
the board along the grain, where it is not needed while supplying 
less and less stiffness across the grain, where it is needed.  In 
order to increase the stiffness to a determined degree at the treble 
end of the board you are bound therefore to use a heavier rib than 
would be needed if the ribs were at a right angle to the grain, you 
are needlessly stiffening the board in the direction of the grain and 
you are adding more mass than yould otherwise be needed.

2.
Your masking of the edge of the board for the glue suggests that a 
full inch or more of the right hand end of the board is glued down to 
the inner rim, which brings the right-hand boundary of the sounding 
board say 20 mm closer to the end of the bridge than it would be if 
the inner rim were cut away to give a glue line 6 mm wide instead of 
the full width used for the rest of the perimeter.  In other words a 
significant proportion of the board's area in this region is disabled 
when it could be used to advantage for the vibrations received from 
the highest strings.  A second effect of this arrangement is that the 
board is stiffened, which brings me to...

3. To my eye, not only is your highest rib doing work in an 
unnecessary direction but it is supplying stiffness, together with 
the rim support, to the unsupported area of board above it, which in 
my design would have another rib.  In other words, where you have one 
rib at 55 degrees to the grain, I would have two lesser ribs at a 
right angle to the grain.

These ideas of mine are not some new-fangled approach I've dreamt up 
but can be seen applied in numerous pianos that have an excellent 
treble without even the aid of tuned partials or even capo bars -- 
how they might sound with these additions is quite a thrilling 
thought.

You might answer the splaying question by saying you want the bars 
roughly at a right angle to the long bridge,  in which case I would 
ask "Why?".

As ever, the Devil's advocate! --

JD