[pianotech] Fwd: Measuring Crown

[Noah Frere]

Note: this was too big with 5 pictures of the kind to go through. So here's
the message but with only the first picture. I can send the other 4 pix
later when i'm home and can reduce the size.

---------- Forwarded message ----------
From: Noah Frere <noahfrere at gmail.com>
Date: Wed, Jan 6, 2010 at 1:02 PM
Subject: Measuring Crown
To: pianotech at ptg.org


I am trying to measure crown in this 1906 Spector & Sons Baby Grand. When I
run a string, 1/32" diameter, along the length of the 7th of 10 ribs
(counting from the front treble area towards the back), underneath the
soundboard, I see noticeable crown (Picture 8852). I got similar results
along most of the ribs. My method was to tape one end of the string at the
edge of the soundboard and hold a chopstick to which the other end of the
string was glued.

When I use a short straightedge, there is very little gap (Photo 8875),
which I expected. In order to accurately see the gap, I shined a light
directly up towards the string or ruler, and took the photo along the side
of the string/ruler. If the light were held off to the side, the shadow
would not be aligned correctly. There may be some element of error in this
set-up, but the gap was clearly visible with the string.

Above the soundboard, I laid the straightedge adjacent the lowest bass
string, right atop the soundboard, at the corner. Picture 8883 shows this.
At the other end of the ruler, there is a large gap (Photo 8884). You can
rock the ruler back and forth a lot. What does this mean? I have not
calibrated the ruler, so am not certain it is 100% straight. But it's pretty
new and I believe straight enough to warrant the reality of the rocking atop
the soundboard, since the rocking is considerable. This seems to indicate
the soundboard is falling, that is, has negative crown. I tested this with
the string and found the same thing. Is this measurement meaningful?

On the other side of the place I measured atop the board, I measured again
underneath. I found very little gap, but nonetheless a little, with the
string.

Anther questions: is the string too thick? You can see it in the
"crown-string" picture.

Does all this points to an undulating soundboard?

Next up: Downbearing
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