Seasonal pitch change:

[Andrew and Rebeca Anderson]

I've been suspicious of soundboard deflection being entirely 
responsible too, for different reasons.  Thanks to Ric we know we 
need to look elsewhere.  I've also noticed a pattern to seasonal 
pitch changes that involves the unisons, especially the three string 
unisons.  It usually don't go out entirely the same way.  Sometimes 
in opposite directions.  Think about it.  There's not a whole lot of 
board and rib to push up or sag down, but there is a lot of board in 
the horizontal positions.  I wonder how much bridge movement back and 
forth we get with hydration and dehydration?  By the way, the pattern 
is the same on uprights too.

Andrew Anderson

At 06:26 PM 3/7/2007, you wrote:
>So has anyone measured plate expansion and contraction with 
>temperature changes? Wouldn't that be the most likely suspect given 
>this newfound innocence of the soundboard?
>
>Jason
>
>On 3/7/07, RicB <<mailto:ricb at pianostemmer.no>ricb at pianostemmer.no> wrote:
>Hi List
>
>I see I sent the wrong post, a copy of one earlier sent. Please excuse.
>What I meant to write was the following.
>
>
>I've been running some numbers and thinking a bit about this traditional
>idea that vertical deflection of the strings is the main cause of pitch
>change and thought some of you might find this interesting.
>
>Given the following string lengths, all with identical back lengths
>(50mm), and lengths from front termination to tuning pins (200mm) (to
>make the example simple) and assuming a 1 mm string deflection as the
>starting point for all strings (also for simplicity) and calculating for
>a roughly 50 cent pitch rise we get:
>
>A string length of :
>
>1400 mm needs 6 mm additional deflection which results in about 22 lbs
>of downbearing.
>1000 mm needs 5 mm additional deflection -->19 lbs downbearing
>800 mm needs 4,5 mm additional deflection --> 18lbs downbearing
>500 mm needs 3,5 mm additional deflection --> 15 lbs downbearing
>250 mm needs 2,5 mm additional deflection --> 13 lbs downbearing.
>100 mm needs 1,75 mm additional deflection --> 13 lbs downbearing
>50 mm needs 1,2 mm additional deflection --> 13 lbs downbearing.
>
>As you can see neither the amount of deflection needed to exact the
>actual 50 pitch rise to begin with, nor the resulting downbearing
>figures are within reason... which leaves one no choice but to admit
>that something else is primarily responsible for seasonal pitch change.
>
>Its also good to note that if we are starting with a 1 mm deflection to
>begin with.. which btw yeilds reasonable enough string deflection
>angles, then the absolute most downward pitch change possible is when
>the panel flattens out and bearing becomes 0.  In this case the same
>string lengths yeild :
>
>1400 mm length --> -1 cent
>1000 mm length --> -1,4 cent
>800 mm length --> -1,7 cent
>500 mm length --> -2,4 cent
>250 mm length --> -4 cent
>100 mm length --> -7 cent
>50 mm length --> -10 cent
>
>These examples are simply illustrative of the kinds of things that
>actually has to happen if the soundboard rise and fall is to account for
>most of the pitch change.  I apply all the resulting change on the
>speaking length itself and do not account for any friction.  This is a
>best case scenario. In reality the string will disperse some of any
>change in tension caused by a change in vertical deflection... lessening
>the frequency change and resulting downbearing... but necessitating even
>more vertical change for any give change in pitch.
>
>I can not help but conclude... looking closer at the consequences....
>that vertical rise and fall of the soundboard simply doesnt have much to
>do with the seasonal pitch change at all.
>
>Tension change, and hence pitch change can come from an altering of the
>relative positions of end points to each other... i.e. hitch pin and
>tuning pin.  Pitch change can also happen without tension change if the
>speaking length is somehow altered.  Seems to me that ruling out
>vertical deflection... one has to look to these to general conditions
>for the explaination.
>
>Cheers
>RicB
>
>
>
>
>
>
>
>
>
>
>
>
>--
>=cell 425 830 1561=

-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/20070307/cf7c671a/attachment.html