[CAUT] pre-stretching new string?

[David Love]

Sorry, but it's not quite a complete enough formula for purposes of this
discussion.  When comparing two strings that produce the same pitch but with
different tensions, either the original length will be different or the
diameter will be different (or both), thus a similar change in length will
yield a different change in tension and thus pitch.  

 

David Love
davidlovepianos at comcast.net
www.davidlovepianos.com 

-----Original Message-----
From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] On Behalf Of
Richard Brekne
Sent: Sunday, June 10, 2007 4:00 AM
To: caut at ptg.org
Subject: [CAUT] pre-stretching new string?

 

The degree any string changes tension due to a change in length to any of
its segments is calculated by the formula given by Dr. Galembo. I've posted
this several times now.  Again. it is  

 Change in Tension = (E*S*Change in Length) / Original length.

Where E is Youngs modulus and S is the cross section of the string.

You first figure the length change, then figure the change in tension, then
add the change of tension to the original tension... then with both the new
tension and new length you can figure change in pitch.  This works for plain
wire.  If you actually apply this formula find out what kinds of things need
to happen to a given string for it to experience the kind of pitch changes
we observe... you immediately see that the pitch changes we see at the tenor
bridge have no (at present) good explanation. 

Cheers
RicB



The degree to which a string goes out of tune will be a function of the
break point percentage (BPP) when it is at pitch (thanks Ron N. for
clarifying that).  The lower the BPP, the greater the change in pitch for a
given change in length.  The top end of the bass section is generally
considerably higher BPP than the low tenor, and the low tenor is usually the
lowest BPP in the entire piano and thus goes out of tune the most with
seasonal changes. 

David Love

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