tuning instability/plate struts

[Overs Pianos]

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Tom,

At 3:17 PM -0500 17/1/04, Tom Sivak wrote:

>Certainly I think we can agree that strings near the plate struts go 
>out of tune more than those in the center of their sections.

Often, and primarily because the string tension (when expressed as a 
percentage of breaking strain) goes low on the treble side of the bar 
and high on the bass side.

>   And when it comes to the tenor break, I think I can understand why 
>this would happen.  The treble bridge ends there, often the 
>stringing scale goes to copper-wound strings for the last couple of 
>unisons

Because the designer has run the scale too low, for the particular 
length of piano, before crossing to the bass bridge. This results in 
a low tension problem, which the manufacture sometimes attempts to 
address by using copper wrapped bichords for the most disastrous 
offenders.

>  and I imagine the tension of those strings differs from their steel 
>neighbors.

Certainly, but if the scale had been thought through properly at the 
design stage, the tuning problem wouldn't exist and the wrapped 
bichords would be unnecessary.

>   And right on the other side of the break, the strings are strung 
>across in another direction; all of those things could probably 
>contribute to instability, although I say this not out of knowledge 
>of the situation, but just looking at it in a logical (but basically 
>uninformed) way.

Very often there is a huge variation in the percentage of breaking 
strain at the bass/treble break. For example, the chart below shows 
the percentage of breaking strain for a Steinway model C grand piano.


(I could have chosen the Yamaha U1H scale - amongst many others - to 
illustrate this trend, but I chose this model to show that the 
problem is commonplace, even for high end instruments)

Note that the % brk has dropped down to the mid 20s in the treble 
before crossing to a bass bichords in the high forties. This is a 
consequence of setting the break at the same place as for the concert 
grand. While E20/F21 will work with a speaking length of 183 cm for 
F21 on a concert piano, you cannot accommodate a respectable plain 
wire F21 speaking length into a 225 cm piano. Notice also the % brk 
glitch between notes 49 and 55. This occurs at the bar between the 
last agraffe section and the first capo section in this piano. This % 
brk variation is the cause of the tuning stability problem at the bar.

>But why does this also happen in the treble break?  Often there's 
>one continuous bridge.  Steel strings on both sides.  All strung 
>parallel.  And yet notes on either side of this break generally go 
>out quicker and farther.
>
>Why should those strings be less stable? 

When recapping bridges, we analyse the % brk across the breaks, 
adjusting the speaking lengths to achieve a uniform percentage of 
breaking strain. Often it will be necessary to build a slight 'dog 
leg' into the bridge cap to accommodate the adjusted speaking lengths 
(but the dog leg must not extend down to the bridge/panel contact). 
We have been making these changes to pianos since 1990 and the 
improvement in tuning stability is noticeable.

At the lower end of the treble bridge a short tenor bridge scaled 
with bichord covered strings is a good solution (apart from the 
obvious one of designing the piano properly in the first instance). 
Typically, the first note on the tenor bridge (when moving down from 
the treble) can be shortened in speaking length 12 - 14% relative to 
the speaking length immediately above it, with a core wire two 1/2 
gauges smaller and wrapped with a 0.2 mm copper cover.

Unfortunately, after shortening the speaking lengths on a new tenor 
bridge to fix a scaling disaster, you will have created a strike 
ratio problem. The shortened notes will now have a lower strike 
ratio. If the plate permits, you could relocate the agraffes to 
correct it, or you could shorten the hammer position on the shank. 
However, shortening the hammers will lower the hammer/key leverage 
ratio.

So, relative to the ease of addressing design issues at the time of 
manufacture, rectifying it later is a lot of trouble. If only 
manufactures would commit a little more effort to design, and a 
little less to marketing spin. Such is the problem that I suspect 
worthy design might go unnoticed amongst the general background noise 
of marketing hype.

Best,
Ron O.
-- 
OVERS PIANOS - SYDNEY
    Grand Piano Manufacturers
_______________________

Web http://overspianos.com.au
mailto:info@overspianos.com.au
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