Duplex

[Ron Overs]

Ron N wrote;

>  >However, our experience has been that the rear duplexes exhibit quite
>>good tuning stability.
>
>Meaning what? That they don't go out of tune with humidity swings, or don't
>go out of tune farther than the longer speaking lengths do, as they
>logically should?

They tend to go out of tune to about the same degree as the speaking 
lengths in the extreme treble, to a bit more towards the lower end of 
the second top section.

>  >One added benefit we have found with tuning
>>the rear is that the tuning stability overall seems to be slightly
>>improved. I don't attribute this to any 'magical' qualities. But I do
>>suspect that the process of actually tuning the duplexes also causes
>>the back scale tension to remain closer to that of the speaking
>>lengths.
>
>Ok, are you speaking here of the initial positioning of the aliquots, or
>the measurement and adjustment of duplex pitches during the course of
>regular tuning?

The initial positioning will always have errors of measurement, and 
will require adjustment. The act of tuning will, in the initial 
months, require a combination of moving the block and adjusting the 
duplex tension. As the block positioning improves (by being adjusted 
and refined at the time of tuning) the duplex tuning will come to be 
accomplished more by tension adjustment and less by block 
repositioning.

>We have different factions using the term interchangeably.
>They are entirely different concerns, and I'm trying to separate them here.

I'm not sure I follow you. If I didn't explain this adequately, I can 
rephrase. I have found that duplex tuned pianos tend to stay in tune 
better with heavy handed pianists during the course of a performance. 
I attribute this to the string rubbing and therefore tension 
equalising across the bridge, and not to the fact that the duplexes 
are tuned.

>  >Over time, for pianos in which only the speaking lengths are
>>tuned, the back scale tension falls as the sound board gradually
>>takes on compression set and the 'crown' recedes slightly.
>
>How can this possibly be? Haven't we already determined that strings will
>render across bridges?

No. The wire only renders across the bridge when the speaking length 
or duplex length is 'rubbed' down, or when the differential becomes 
large enough for the string to pull through the friction created by 
the bridge pin offset angle.

>Didn't you just indicate that you are sure
>longitudinal vibrations make it from the speaking length, past the bridge
>pins and across the bridge to the duplex?

Yes, but while the vibrations make it through the string remains in 
contact with the bridge pin and bridge. These vibrations are of a 
very small amplitude, small enough not to cause the wire to move 
relative to the bridge pins. You will recall that I mentioned that 
the bridge pins yield a little when the energy travels through from 
the speaking length to the duplex length. The strings and bridge pins 
will tend to flex as a unit (as will the bridge cap wood on a 
microscopic level also). If the bridge pins were incapable of flexing 
in response to forces placed upon them, then they would prevent any 
longitudinal energy leaking through to the duplex lengths.

>How can they do that if the
>string refuses to move on the bridge to the point that the back scale
>tension falls as the soundboard crown deteriorates? How much time are we
>talking about here?

Over longer periods the board gradually recedes, but it will be over 
a couple or several years. You know when you find a piano with low 
back scale tension. You arrive on location for a tuning, and while 
playing through the compass of the piano to check for miscellaneous 
problems before commencing the tuning, the back scale segments are 
'clicking' across the bridge as you strike each note.

>  >After some
>>time, when the big hitting pianist comes to town, the tuning
>>stability goes horribly wrong as the speaking lengths are pounded,
>>pulling the tension of the back scale up and killing the tuning.
>
>I've found this likely to happen in the Winter months, when the piano is
>pulled up to pitch for the tuning, leaving the back scale somewhat low in
>tension. I've also seen individual strings go sharp after play in the
>Summer months, when the pitch is lowered during the tuning, leaving the
>back scale somewhat higher in tension.

Agreed, but whether the tension is different in the back length 
through short term climatic variation or through longer term 
compression set, the net effect on tuning instability will be the 
same. In either case, tuning the rear duplex prior to a performance 
(or more correctly, equalising the tension on either side of the 
bridge) would tend to reduce any pre-existing tension variation. 
These days, when I pitch raise a piano I also rub it down with a 
hammer shank. It seems to help stability a lot.

>This is exactly the point I'm trying
>to make about the duplex segment not maintaining the same tension as the
>speaking length, regardless of their length ratios.

Yes I saw your post on this. Interesting.

Before this thread gets totally out of hand, if it hasn't already, I 
must qualify my posts by mentioning that I'm not saying that the 
views I've been posting are correct - they just appear to be at 
present. One never quite knows about the validity of conclusions 
drawn. We may recall Cromwell's famous words. Off the top of my head 
I think they are, "I beseech you in the bowels of Christ, think it 
possible that you may be mistaken". I now step down from the witness 
box.

Ron O.
-- 
Overs Pianos
Sydney Australia
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Web site: http://www.overspianos.com.au
Email:     mailto:ron@overspianos.com.au
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