Duplex

[Overs Pianos]

Ron N wrote:

>I understand what you're saying about the initial positioning of the
>duplex, but how can these segments possibly stay in relative tune with one
>another with any sort of humidity swings in actual use?

The higher the downbearing angle, the greater will be the pitch 
deviation due to humidity swings. This applies to both the speaking 
length and rear duplex alike. We are particular about not using too 
much downbearing, and also being careful to ensure that the bearing 
is uniform from note to note. This reduces the effect of humiditiy 
swings on tuning stability to a minimum, but nevertheless, the 
duplexes will still go out of tune just as do the speaking lengths. 
However, our experience has been that the rear duplexes exhibit quite 
good tuning stability. 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. 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. 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. This 
is, I have no doubt, the reason behind this crazy technique of 
pounding a piano to get stability prior to a concert tuning. A 
similar result can be had, without ruining the rollers in the 
process, by gently rubbing down the speaking lengths with a piece of 
hammer shank.

>  >Nevertheless Ron N, I have to tell you that when we
>>compare two pianos, one with and one without tuned rear duplexes, the
>>difference is immediately obvious. I really appreciate the lack of
>  >low-chorus effect on the tuned back scale piano.
>
>So why not an intentionally de-tuned rear duplex, like is done with the
>front? Surely the tolerances qualifying a duplex to be out of tune are
>wider than those for tuned, with a similar lack of extraneous noise.

An excellent idea. I can't see why deliberately detuning the rear 
duplexes wouldn't work just as it does for the front duplex. The 
problem with 'chorus effect' occurs when the rear duplexes are nearly 
in tune. Your rear detuning idea should work very well for the 
vertical hitch pin designs (controlling the rear lengths has been my 
only reservation with regard to using vertical hitches). However, 
when dealing with loop stringing and vertical hitches, it would be 
necessary to ensure that the chosen hitch pin position resulted in a 
detuned length for both of the adjacent back scale lengths. But since 
the tolerance for detuned segments is wide, this should not be a 
problem. Excellent idea Ron N.

Ron O
-- 
______________________________

Website:  http://www.overspianos.com.au
Email:        mailto:ron@overspianos.com.au
______________________________