duplex position

[pianoguru at cox.net]

David, et al,

I cannot speak to the specific make, model, and era of your piano, but typical of pianos with this type of duplexer, the distance from the back bridge pin to the duplexer at note 88 is equal to the speaking length.    One can go a step further, and after stringing, nudge the duplexer in or out, to "tune" the duplex length to sound in unison with the speaking length at note 88.  Whether one believes that "tuning" the duplexer is effective, it was probably the design intent, if you find that the original position of the duplexer suggests this relationship between the duplex length and the speaking length.

This 1:1 relationship can only be maintained for a few note below 88.  You can clearly see where the duplex length suddenly becomes shorter, around note 80, or so.  Measuring the speaking lengths and duplex lengths, you will likely fine a whole-number relationship, with ratios following a sequence something like 1:1, then 2:3, 1:2, 1:3, and 1:4.  The rationale behind these relationships is to make the fundamental frequency of the sympathetic vibration of the duplex length correspond to a higher partial of the speaking length, or at least to have a harmonic relationship to the speaking length.

There are exceptions.  Some builders make this distance significantly shorter to effectively add to the stiffness of the soundboard in the high treble.  Bear in mind that significantly shortening the tail length has implications for the balance between front and back downbearing, not so much at note 88, but throughout the scale.  Changes in the crown will have a huge impact on a very short tail length, while having a negligible impact on a relatively long speaking length.

Other makers would prefer to use the position of the duplexer to refine the downbearing, moving it in or out to increase or decrease downbearing.  If this were the intended function, then the term, "Duplexer," would be a misnomer.  

It is not uncommon to see three sections with "duplexers," with the lowest (tenor) section being muted out with stringing braid or felt.   In these cases, it may have been the intent that the duplexer in the top two section truly function as duplexers, while in the tenor section its function is simply that of a string rest to facilitated controlling downbearing.

I have seen Steinways with hardwood tapered shims under the duplexer, and cut to the same shape as the duplexer.  Whether this was done in the factory or by a rebuilder, presumably the shim was added to control downbearing, while preserving the duplex function of the duplexer.

Frank Emerson