[pianotech] Tuning the duplex sections

[Delwin D Fandrich]

1)  With an inefficient termination--i.e., a V-bar with shallow string termination angles and "tuned" front duplex string segments on the other side--energy can be transferred back and forth across the V-bar. Because the duplex string segments are tuned (in theory, at least) to some calculated partials of the speaking lengths, vibrating energy at or around the resonant frequencies of the duplex string segments will pass back and forth across the V-bar with relative ease. Among other things, this has the effect of increasing the rate of decay in the desired speaking lengths because some of the energy that is transferred across the V-bar to the duplex string segments is absorbed into the plate at the front bearing bar. This loss is in addition to whatever amount of energy is absorbed into the plate at the V-bar.

Since the so-called "tuned" duplexes are rarely, if ever, accurately tuned, these vibrating portions of string can, and often do, produce sounds at undesirable pitches that are close to, but not quite on, some harmonic of the normally speaking strings. As well, because the string deflection angles are shallow and vibrating energy is being transferred back and forth across the V-bar, any slight imperfections in the shape or surface texture of the V-bar can, and often do, create undesirable vibrations or "string noises." In an effort to control both dissonant vibrations and the string noises it is a common practice to mute the front duplexes. This damping absorbs some of the vibrating energy in the duplex string segments but it does not stop the transfer of vibrating energy coming from the desired speaking lengths of the strings to the duplex string segment. That transfer continues but it's now a one-way street; energy is being dampened on the duplex side of the V-bar and is now unavailable for any transfer back into the speaking side of the V-bar so the decay rate in the speaking string increases even more and the sustain time further decreases.

2)  When the string angles are greater than about 15˚ to 18˚ and the duplex string segments are kept reasonably short energy is not freely transferred back and forth across the V-bar between the speaking string segments and the duplex string segments. The termination of the speaking strings at the V-bar is more efficient and most of the energy arriving at the V-bar is blocked and reflected back into the speaking string segments. Some energy, to be sure, is still absorbed into the V-bar and/or capo tastro bar but very little makes past the V-bar it into the duplex string segments. In this case damping the duplex string segments makes little difference because there is little energy there to be damped. 

As an added benefit, because the strings are not "rocking"  back and forth at the V-bar, its shape is less critical and string noises are virtually unheard of. 

3)  The back scale is a whole other issue. Energy is not being transferred from the speaking strings across the bridge terminations and to the backscale portion of the strings. The backscale is set in motion by the motion of the bridge(s). Whether or not the backscale string segments are tuned does not materially affect the vibrating energy in the speaking portion of the strings or their decay rates and, hence, will have little, if any, effect on how they vibrate or how long they vibrate. 

Whether tuning the backscale string segments has any positive effect on overall piano performance is, for me, at least, an open question. I have yet to see it conclusively demonstrated by even the most ardent supporters of the scheme. Keeping an adequate backscale length is clearly important but deliberately tuning the backscale to some partial lengths of the speaking strings has long seemed an exercise in futility. Besides, given the broad spectrum of fundamental waveforms and partials driving the bridge(s) at any given moment (while the piano is being played) it would be well-nigh impossible to avoid finding backscale lengths that are not "tuned" to some fundamental or some partial of some note or other. 

ddf

Delwin D Fandrich
Piano Design & Fabrication
6939 Foothill Court SW, Olympia, Washington 98512 USA
Phone  360.736.7563 — Cell  360.388.6525
del at fandrichpiano.com — ddfandrich at gmail.com


-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of David Love
Sent: Tuesday, March 15, 2011 4:10 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Tuning the duplex sections

Yes, that's right of course, went too far with that idea.  

So then why exactly does muting the front duplex kill the tone?  Even a light muting in which the rocking motion is presumably unimpeded.  What does the front duplex contribute in that case (when it's unmated) and how does it contribute.  Similarly, why does muting the front duplex in a piano with a very short duplex not kill the tone nor does the tone suffer, seemingly, from having a very short front duplex.  

David Love
www.davidlovepianos.com