Duplex

[Overs Pianos]

Yesterday I wrote:

>  > I realise that most of the excitation of the rear duplex will be in
>>  the form of vertical excitation in response to the movement of the
>  > bridge relative to the hitch plate, BUT, I have found that there is a
>>  small amount of longitudinal excitation which gets past the bridge
>>  pins from the speaking length to the rear duplex. If a bridge agraffe
>>  is used, as in for example the Australian Stuart piano, there will be
>>  virtually zero longitudinal excitation of the duplex.

Del replied:

>And you have determined this how?

A rear duplex behind the actual speaking length and an adjacent rear 
duplex (ie. on an adjacent note) are tuned to the same harmonic of 
the speaking length being tested. Everything else on the piano is 
muted out (tape is the best for this purpose). One of the rear 
duplexes under test is muted while the speaking length is played 
heavily then damped. Repeating this experiment with each duplex will 
demonstrate that slightly more energy gets through to the duplex 
which is connected directly to the speaking length. I must add 'hear' 
that this type of experiment has to be middle of the night stuff when 
things are very quiet. The amount of energy that crosses over is 
pretty damn small.

As I mentioned yesterday, I suspect that the bridge pins are flexing 
in the bridge cap as the string tension cycles in response to the 
string position (as it vibrates). I believe that this allows a small 
dynamic tension deviation to leak through to the rear duplex. I am 
only speculating that less energy would get through with the a bridge 
agraffe (I have not tested one), but I think it is reasonable to 
assume that it would be less inclined to flex in response to minute 
changes in string tension as the string vibrates. I hope this makes 
sense, its hard getting concepts across without being able to wave 
our arms about as we speak.

>How did Stuart manage to stabilize the bridge to such an extent that he was
>able to eliminate its 'rocking' motion?

He can't eliminate it.

>If you are evaluating this strictly
>on the basis of sound, there were so many other things limiting the
>acoustical performance of the Stuart piano I examined I'd hesitate to blame
>much of anything on the bridge termination device.

I agree entirely with you Del. When I returned from the US after the 
convention, I had a good look over a recent Stuart. They are so many 
factors in his piano which go against my own tone building philosophy 
(I made notes which occupied an entire A4 page). Ah well,  its 
certainly not status quo, and its certainly not a clone of something 
else.

>The device he is using is nearly exactly like one I experimented with for a
>time in the late 1980s at Baldwin. (Nearly, but not quite. Mine was machined
>out of bronze. I think his is cast something or other, probably steel.)

His is brass with steel pin inserts for the string bearing points. 
His earlier agraffes used ceramic as the bearing points. But I 
haven't seen the piano you looked at, so he may have used steel for 
that piano.

>The
>side-by-side performance of the device compared with bridge pins was
>measurably identical. That is, when measuring adjacent unisons in the same
>piano, one with standard bridge pins and the next with the bridge
>termination device.

I have not tested this, but your observations are exactly what I 
would have expected. Interestingly, Wayne Stuart said that he tested 
agraffes against standard bridge pins, finding that the agraffe was 
superior. But to this day, I have wondered if he bothered to repin 
the standard pinning of the Yamaha C3 notes he used for the test. If 
the pins were not tight in the bridges he would get the results he 
got wouldn't he?

>What measurements have you taken?

None at all in the case of the Stuart, just a 'seat of the pants' 
belt it and see what comes out.

One comment I would make about the way Stuart installed his agraffes. 
His agraffe has a height of 8 mm from bridge top to string. His 
string plane (for the long bridge) is nominally 32 mm higher than the 
sound board panel. Therefore, his effective bridge height is only 24 
mm. This is, I suspect, one of the principle reasons why his piano 
sounds a bit 'honky' in the middle (the extremely low 18 - 19 mm high 
ribs would also be a contributing factor also). I can't understand 
why he didn't fit an additional bridge under the belly to achieve a 
higher bridge stiffness factor, like Grotrian did with some of their 
early pianos. This would have been a logical way to achieve a 
'normal' degree of bridge stiffness while employing agraffes. I saw a 
Samick concert grand last year which also had 24 mm high bridges, it 
'honked' like a duck too. So the moral of the story is; not 
everything that quacks like a duck is a duck.

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