Unison coupling

[Roger Jolly]

Hi Ron
>* This is contrary to what Ric's quoted article said. What's your source?
>This isn't a trap, honest, I'm just trying to resolve conflicting
information.

Yes, and I was suprised. Remember the class in KC. when I detuned Gina's
octave by voicing a note down. Softening the hammer reduced the
inharmonicity and octave started to beat.
The whole class heard it, and those in front saw the results on the
spectrum analyser.
Sorry for those standing in the corridor.  <G> This years voicing class I
will be dealing with this effect in greater detail. In the tenor break, and
killer octave areas.
Getting reproducable, and accurate results has been a problem, but I think
I have that licked with the new 'Jolly Thumper'.  But the trend has been
consistent. higher inharmonicity and shorter sustain of partials with hard
hammers and high velocities.
Now that I have a device to control mass and velocity, I will soon be
starting a new series of test.
>
>
>>I'm convincinced that many factors conspire, damping or impedence of the
>>board, the wire, the hardness of the hammer, the velocity, the regulation,
>>and so the list goes on.
>>Just some half baked musings.
>>Roger
>
>Musings from just inside the door of the Mezzo-Thermoneal Stabilizer, eh?
>Alright, let's bake a little. 
>
>The generally accepted explanation for the pitch drop moving from the
>attack phase of the envelope, to the dwell, is roughly this: At the
>beginning of the attack, the string vibrational amplitude is at it's
>widest. At the strings widest deviation from a straight line between bridge
>pin and, say, agraffe, the string's path is longer than the supposed
>speaking length. That means the string has to stretch, which raises the
>tension, which raises the pitch. As the soundboard assembly, duplex
>segments, air resistance, internal friction, and whatever else you might
>think of reduces the vibrational amplitude, the string assumes a much
>straighter line as it enters the dwell. Since it isn't stretched as much as
>in the attack, the pitch drops to what you've "tuned" it to for the bulk of
>the duration of the dwell, then further drops (less dramatically) through
>the decay as the string straightens out still further. It seems to me that
>this requires that the elastic recovery rate of the string increases as the
>tension gets closer to the yield point. I would assume that it does, since
>the elongation factor changes proportionally to the proximity of tension to
>yield point. Otherwise, the string would act like a pendulum, and the
>oscillation period (frequency) would remain constant whatever the amplitude
>of oscillation.

I would support this
>
>I still like this explanation. It's simple, logical, and doesn't require a
>cast of thousands, so it's understandable. That's not what we're after
>though. The original question here, is what makes the pitch drop when a
>second string is tuned in. Is the perceived drop just a difference in the
>timing of the attack/dwell transition, or is there actually another pitch
>drop in the dwell phase, in which case we have TWO pitch drops occurring.

My original theory, One string, the bridge and board moves x amount, add
the second string and the board excursion will be x plus, due to the
increase in energy being transmitted tho the bridge, The wider excusion of
the board cause the sum to go about .3 cents flat.
Add the 3rd string and we get a proportional dop in pitch .2  cents for a
total of .5 cents.
We tend to think in terms of the energy of the string effecting  the board.
But I am starting to think backwards, what effect has the board on the
string, since one end of the string has a mobile termination point.
The movement is very small but so is the pitch drop.
Each time we discuss the coupling subject, I just end up with more
questions, and no valid answers.
I wonder what the effect would be on pitch, if the board was made stiffer
in this area?
You make boards, any observations in this area?
This tuning deviation could be a pointer to an ideal Z in this critical
area. Or is that off the wall.

Roger> 
Roger Jolly
Saskatoon, Canada.
306-665-0213
Fax 652-0505