[pianotech] Tuning the duplex sections

[David Love]

One thing to add here, I wonder how compression crowning that forces a bend
in the ribs might contribute to this in some way.  Consider this as an
experiment.  Take a ball and attach it to two springs pulling the ball in
opposite directions.  Take another ball and attach it to two springs pushing
in on the ball.  Assuming you could set them up with some measureable
equality, when you set the two balls in motion which one (if any) will
continue to move for a longer time?  My hypothesis would be that the ball
being pulled in opposite directions would stay in motion longer than the one
being pushed from opposite directions.  The compression crowned, rib bending
board might be more akin to the ball being pulled and a non compression
system might be more akin to the ball being pushed.  That might describe
some differences in soundboard systems and perhaps this bloom you are
talking about as well.  I don't know really but I've wondered about it.  

David Love
www.davidlovepianos.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 7:20 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] Tuning the duplex sections

Sorry, I was commenting on two different things.  And, of course, you're
right the clapping of hands impacts the soundboard first (my mistake) and
strings indirectly as the soundboard moves.  However, if the clapping of
hands moves the strings via the soundboard that implies that the soundboard
is just as capable of transmitting energy to the strings in the same way
that the strings provide energy to the soundboard.  The transduction process
is not one way but there's a feedback loop, of sorts in which the strings
are providing energy to the soundboard and the soundboard is also providing
some energy back to the strings.  In terms of your question about the
differences perhaps it has something to do with the relative stiffness or
freedom of the assembly itself.  An assembly that has more freedom will,
once in motion, tend to continue to move more freely and that may have some
impact on its ability to transmit energy back to the strings in this loop.
Yamaha boards are relatively much heavier than Steinway boards.  Their
ability to be driven requires a much higher tension scale and heavier
hammer.  The higher tension scale with greater stiffness (or more mass or
both) may not react as easily to the feedback loop of energy being
transmitted back to the strings.  When you combine that with a relatively
heavier assembly that is somewhat more restricted, then the input from the
string driven side of the equation dominates and you won't get the bloom.
The bloom may be the consequence of this back and forth energy transmission
between the two.  It also might be that the more equal are those two springs
(string and soundboard) the more this feedback loop will be self
perpetuating and the more out of balance they are the less.  There may be
several combinations of scale, assembly, downbearing, backscale length, for
example, that might contribute to the soundboards ability to transmit energy
back to the string or put these two springs more on par.  Thinking out loud
here, hope that was clear.  

With respect to the other comment by Frank about the unmuting the backscale
and that changing the tone of the piano it does seem to happen.  I've had
the experience recently of unbraiding the tenor section of a D and noticing
a difference.  If there is this feedback loop (and certainly there is some)
why wouldn't the backscale lengths be part of that loop as well?  Very short
lengths in the upper end of the piano might be too stiff to be set in motion
by soundboard energy but the longer lengths in the tenor and bass certainly
are.  

David Love
www.davidlovepianos.com


-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of John Delacour
Sent: Tuesday, March 15, 2011 1:43 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] Tuning the duplex sections

At 19:05 -0700 14/03/2011, David Love wrote:


>Why would that be a surprise?

David, I don't think you can have read my message very carefully.  I 
contrasted two opposite cases, one there the pedal causes the tone to 
'blossom' and the other where it causes it to wither.  If the same 
cause produces opposing effects in two different cases, surely one of 
these effects must be a surprise.

>If you go to the piano, lift the dampers and then simply clap your 
>hands over the strings you'll get some excitement of the strings. 
>That certainly comes from the change in air pressure that follows 
>the clapping of your hands.

No argument there, except that I think it's important to repeat that 
without the soundboard and bridge you would hear almost nothing. 
Your clapping does not excite the strings directly; it produces sound 
waves in the air that impinge on every square inch of the soundboard 
and produces flexural waves in the board that are transferred through 
the bridge to the strings and cause them to vibrate.

>...The front duplex by the same token probably receives some energy 
>from the changes in air pressure from the surrounding air.  The 
>difference is that the front scale is not attached directly to a 
>flexible diaphragm as the soundboard but rather the plate which is 
>acoustically dead, in effect.  Doesn't that make sense?

I'd say, as I think others have recently said, that the vibration of 
the front section of a string comes from direct mechanical 
transmission of stress in the speaking length of the same string 
across the front bridge by what has been called a 'rocking' motion or 
see-saw effect and that any direct excitation by the sound waves in 
the air is infinitesimal.  And this can quite easily be demonstrated 
by experiment.

So my question (and my surprise) remains.  Why does good piano x 
bloom when I lift the dampers and good piano y wither?

JD