impedance for 99

[Doug Richards]

Ron,
I might be able to save you some time.

BUT...

I've been doing FEA stuff for the past 6 or so years, and to be honest, the
first two or so years didn't contribute much to the improvement in disk
drive design.  Even with the experience I have now, I would guess it would
be a few months of serious modeling  and lab work to be to a point where I
would direct design decisions.  Since this is not a funded project, no
promises.

All that said, work (or play) is now underway.  My first cut was a few
months ago with just a string and the end constraint differences.  Next I
will add a flat soundboard with non-homogeneous material properties to study
the first order coupling of string to board.  Then add crown and, followed
by ........

Just remember, NO PROMISES!

doug

PS  Did you ever get the impression that I don't like the word impedance
when discussing mechanical structures  :~)

> -----Original Message-----
> From:	Ron Nossaman [SMTP:nossaman@SOUTHWIND.NET]
> Sent:	Friday, January 01, 1999 10:21 AM
> To:	pianotech@ptg.org
> Subject:	RE: impedance for 99
> 
> > I'm just not comfortable with
> >mechanical impedance.  Impedance to me is an electrical quantity.  My
> >measured or predicted transfer functions of on excitation and response
> ARE
> >complex (real/imaginary or more to my liking is magnitude/phase).  While
> I
> >have studied simple electrical circuit analysis and understand the basics
> of
> >caps, resistors and electrical impedance, I just can't get comfortable
> with
> >describing a mechanical structure using impedance.  My prejudice I guess.
> I
> >also work with a BUNCH of mechanical engineers that have never described
> >mechanical problems using impedance.......  
> 
> * You're among a BUNCH of piano technicians here who, until fairly
> recently,
> didn't either. It's just a one word handle covering the principals and
> phenomenae of measured or predicted transfer functions of an excitation
> and
> response, but it's easier to type. 
> 
> 
> >
> >Next question for you.  Why do you say that pianos are very non-linear?
> >
> >Just to frame the question, in a FEA model, many material properties are
> >cataloged and used.  Most of the time, non-linear analysis is either do
> to
> >exceeding the yield point of a material or a large deflection.
> >
> >Hammer are for sure!
> >
> >The rest?  A bunch of steel that is not near yield except maybe strings
> at
> >the termination points and wood.  You may argue that wood is not linear,
> but
> >that gets into long-term load bearing characteristics and how close the
> >designer pushed the envelope.  Other that those, what material properties
> >fall into the non-linear category?  
> >
> >doug
> >
> 
> It's not the material response that's non linear, it's the assembly
> response
> at different points along the bridge. In use, a soundboard assembly is a
> driven diaphragm, sort of like a speaker cone, that has to accommodate and
> respond to frequencies ranging from roughly 27Hz-4200Hz. That's woofer,
> tweeter, and crossover, all in one unit. Let's see the audio engineers do
> that in a high performance speaker! It's mounted on a rim that isn't
> exactly
> the ideal shape for naturally conferring the proper frequency responses to
> the corresponding area of the board. Designers use cut off bars to
> minimize
> this problem to whatever degree they can. Compounding this (in an over
> strung scale), the bass bridge is in roughly the same area of the board as
> the low tenor, so this area must respond to a wider range of frequencies
> than are required in other areas. The designer has to work with what he's
> got to try to achieve the impedance characteristics necessary to the sound
> he wants, at every point in the scale. He can re-define the perimeter, to
> some degree, with cutoff bars, juggle rib lengths, height, width, crown,
> angle, feathering, choice of wood, and crowning method, establish panel
> thickness, grain angle, grain density, taper and diaphragming. He can
> determine bridge placement on the assembly, construction method, height,
> width, stiffness, and mass. He can change the impedance load on the
> assembly
> by designing a different string scale, and/or downbearing schedule. All of
> this affects the impedance balance between strings and soundboard, without
> any changes whatever in the properties inherent to the materials used. If
> we
> could estimate the impedance response in slices, a note, rib, or section
> at
> a time, configure it as necessary to accommodate the string impedance load
> to produce whatever sound quality we're after, and blend them (which is
> partly what the bridge and panel stiffness do), we could, hopefully,
> eliminate some of the guesswork. I have been considering trying to set up
> a
> crude outline of this process in Pascal to try to establish some rules and
> procedures, but the amount of time I anticipate it would take for me to
> get
> to even a Stone Age level tool have, so far, kept me from trying. So, you
> see, I'm hoping you can save me from myself by defining some of this stuff
> in an existing tool. It's purely selfish.  %-)
> 
>  Ron