RC&S question in general Kent

[David Love]

I'm not sure that trying to figure out "compression" isn't chasing the
untamed ornothoid (i.e. the wild goose).  The question is more one of
stiffness, or as has been mentioned by various people, the relationship
between stiffness and mass-at least in certain parts of the scale.  As I see
it, compression is used to achieve stiffness, or resistance to deflection,
and therefore is a means to an end, not the end in itself.  Understanding
deflection in absolute terms (not as a function of residual percentages
which really tell you nothing without first knowing what your starting point
was) will tell you more about the load resistant properties of the assembly
and eventually (one hopes) how effective that section will capture and
transduce specific frequencies or sets of frequencies.  If that's a goal
(which I think it is) then one of the biggest arguments for RC&S assemblies
is being able to duplicate with some degree of predictability and
regularity, degrees of stiffness without having to rely nearly as much on
compression.  Even then, you don't escape dealing with compression
altogether but it is modulated through downbearing adjustments rather than
wondering what the panel delivered during the CC process, what it still has
after a month and what it might have after several years.  Without that, you
have little hope of teasing out the variables that give you a better
opportunity for reliable repeatability.    

 

David Love
davidlovepianos at comcast.net
www.davidlovepianos.com 

 

Symantecs aside, it seems like a fair and interesting question to ask to me.

 

I've built my rib data spreadsheet to calculate the sag by entering the
pressure of the string bearing force, rib dimensions, modulus of elasticity
of rib material, bending and resisting moments & moment of inertia. Many of
these factors come from static tables in old industrial standard texts for
the given material (ie sitka spruce, sugar pine etc.) and are based upon
some sampling once upon a time. I know it's been brought up that the modulus
of elasticity, for example, will vary from rib to rib even with the same
species and dimensions (this by the way is definitely one of the advantages
of the laminated ribs in that the elasticity coefficient is averaged out). 

 

Here are some potential drawbacks:

 

1.      Even with all this data, some of the static values and constants are
averages or aproximations. 

2.      Everyone I know, including myself, uses the formula for center
loaded beams, which isn't exactly the case in the piano, where there may be
two to three different loads on a given rib and even when there is only one
load it is not necessarily in the center. This does make a good case for the
symetrical design though, if for no other reason than to make the math
easier.

3.      A judgement still has to be made as to how stiff you want your
assembly to be. 

Nevertheless, it's still a pretty good tool, a point of departure shall we
say. I don't know how much compression this translates into but I would sure
like to know. And I would like to experience how this affects the sound. All
in due time I hope, unless this is my Moby Dick.

 

Ron N. makes some very good points about staying out of the "swamp of
details" and staying aware of the diameter of the hair we are trying to
split (definitely goes in the Ron N. Greatest Hits of List Quotes), but
there's no harm in asking questions.

 

Cheers,

 

Jude Reveley, RPT
Absolute Piano Restoration, LLC
Lowell, Massachusetts
(978) 323-4545

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