Resilience and energy transfer, was Acetone

[Sarah Fox]

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Hi all,

Interesting question:  What happens to hammer felt when it is doped?

The same question can be asked about open-cell foam rubber.  Normally =
it's nice and squishy (resilient), and although it can impede sound =
reflection somewhat, it isn't that effective an acoustic material.  =
However, the stuff is commercially treated (doped with something, I =
believe) to make it a bit stiffer and very slightly "crunchy," and that =
stuff, when cut into wedges or egg crate, has great antireflective =
properties and can be seen lining the walls of the world's finest =
recording studios.  That's because it absorbs sound energy, rather than =
reflecting it.

Of course we want to do the opposite with a hammer.  We don't want the =
hammer felt to absorb the mechanical energy, but rather to transfer it =
to the string.  In a perfectly elastic collision, no energy is wasted.  =
Either it goes into the string, or it ends up in the kinetic energy of =
the hammer as it flies into the rep lever and backcheck.  Either =
application of energy is good.  To the extent that the collision is =
inelastic (e.g. when the hammers are doped and crusty, at least at a =
microscopic level), some of the energy will go into friction.  The =
remaining energy will be split between the string and the hammer's =
return.  Of course if the hammer were doped to the point of being a =
wool-reinforced mass of plastic (similar to fiberglass), I suppose it =
might be pretty elastic, but it would take a LOT of dope to start =
increasing elasticity, rather than decreasing it.

Here's an interesting experiment, though.  I have no idea how it would =
come out.  Most dopes "crust up" the felt, as evidenced by the "sugar =
coating" y'all talk about.  Thus, you have a plastic grit that rubs =
against the hairs of the wool, increasing friction and decreasing energy =
transfer.  However, if you could find something that sticks to the =
fibers, doesn't flake off, and doesn't crust up, that could increase the =
stiffness of the felt without diminishing the resilience.  This has been =
an ongoing technical issue with regard to hairspray products.  I don't =
really know the state of the art in this industry, as I don't use the =
stuff, but hairspray is a lot less "crusty" than it used to be.  In fact =
I think some of it is supposed to have elastic properties, perhaps =
providing elastic ties between hairs to stick the hair together, while =
preserving "bounce."  I wonder if some brand of modern hairspray, =
dissolved in a solvent vehicle and applied in the same manner as your =
traditional hammer dopes, would improve the performance of mooshy =
hammers without sacrificing power.  Hmmmm.....  You guys could probably =
hit the local beauty salon for tech support.  LOL!

Peace,
Sarah


----- Original Message -----=20
  From: Michael Spalding=20
  To: oleg-i@noos.fr, Pianotech=20
  Sent: Friday, April 23, 2004 7:56 AM
  Subject: RE: Resilience and energy transfer, was Acetone


  Isaac,

  I'm relying on decades-old memories of college physics for this, so =
the details may not be exactly right, but I think I have the general =
principals correct.  Resilience is a measure of how little energy is =
absorbed, and lost to conversion to heat, by a material when compressed: =
 the more resilient the hammer, the less energy it absorbs during =
collision with the string.  As to the other question, how much energy is =
transferred to the string vs how much is used to propel the hammer into =
the backcheck, that is influenced by several factors more important than =
resilience, including the mass and compliance (inverse of stiffness) of =
both the hammer and the spring.  The example of the ball rebounding from =
the pavement illustrates an extreme, where the ball is light and =
compliant, the pavement is massive and stiff.  Heavy hammers transfer =
more energy to strings than light hammers.

  hope that helps

  Mike


    ----- Original Message -----=20
    From: Isaac OLEG=20
    To: Pianotech
    Sent: 4/23/2004 2:46:01 AM=20
    Subject: RE: Acetone


    So the resilience may be adapted in regards of  the hardness, the =
weight and also the resilience of the object that is contacted , Indeed =
if the ball rebound, that mean the energy get reflected to the ball. In =
the hammer we want some to be given to the strings as well is not it ?

    Best

    Isaac OLEG

    -----Message d'origine-----
    De : pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]De =
la part de Erwinspiano@aol.com
    Envoy=E9 : vendredi 23 avril 2004 00:30
    =C0 : pianotech@ptg.org
    Objet : Re: Acetone


    In a message dated 4/21/2004 10:58:19 PM Pacific Standard Time, =
Richard.Brekne@grieg.uib.no writes:
      btw... Whats Delwins take on hammers and laquer ?  Seem to =
remember he=20
      went further then we do over here for the natural  hammer =
resiliancy=20
      side.  No... this is one of those typical <<opinions vary>>=20
      questions....and so it should be :)

      Cheers Dale !

      RicB
      Ric
      Right you are.! It all depends on whos defining resieliency Know =
what I mean. A steel ball is the most reslient when bounced off cement.  =
It probaly expends the least amount of energy per bounce than the super =
ball or a felt hammer.
      Cheers back at ya
      Dale
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