Re: [Harp-L] Soaking harps

["Rick Dempster" <rick.dempster@xxxxxxxxxxx>]

Vern;
         When the plate expands on being heated, will the slot grow
smaller, or larger; and then, in either event, singularly in length or
width, or both?
Thanks as usual for your input on these most interesting (to me, any
way) subjects.
RD
        

>>> "Vern Smith" <jevern@xxxxxxx> 19/07/2007 2:41:21 >>>
----- Original Message ----- 
From: "Ken Deifik" <kenneth.d@xxxxxxxxxxxx>
To: <harp-l@xxxxxxxxxx>
Sent: Tuesday, July 17, 2007 10:56 PM
Subject: Re: [Harp-L] Soaking harps


> Joe Leone wrote:
>>Another theory of mine is that since the reed plates have a much
>>larger mass than the reeds, cold ANYTHING will cause the modulus of
>>contraction to differ between THEM and the much much smaller mass
>>reeds. This also may close up the tolerance(s).

The coefficient of linear expansion is not a function of the mass of
the 
part.

> Well, I started this with my question, and with full faith that youse
all 
> would come up with the goods and you haven't failed.  I thank
everybody. 
> I've learned alot.
>
> I'm totally intrigued by Joe's reply.  I have a harp whose
responsiveness 
> leaves alot to be desired, and which I unfortunately need to use. 
I'm 
> going to stick it in the fridge tonight and see how it performs in
the 
> morning.
>
> I will report back.

This is a case where an analysis of the physics involved can give the
answer 
without resorting to experiment.  If the experiment produces results 
inconsistent with the theory, then you need to carefully review it for

subjective errors.

The linear coefficient of expansion of brass is about 10
parts-per-million 
per degree F.    Moreover,  it is the same for the reeds as for the
plate. 
Small differences of alloy composition have little effect on this
property 
of brass.

The whole plate, reeds slots & all, grow and shrink proportionally with

temperature.... but not very much.  For instance if the temperature
drops in 
the frig  by 50 degrees F, the plate (and all of its dimensions) will
shrink 
by  500 ppm or to .9995 of its previous dimension.  That means that the

.001" clearance between reed and slot will decrease to .0009995"...a 
miniscule, imperceptible amount.

I anticipate that someone will advance the hypothesis that the modulus
of 
elasticity will affect pitch.  Here again it is imperceptible for two 
reasons.  The elasticity changes hardly at all near room temperature
and the 
pitch is proportional to the square root of elasticity.

There is a perceptible effect on steel guitar strings because the
linear 
expansion which affects string tension is different from the wooden
neck.

Vern

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