Re: [Harp-L] Soaking harps



----- 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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