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problems with salt test - Discussions with Anthony

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-----Original Message-----
From: Anthony
Sent: 28 April 2010
Subject: problems with salt test
Hi Jim,

Since our last e-mails I'm pleased to let you know that I did indeed manage to make some outdoor instruments, and had a great summer last year taking them to festivals, often locate dnearby the childrens' space.

Now we're in year 2, and I'm making bigger and better things. So far Ive made a giant copper chime, a pvc batonka, melody the cow bell cow, whispering pipes, tongue drums, a giant triangle, a giant rainstick. Today I finally finished my first giant xylophone. Hooray!!

It's pretty straight forward- 6 notes, A major pentatonic. Made from sapele, 40mm thick, 150mm wide.

I watched your you tube video for finding nodes with salt- something i'd read about. The next day I did exactly as instructed but had some interesting reults--

1- the nodes were not equi-distant from the ends of the bars on any of the 6 bars.
2- the lines of salt collected in lines not perpendicular to the sides of the bars, but also not parralel with each other.

In the end I drilled perpendicular holes 22.5% in from the ends of the bars and this seems to work fine for all but 1 of the bars- the lowest note (A). It was in tune before we drilled the holes, now it's a fraction sharp.

Question- If I sand out a little arch will the pitch be lowered?

Hope you can help.
All the best

Anthony James Brown

Hi Anthony - great to hear from you, and thanks so much for taking the time to get in touch and send a photo. Great work by the way!

Ok now for your salt test result findings.
So your point number two, is perfectly normal. The nodes will always go across the bars, but it is quite common for them to be angled. They will certainly vary from one bar to another depending on the exact grain direction and type as well as the type of base cut used to mill the lumber from. You will often find that the angle is a little different on one end of the bar to the other because of the variances in grain - although the angles should at least be non-opposing directions, with just the DEGREE of angle changing.

Point one - well the nodes can be a little different in distance from the ends of the bars, although they should not be a great deal so. If they Are different, it might be partially due to those same grain variations but more likely because of an uneven arch shape - but even this should not make a BIG difference between the two ends. If you find there IS a big difference, you may find that you are actually reading one of the nodes in the bar from a different mode of vibration - one of the overtones.

And to answer your last question - yes - sanding a little out of the middle of the bar will definitely lower the pitch - sand just a little though as it should not take too much.
Hope this helps Anthony.
All the best again.

Jim McCarthy

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answers by Jim MCCarthy - 28/02/2008

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