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On the Acoustics of "Rhythm Bones"

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----- Original Message ----- 
From: Steve Wixson 
To: jim@percussionclinic.com
Sent: Friday, February 22, 2002 1:24 AM 
Subject: Marimba Bar Theory and Rhythm Bones
Jim,
I am part of a fairly new group called the Rhythm Bones Society, and you can learn more about us at (of course) rhythmbones.com. We are trying to understand how sound is produced by the rhythm bones and I found your Honours Essay on page http://www.percussionclinic.com/art_mbasa.htm. I wonder if marimba bar theory applies to rhythm bones.
Thanks, Steve.


Hello Steve!

Its great to see someone who's so dedicated to their art and passion. I'm not sure how much I will be able to help you. The theories of applied acoustics as it affects all instruments will of course also be applicable to rhythm bones. There are some similarities between marimba bars and the bones, but quite a few differences as well. As I looked at the website it struck me that there were not only more than one material that the "bones" could be made from, but more than one method of holding them and playing them as well. Both these things can/will affect the sounds produced a great deal. Perhaps the best thing is to ask me some fairly specific questions, so we have a starting place to deal with the issues involved.

The first interesting area that springs to mind would be that - the pictures seemed to indicate bones players holding the bones at one end, so that each pair strikes each other at the other end. his creates an interesting situation where a nodal position is being forced (by holding) at the very end of the bone. This tends to force the bone to vibrate in a way such that the length is 1/4 the wavelength of the note produced. If the bone was played in say the same way as the Central American "Clave" where the stick is allowed to vibrate freely, it would tend to revert to a natural vibrational mode similar to that of a marimba bar, where the length of the bone would be 1/2 the wavelength of the note produced. So right off the top - we have a situation whee the grip used tends to lower the pitch (although there would also tend to be a greater number of high harmonics) and shorten the duration of the note. It looks a little in the pics like some players hold the bones so that they touch each other about 1/4 of the length of the bone from the end - In which case - assuming the performers could restrict their hands and fingers as much as possible to the same part of the bone (1/4 from the end) the opposite effect would occur - that is that the note would be higher, purer, and longer in duration.

Anyway - as you can probably tell, it is possible to get quite involved in these issues so feel free to get specific, and I'll see If I can help.
Thanks again for your emails.

Jim McCarthy


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Jim,
A big thanks for your response.

It seems one problem in analysing bones is that two of them are used each one with its own properties. For testing I am making a bone with a solid ball attached to the end to reduce the number of variables. I have also thought about making some fixtures to hold the bones for testing.

Many of us tune our bones. The bones are held as you described and the stationary bones is moved up to produce a clearer sound. I have done that and then measured where the strike point is - about 1/4 the length of the bone. So that seems consistent with what you said in your last paragraph.

I have done this experiment documenting it with my computer using the Cool Edit audio program and its frequency analysis plug-in. While the frequency curve is not as clean as I would like, I can definitely see an increase in the amplitude of the lowest frequency shown when the strike point is about 1/4th.

Steve.


Hello again Steve.

With no hurry of course, I would be keen to get a close up photo of the bones in the hand so I can see the exact grip etc. If such a thing is easy to do. (might be a good one to stick on the website anyway!) Cool edit is a good audio program, and the analysis feature can work well - make sure you remember to use the log scale not the linear scale, to see meaningful results. It would be interesting to compare frequency analysis with differing strike points but also differing clamp points. The theory is that wherever you hold or clamp the bone should be forced into being a nodal position (position where there is no movement) - you cannot force an antinodal position (position of maximum movement) in a vibrating bar, but you can increase the resulting vibration by striking the bone at the exact antinodal position. This position would be determined by the forced nodal position.

The two possible fundamental modes of vibration are 1/4 wavelength (like plucking a ruler which is clamped on the edge of a table) or 1/2 wavelength (like a harp string) The 1/4 wavelength mode would probably be accentuated with the bone with the ball on the end, as the big mass would tend not to vibrate as easily as the rest of it. Try clamping this ball in a vice or something. In this mode the best note SHOULD be attained by striking as close to the free end of the bone as possible. If this is not the case - or if the resulting tone is not what it should be to the ear when the bone is clamped like this by the ball (no joke intended!) - then The sound you are probably after is that produced by 1/2 wavelength mode. I would suggest that this is likely, as that is the most natural mode for a bar to vibrate in. You might try clamping a bone (one without the ball on the end) at a point 1/4 from the end. The best tones should then be achieved by striking either at the very ends (either one) or the exact center. If this seems to work, or only partially works - you could further experiment by putting a second clamp on the other end - also 1/4 from the end so it looks like so:

____c________c____
If this doesn't seem to effect the result a great deal then you know 1/2 wavelength is the winner. If it does - then there is something else happening - maybe 1 1/2 wavelength. for that you could try clamping 1/3 from the end and strinking in the center.

looking forward to hearing about your progress!

Jim


Jim,
Attached is a snapshot of me holding bones. The middle finger presses the stationary bone against the base of the thumb. The other bone is held loose enough so that it strikes the stationary bone when the arm and wrist are moved.

Steve


Hello again Steve.

Thanks for the picture you sent - I can see the important things very clearly.

Two things spring to mind - firstly if it is indeed the stationary bone producing the sound you are aiming for it looks a little like you have nodal positions roughly 1/2 way and 1/4 way along the bone - this would force the vibrational mode to wavelength x two (a pretty strange and un natural mode) and the best striking points would be 1/8th from the end or 3/8ths from the end. - Secondly it rather looks just at a glance that the moving bone is actually the one that is in a better situation for good vibration. The ring finger would be the main nodal position at 1/4 along the bone (as long as the grip was loose enough between the ring finger and the middle finger) and the best sound would occur when the moving bone was striking at or close to its very end. Does any of that sound about right?

How are your tests going by the way?
Best wishes - Jim.

Jim Reccomends www.makeamarimba.com for comprehensive blueprints and building guides to make your own marimbas.

answers by Jim MCCarthy - 21/06/01

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