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music box resonating sound piece - question from Grant Watson

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-----Original Message-----
From: Grant Watson
Sent: 12/02/08 3:32 PM
To: jim@percussionclinic.com
Subject: music box resonating sound piece
Hello Jim,
I came across some of the articles you have written regarding resonator tube length and I was wandering if I could ask your advice about a sound piece I am working on.

I have horizontally suspended a 3ft.x8ftx1in old growth cedar plank and attached at bottom centre a tiny wind up music box mechanism. I have also suspended a 3ft dia. x12ft resonator tube horizontally behind the centre of the plank. I have capped off one end of the tube and have held it back approx.. 1inch from the plank face ( very similar to the standard marimba arrangement but horizontal)

The effect is very rich in tone and in volume but I was wondering if you could offer any advice on further enhancement. Could I attempt to tune the plank, locating nodes and internodes like a marimba key? Also should I then try to adjust and tune the resonator. The music box mechanism plays "Happy Birthday" starting in the key of E

Thankyou for your time Jim
All the best Grant Watson


Hi Grant... Thanks for your email...

Sounds like an interesting project. For my money, I would hesitate to play too much with the timber plank, as any changes are undoable, and it rather sounds like there is more going on than a simple marimba type systems - and if it sound good now then you don't want to ruin it.

As the plank is so large, it is probably not being activated by the music box at any of the primary frequencies that the resonator would amplify - in fact it is doubtful that the tube is actually resonating significantly at any of the frequencies that the music box is producing either.

Your plank and pipe seem in proportion to each other for a big and very low marimba note, so you could certainly find the plank's nodes and tune its modes to suit the resonatotor. You would then significantly increase the effectiveness of the resonator by turning the mouth towards the flat of the plank. If the music box was on the plank it would in theory act to lower the pitch of the main vibrational modes by adding to the mass in the bar's center. Therefore you would tune the bar - glue the music box - THEN further tune the plank.

I would suspect that the current system has the plank acting as a sounding board amplifying the music box. A bit like putting a tuning fork on a table. In this system the music box transmits it's phone vibrations to the plank which effectively increases the surface area which is radiating the sound. The pipe is probably having virtually zero resonant effect at all, but may in fact simply be acting as a reverb chamber. The high frequency sound from the music box is easily reflected by the hard plastic surfaces of the pipe, so it becomes trapped in the pipe once it enters, and bounces around until it hits the end cap and comes back out the pipe with a delay - the longer the pipe the longer the delay. This is a bit like singing in the bathroom - always sounds fuller because of the high frequency reflections - now image a bathroom built by tiling a railway tunnel!- the reflection time is uniform in all but one dimension which is the one open to the air - so you get that unique "tunnel reverb" sound.

Now if my theories about what is happening are correct you should be able to tune your plank without effecting its amplification lf the music box. If i'm wrong though you could destroy your current good effect. If it was me I would temporarily remove the tube from the equation then compare the music box on another plank similar but different. If i'm correct another plank should have a similar effect - it may not be quite as good because a nice seasoned piece of hardwood will transmit vibrations better than any old wood - but similar. That will let you test my theory without ruining your cedar.

Now that being said there's probably no real point in tuning your plank like a marimba unless you can activate it's fundamental mode somehow. Unless your music box is a contra-bass music box and the low e is the same as the low e you have tuned your bar and tube to - it cannot activate the bar.

A plank vibrating like a marimba bar has 99% of its energy transmitted from its biggest faces, which is why we face one towards the resonator. Most likely a plank vibrating like a sounding board also has most of the enegry transmitted from its biggest faces - depends a bit on the notes and music box orientation though - this means you might find turning the plank towards the tube more increases the reverb effect.

I sincerely hope these thoughts are of some use to you.
All the very best.

Jim McCarthy


answers by Jim MCCarthy - 15/02/2008

For more help on instrument building you can email Jim.

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