Orff-style box resonators V 1/4 wavelength - (Discussions with Jim Cook)
-----Original Message----- From: Jim_Cook Sent: Friday, 22 August 2003 5:13 AM To: email@example.com Subject: Orff-style box resonatorsJim,
Do you have any information (dimensions, theory, etc.) about the Orff-style box resonators? I have seen Orff designs with a simple open top box on which the bars are mounted to five chamber designs with each having its own resonating frequency. I realize that individually sized resonators are ideal, but the Orff design is certainly easier to build. Would the Orff design be suitable for a home practice marimba with the bars carefully tuned and spaced as a performance instrument?
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Considering how easy it is to use PVC pipe to construct individual resonators, I would suggest it is your best option for any instrument of any seriousness. The Orff design really only works at all in the upper register where resonators are not that important anyway - most of the sound required comes straight from the bars. As you go lower the wavelengths get too far apart to work in a single chamber. You will notice even on Orff instruments that the lower ranges tend to have less bars per chamber.
Choose a diameter pipe roughly the same as the bar width and cut it to length - 1/4 of the wavelength of the note the bar makes. Use v=fw where v is velocity of sound in air - f is frequency of note and w is wavelength. This length of pipe will be a bit longer than what you need, but is the best place to start. Get a push on cap for the pipe. Put it under the bar and hit the bar, and listen for the resonance in the pipe. You will have some room to work with as the cap can slide up or down a bit to adjust the length. Chop the pipe down bit by bit with a hacksaw till you are able to get a good resonance - mark the cap position with a texta, then glue it on to line up with the mark.
If you want my thinking on the orff style: - here is a cut n paste from an answer to a similar email a while back!
"The important thing to understand about box resonators is that they do not really have a precise resonant frequency. Usually you want a resonant chamber for every few notes. If a chamber can service one note that's best obviously, but a whole bunch of construction effort. For 15 notes, I would look at separating the box into at least 3 or 4 chambers. For each chamber, pick out the note in the middle of the group of notes it services, and match the depth of the chamber to 1/4 of the wavelength of that note. Usually you will find that is about right. It might be a good idea to have a look a couple of commercial models if you have the opportunity - primary schools often have a number of this type of instrument.
The width of the chamber is just whatever is dictated by the walls of the box that the bars are mounted on - the length by the width of the bars and the number of bars over it.
Lower notes are usually more critical than higher ones. You often see little plywood plates mounted flat just under the lower bars with oval shaped holes cut directly under the middle of the bars. Because this closes off the aperture of the chamber a bit, it lowers the resonant frequency. If you find that the high bars for any given chamber resonate, but the lower ones do not - you might like to try the plates for the lower bars."
Hope this is helpful.
Absolutely perfect response! I couldn't find _any_ information on the Orff style resonator boxes and a ton of information on the quarter wave tubes. You verified what I suspected. I'm going to look at using clear acrylic tubes for the resonators. They're as inexpensive as PVC, available in just about any size and completely transparent. I have an idea for a simple to construct adjustable plug to allow easily tuning the resonator.
Thanks for your help!
WOW! clear. That would look great. You could do some really cool lighting things in shows with clear resonators.
Yeah, I saw an "art" instrument builder's website http://www.chrysalis-foundation.org/bass_marimba.htm and he had a couple marimba designs with clear acrylic resonators. They did look really cool and like you say, with a little lighting would really look great! I checked the pricing locally for clear tubing and they're very inexpensive and available in 1/4 inch diameter increments so matching the resonator diameter to the bar width should be easy.
How 'bout piezo triggers under each bar to flash a light behind that bar's resonator when the bar is struck?
Nice idea! you could probably fashion the end plug with a globe built on. The downside is of course, that you would be forever cleaning the dust from the bottom :-) Thanks for the link too!
-----Original Message----- From: "Jim_Cook" Sent: 13/11/03 7:05:30 AM To: "firstname.lastname@example.org"Jim,
As I begin tuning my daughter's marimba bars I'm having problems getting the sound, particularly the harmonics, reliably into her Korg DT3 tuner.
Would a contact mic be a good idea? If it were in direct contact with the bar would it adversely influence the pitch? Is there a better tuner? I've been considering a Petersen electronic strobe-type tuner.
I have been holding the bar near the tuner at a node and striking it with a semi-hard mallet. It just doesn't seem to be loud enough for the built in microphone. I've been considering building an adjustable jig that supports the bar at two nodes (as close to the location of the string holes as possible for the fundamental) with some form of soundboard underneath to raise the volume a little bit.
Thanks for your email ..
The tuner you are using is probably the best quartz type tuner around although I think the stroboscope would certainly solve your problems - they really are much better, and all the professional instrument makers and tuners use them. As the notes you are trying to measure get higher they also tend to get shorter, and I think this is the main problem with any quartz tuner - they need a bit of time to recognize the note. With a stroboscope you already know what you're looking for, and your eyes have a very quick response. You may have better luck with a small piezo transducer, but they can be tricky to adhere in a manner which both does the job and adds insignificant mass. If you have a computer setup, you might find it easier and/or cheaper to try a mic on your soundcard, and using some sort of accurate frequency analysis application. I have used the stuff built into cooledit pro often with success, but its an expensive app if you don't need the rest of it. There are freeware things around if you look, but I can't think of them offhand. The jig is a good idea - mainly I think for just keeping the bar relatively still when you strike it, making it easier to get a mic or tuner in close. Two little H frames would be the go, with string or rubber bands as the horizontal. You could use a sheet of ply or something to attach them to, with a long bolt hole at one end, so one of the H's can slide. I suspect putting effort into a sound board would be time wasted - just too much variety in the frequencies you need to measure.
Hope some of this is helpful...
Thanks! Yes, very useful! I completely forgot that I would be telling the Petersen tuner which pitch I was looking for and the Korg has to figure it out first.
Now, will Santa bring me a Petersen tuner for Christmas...
Glad to help! - got my fingers crossed for you - lets hope Santa's name is Pete this year!
answers by Jim MCCarthy - 16/11/2003
For more help on instrument building you can email Jim.