On Super low pitched Marimba notes - (discussions with Michael Sparrow)
-----Original Message----- From: Michael Sparrow Sent: Wednesday, 12 May 2004 7:42 AM To: email@example.com Subject: Marimba ResonatorsJim,
I recently discovered your website and have found your advice on instrument building to be of great value. I'm working on a Theatre project for which I'm planning to build a single bass marimba bar with a frequency somewhere around 24-26Hz. The thing is that the bar will be mounted in the deck of the stage (level with the stage) with only a few inches below to accommodate a resonator. I thought about using the open space under the stage as a cavity resonator but I don't think I'll get a good result. I'm planning for the bar to be 23cm wide by 120cm long and I'm hoping to use 15cm PVC for the resonator. I plan to use an L-shaped PVC joint just below the bar to run the sound into a 1.7m section of PVC that will have a U-shaped section linking it to another 1.7m section of PVC with an adjustable end cap. I'm worried that one 15cm resonator won't create enough amplitude to fill the theatre so I was thinking about having two identical resonators side by side. My questions are 1) will two bends in the resonator drastically reduce the amplitude as opposed to a straight 3.4m tube? 2) will having two identical resonators boost the sound or will they take sound away from one another?
Lastly, I've heard conflicting reports about where the fundamental nodes are in a marimba bar and I'm guessing that that has something to do with the cut-out below the center of the bar. I've read three different percentages saying that the nodes are at 20%, 22.5%, or 25% of the bar. Which is correct? Should I tune the fundamental before deciding exactly where the nodes are?
Thanks so much for your help,
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Hi Michael - thanks for your email.
A project like this is problematical to say the least - but also right up my alley! You probably came across one of my beasts online called GUS - a similar 1-note marimba with a frequency of around 25Hz. Now in many ways I made Gus as ideally as possible, and whilst it worked, there were still many problems getting an acceptable result in a performance situation - even with amplification. (amplifying 25 hz is not that easy)
Firstly your bar:-
2ndly - your resonator/s:-
All of the stuff I've said so far is about getting maximum volume from your note - believe me you will need every bit you can get. I'm not sure how much knowledge you have about this kind of thing, but if you didn't already know - a marimba bar's note has pretty much all its energy at the fundamental frequency (ie. 24-26Hz in this case) Well that's right down there where human hearing is just not that sensitive. To make people hear it you need just truckloads of sound energy - just to hear it at all. Its easy to get fooled by other instruments that produce low notes, but other instruments make their sound with a more complete set of upper harmonics which are related to the fundamental. For example the lowest note on a piano is around 27.5 hz fundamentally - but the human ear doesn't actually hear this - there's no way that small string is radiating enough energy at that frequency. The human brain is very clever though because it hears all the upper harmonics that the string is also producing and it calculates the ratios between them and puts back in the fundamental for us inside our imaginations. I other words we think we hear it. This is part of the psycho-acoustic effect. A marimba note pretty much just relies on the fundamental inside the resonator - and in fact the upper partials that we tune in the bar are tuned so that they DON'T also resonate in the resonator. All of that being said - a great deal of how well this kind of thing works is dependant on the room it is in, and the placement of both instrument and audience. Its sometimes hard to predict the effects too. A theatre should have enough size, but ideally there should be lots of reflective surfaces, and not so many parallel ones to introduce standing wave problems. Also be aware from a musical or compositional perspective, that it may take a bit of time for the sound to propagate. We usually need to hear at least 3 or 4 full waves before we recognise its frequency - at 25Hz that's getting to over a second.
Certainly there's lots to think about - but its all good fun stuff.
The nodes for the fundamental vary a little with lots of things - probably accounting for the varied reports you have heard. Timber type and bar thickness can make a bit of a difference, and the exact position can actually move a bit after tuning, so yes don't drill the holes until after a rough tuning has been done. You will need to do the final tuning after the drilling though. The best way to find the node is to get the bar horizontal and suspended a bit - even if you just get a mate to hold it from the sides or roughly rig up a couple of horizontal ropes for the bar to sit on. Sprinkle some of the sawdust that you are likely to have after rough tuning over the nodal areas and start gently tapping the bar in the middle to get it vibrating. You might also need to blow gently. What tends to happen is that the dust starts to gather at the nodal points because that is the only part of the bar that is not moving and causing the dust to dissipate. You have to play with it a bit, bit you get an exact node by doing this. Do it for each end, because they might be slightly different measurements from the ends. With a one-note instrument you don't have to worry about matching up the holes with the next bar, so you can make it ideal.
Hope this helps..
answers by Jim MCCarthy - 17/05/2004
For more help on instrument building you can email Jim.