Frequencies of wood resonance

[freeztar]

I'm a recording musician, and as such I have to deal with resonances within my home studio. I have a subwoofer placed under my desk which is constructed of laminated pressboard about 3/4" thick. I realize that some of the sound waves are reflected off this surface and some are absorbed. The absorbed frequencies (at least the low ones) can be felt by placing my palm on the desk. My question is: Does anyone have an estimation of what frequencies might be resonating through my desk or a way I can do a rough calculation?
I can't remember where I found it, but I remember seeing a chart showing the resonant frequencies of different types of wood. My aim is to help absorb these resonances in some way so that they do not interfere with what I'm hearing from the speakers. Any help on how to go about that (short of getting a foam desk )would also be appreciated.

[orbsycli]

welcome! as am i, although i am more of a mad scientist when it comes to these things, than a professional.
what i do (when it comes to absorbing that stuff) is i place some foam underneath the monitors.
it helps, enough for what i'm looking for...but probably not good enough for someone who is REALLY attentive to these things.
as for the frequencies, i have no idea! but i am looking forward to finding out:

[Boerseun]

Hi, and welcome!

More important that the kind of wood you're using, I'd say the length of your desk surface is probably the most important thing here. If you've got the bad luck to have a desk with the required length to accommodate the resonation you don't want, I'd say either move the stuff on your desk so that nothing is standing in the centre or at quarters, or cut a length off your desk (which you probably don't want to do).

I had the same problem a few years ago with a home theater system. I took duct tape with weigths and taped them to the bottom of the desk. True story! Worked like a charm, and broke the resonation.

[freeztar]

Originally Posted by Boerseun

Hi, and welcome!

More important that the kind of wood you're using, I'd say the length of your desk surface is probably the most important thing here. If you've got the bad luck to have a desk with the required length to accommodate the resonation you don't want, I'd say either move the stuff on your desk so that nothing is standing in the centre or at quarters, or cut a length off your desk (which you probably don't want to do).

My desk is approximately 2 ft. x 3 ft. x 3/4"
Is there an equation to calculate the resonances created by such dimensions?

I had the same problem a few years ago with a home theater system. I took duct tape with weigths and taped them to the bottom of the desk. True story! Worked like a charm, and broke the resonation.

What did you use for weights? It seems like they would be subject to rattling unless they were suspended. Very interesting approach. I wonder what the actual physical properties that underly that are?

[Boerseun]

Not sure about the formula, doubt that there'll be, seeing as your table will only rattle at certain frequencies. Basically, if the wavelength is the same as the length of your table (or a fraction thereof, like either 1/2, 1/4, 1/8) then harmonic resonance will ensue. But only at those frequencies. You sometimes hear stuff rattling as the sound frequency goes up the scale, and then stop rattling - regardless of the volume. But once the required frequency hits again, the rattling starts again. Same thing that cracks a glass.

What I used for weights was 6mm flatbar. Basically what it does, is to absorb the wave that forms on the desk due to the flat bar's inertia. But try to weight it 'randomly', for want of a better word - if you weight it down in the centre or anywhere a standing wave can get through, the rattling will continue. So, for instance, weight it down in a strip from the left front of the desk to the right back, or basically anywhere, but not symmetrically.

[freeztar]

Originally Posted by Boerseun

What I used for weights was 6mm flatbar. Basically what it does, is to absorb the wave that forms on the desk due to the flat bar's inertia. But try to weight it 'randomly', for want of a better word - if you weight it down in the centre or anywhere a standing wave can get through, the rattling will continue. So, for instance, weight it down in a strip from the left front of the desk to the right back, or basically anywhere, but not symmetrically.

I'll try that, thanks!

[cwes99_03]

If you truly want to understand it, I suggest contacting Dr. Thomas Rossing at Northern Illinois University in DeKalb, Illinois. He has been studying resonance in all shapes of percussino instruments. Seeing some of the images that he has created that allow you to physically see the modes for standing waves in the instrument (or in this case desk) are quite interesting. You are working with a 2-d object and standing waves are created in 2-d.

As for solutions, foam and rubber isolators are your best bet. Coat the areas around your speakers, the parts nearest the speakers, with that foam, to deaden the pickup of the wood. Then the weights to break up the desk into an irregular vibrating surface will cut down the modes of vibration of the wood. You should do this with your walls as well, if you are really serious, as well as shaped foam on the walls to break up the resonance pattern of foam. You could also cut strips out of the wood (not the full depth of the desk, but enough to break up resonances into higher frequencies of standing waves.)

[Turtle]

___The solutions given look sound. On the question of different woods having different resonant frequencies, most assuredly yes. Imagine an 8 foot 2x4 of wood dried to 7% moisture content. If it is Balsa & you strike it the frequency is very low compared to striking Doug Fir which rather has a distinctive 'ring'. Walnut gives a 'thunk', as does Redwood.
___A frequency spectrum analyzer is the tool for establishing a particular resonant frequency.

[freeztar]

Originally Posted by cwes99_03

If you truly want to understand it, I suggest contacting Dr. Thomas Rossing at Northern Illinois University in DeKalb, Illinois. He has been studying resonance in all shapes of percussino instruments. Seeing some of the images that he has created that allow you to physically see the modes for standing waves in the instrument (or in this case desk) are quite interesting. You are working with a 2-d object and standing waves are created in 2-d.

As for solutions, foam and rubber isolators are your best bet. Coat the areas around your speakers, the parts nearest the speakers, with that foam, to deaden the pickup of the wood. Then the weights to break up the desk into an irregular vibrating surface will cut down the modes of vibration of the wood. You should do this with your walls as well, if you are really serious, as well as shaped foam on the walls to break up the resonance pattern of foam. You could also cut strips out of the wood (not the full depth of the desk, but enough to break up resonances into higher frequencies of standing waves.)

I'll look into Dr. Rossing.
Standing waves are indeed 2-D in nature, but their functions, are not. Many factors are involved when creating a standing wave and those factors include three dimensions.
Foam is definitly the keyword here lately. Placement is a different story. Ideally I would coat my room from floor to ceiling with it, but no funds to do so as of yet. I've seen professional studios that implement very light usage of foam, yet it is effective.
My current setup is this: I have two studio monitors at opposite end of my 3.5 x 2 ft. desk. On top of those are two satellite computer speakers with tweaters of approximately 5 cm. Underneath my desk and almost back against the wall is my subwoofer, the cause of my dilemma, which is the main resonating factor. I am either using my monitors or the computer speakers (with sub), never both at the same time.
I have yet to try the weights, but it seems like that will help significantly.

Originally Posted by turtle

___A frequency spectrum analyzer is the tool for establishing a particular resonant frequency.

Indeed. The difficulty in measuring the frequencies of the desk sans the frequencies of everything else is the problem.

[cwes99_03]

Quick question as to the weights. These are used basically to create a chaotic oscillation, so that no single standing wave (or harmonic thereof) can be setup. My question then asks, does not a chaotic oscillation, if strong enough around certain focusing points still pose the possibility of anharmonic oscillation in the wood?