LF desk vibrations even w/ no speaker contact

[jsbeeth]

I work with my monitors atop a double tiered wooden desk (something cheapish -- MDF, particle board or the like with a laminate top veneer), and I'm getting substantial sympathetic vibrations in the wood at certain frequencies, generally around the 139 HZ range. It also seems to correspond strongly with specific pitches in the music playback.

I initially thought it was a speaker/desk contact issue, but even with various barriers and complete decoupling (i.e., friends holding up the speakers), the problem remains. I tried partially covering the desktop with some of my 4" OC703 panels, and this did help somewhat, but was not a silver bullet, nor would it be a logistically feasible solution had it eliminated the vibrations. Putting additional weight on the desk also didn't seem to help.

Any ideas? I've seen recommendations for covering desk surfaces with thin rubber, neoprene, etc, but these seem to address problems of HF reflections, whereas my issues are with low end vibrations.

Thanks!

[phybeau]

just a hunch:

is there any way to increase the mass of your desk?
This might lower the resonance freq. of the desk....

Regards,
Phybeau

[DanDan]

It is likely that you desk is responding to a higher level of airborne sound, most likely a mode. As it vibrates it may be absorbing some of this energy. Unless the desk itself is resonating, a thump will show this, I would regard this as a welcome bit of bass trapping.

DD

[tINY]

You run into these panel resonances when you build speakers too...

There are two ways to deal with them: Mass and Stiffness.

First, I'd try some 1x3 bracing screwed to the bottom in an "X" (1"edge on the underside)

The problem with using this as "bass trapping" is that it doesn't really dissipate the energy - it only re-radiates it back into the room.



-tINY

[jsbeeth]

Thanks guys!

Phybeau - I did try weighing it down with additional materials, which didn't do the trick. Is there another way?

Perhaps bracing it as tINY suggests would achieve this. tINY -- any suggestions for the material of the bracing?

Dan Dan - I believe the desk may be resonating, though it's somewhat difficult to tell. What do you mean by "thump" here?

[DanDan]

There is a thin line between re-radiating resonance and absorptive resonance.
If possible, thump the desk with a rubber mallet or more simply your fist. If it is self resonating you should be able to hear it and even feel it if you keep a finger on it. You could brace it by simply attaching strips of wood underneath, screwed and glued. This is done in speaker cabs. Perhaps even better you could try to get hold of rubberised damping pads such as those used in cars or PC's or inside loudspeakers. This latter seems the most sure solution IMHO

DD

[phybeau]

Jsbeeth,

first of all, I'm no expert, so anybody correct me if I'm wrong.

It seems like there is a peak around 140Hz at the same place as your desk.
This makes your desk resonate.
I don't know what your desk looks like, but if I picture it like some sort of table, you might be able to (firmly) attach a heavy plate of wood (or maybe even a thick steel plate) underneath the whole table.
This additional mass should lower the resonance frequency of your desk.

This, with bracing the desk like tINY suggested, should result in less resonance in a lower frequency.

Am I making any sense?

Phybeau

[jsbeeth]

Gotcha, phybeau.

Dan - the "thump" test reveals that the desk is not self-resonating, so does that definitely mean the desk is primarily reacting to pre-existing room modes rather than creating its own LF spikes? In which case, would bracing the desk and eliminating the vibrations even address the accompanying sonic problems that are really my concern? And if not, what can I do? Just more and more bass trapping everywhere?

Thanks again!

[tINY]

It could be the whole desk moving up and down, or it could be the floor resonating and being carried up to the desk through the legs....

3/4" MDF is cheap. Try screwing a piece underneath with glue or caulk in-between. The worst that could happen is that the resonance moves down.

For braces, steel L channel or furring strips can work. If the panel really is resonating, it doesn't take much added stiffness to change the behavior dramatically.

Got pictures of this thing?



-tINY

[jsbeeth]

Here's a pic.

Though again, since the desk does not appear to be self-resonant, I'm now starting to wonder if the vibration is just symptomatic of a room issue rather opposed to the desk reinforcing/boosting the low frequencies itself... And if so, would bracing the desk even matter?

[phybeau]

Is it possible to move the desk, say 2 feet, towards you, and then 'measure' again?
(I might have missed this, but have you ever done any measurements in your room with Room EQ Wizard or something similar?)

Phybeau

[SAC]

The vibration is either due to airborne or mechanically coupled energy.


You can either damp the mechanical vibration of the desk with mass (or playing with eigenvectors - which is too confusing to explain here) or by mechanically decoupling the desk from the conductive source.

So your job is to first determine the coupling source, be it conductive or airborne. and then take the appropriate corrective action - to mechanically decouple he desk from the floor or surface transferring the mechanically coupled energy, or to damp the desk with mass in order to address airborne conduction. Damping the desk with mass will be most effective if you can locate the eigenvalues on the desk and place the mass there. And easy way is to press your finger(s) against the desk in different places and see where the applied force has the greatest effect in stopping the resonance.

With an accelerometer the process is easy, but one method you might use to determine mechanical conductance is to lay a mic on the floor near where the table sits, and see what kind of response is indicated in a swept sine wave sweep frequency response and waterfall in a program like RoomEQWizard.