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Old 02-06-2009, 02:34 PM   #44
Tedwood
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Location: South Coast UK
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Quote:
Originally Posted by Yogi View Post
EVERYTHING has a resonance. For some materials these can be inaudible, so difuse that they can not be determined, OR (worst case) somewhere below 250 Hz and they tend to ADD muddiness in those frequencies. Anyone want to venture a guess as the the resonant frequency of 1/2" drywall on wood studs at 16" centers? And the bigger part of this is that those cavities wind up being strong bass radiators at this frequency. 125 Hz. Smack dab in the middle of the bass range with the half wave at 62 and first harmonic at 250. Also if those studs happen to be steel studs (all commercial buildings constructed now) the resonance is at 80 Hz. and is almost overwhelming. I've mesaured 80 Hz. resonances in a few churches (I work with lots of churches to solve their acoustical problems) I've seen the resonances add up to 25 db. And no notch filter will remove it either. How can you tell if you have a resonance problem? Pretty easy, run a sweep frequency through your speakers. If you hear a bonk or rumble or sudden increase in volume that is present throughout the room at a particular frequency and it also occurs somewhat at freq/2 or freq*2 you have a resonance problem. In part two I'll give you some ideas on how to fix them.

I'm a builder, trying to retire and be a PC repair man. I have put enough drywall/plasterboard up on walls and ceilings to know the sound off pat. I guessed about 150 Hz, (maybe it's not so big over here )


Two types of construction would have complete different results according to whether the studding was anchored to solid walls or suspended, both present different problems

The resonance will vary according to construction, for instance if the drywall is stuck to solid block work with drywall adhesive the resonance will vary according to how far apart the globs of adhesive are, in exactly the same way a drum goes up in pitch when you press in the center (second harmonic). If you move your finger around on the drum skin it's no longer the second harmonic and the sympathetic resonance would change. In this method the drywall has a much higher resonance. This may help with your drywall resonance but it transmits the vibrations through to the solid wall then and you may then have a problem with the resonance of that solid wall, which is typically very low, if not sub sonic. The analogy here is the drum shell amplifying the resonance of the the drum skin

In the case of the suspended type construction you have a resonance of 125 Hz per board, but only on their own, as soon as you construct something bigger it becomes like a bigger drum and it gets lower, unless you anchor it, and then you might have the first problem of transmission, if you could find a part of the building such adjoining walls that nullify the resonance so much the better. What ever you do though there is a catch, but one thing that could help a lot I can say after years of experience, is damping.

If I had large walls I would be looking at having the studwork floating, and not attached rigidly to the ceiling so you don't transmit and build up resonance problems. The second thing is I would pack glass fiber behind the suspended wall so that it "dampened" the resonance of the suspended wall.

These are just the basics and there may be more structural properties to consider that could cause further problems or could be put to good use in remedying resonances, such as cavities and adjoining walls.

Disclaimer
I'm not an acoustics expert but I am a builder with a good ear for sound, that could have it's advantages
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Last edited by Tedwood; 02-06-2009 at 02:49 PM.
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