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PostPosted: Thu Feb 23, 2012 10:39 pm 
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It comes up again and again that there is no benefit to be had by putting decoupling (foam neoprene etc.) between the floor and wall footers if they are going to be anchored, apart from helping seal floor anomalies (unevenness). The bolts will short-circuit any decoupling methods.
It seems to me that while not perfect isolation, there should still be a substantial reduction of vibration propagation.

If the length of the wall structure is resting firmly on the slab I should think the transmission between the wall and floor would be maximum, whereas if the wall were decoupled, the transmission (energy of the entire wall structure) would have to channel through the few bolts, and if these bolts were anchored into plastic plugs in the slab, and the bolts sheathed such that there was decoupling between the metal bolt and the frame, then a certain amount of decoupling should be had as opposed to if the wall footer were laid directly on the slab.

I always understood that every interface of materials in a given transmission path causes a certain amount of transmission loss - this is the basis of some isolation or decoupling techniques.

Can anyone explain or demonstrate why decoupling measures as described above are generally considered futile and useless, or would you concur that although there will still be some propagation through the bolts, the overall coupling will be reduced.

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PostPosted: Fri Feb 24, 2012 12:29 am 
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and if these bolts were anchored into plastic plugs in the slab, and the bolts sheathed such that there was decoupling between the metal bolt and the frame,
Lots of conditionals in there, Brian! :) "Therein lies the rub", as they say.

So, yes IF you put rubber under the framing, and IF that rubber happens to deflect (compress) to the right degree when the wall is complete and the full load is on the rubber (kg/m2 correct for % deflection), and IF you use suitable isolation collars on the bolts, and IF you apply the correct pressure to the nuts when you tighten them so that they to exert the correct load on their isolation collars, then yes, you can decouple the frame. But if NOT (ie. if any one of those "if's" doesn't work out) then the wall flanks to to the floor.

So how bad can that be? After all, it's just a simple thin piece of metal touching a piece of wood! Well, have you ever tried tapping a tuning fork, and holding it up in the air? Can you hear much? Nope. Now set down the "thin metal" end down on your desk, and see if you can hear any flanking going on... :)

The right way to do it:

http://www.acoustiguard.com/Iso-Sill-Isolation-Pad/




- Stuart -

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PostPosted: Fri Feb 24, 2012 8:32 pm 
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I'm not disagreeing here, I just think there's a lack of quantification with which to decide it's value. We know that many principles and cause/effects in acoustics don't seem to follow logic (in reality I'd say it's more a lack of understanding of the logic involved), and this is an area that no one seems to explain - why is it not possible or rather not worth taking steps to detrimentally influence the efficiency of transmission?

Consider the situation from the opposite point of view. Assume we want to build a wall-floor construction with the intent of maximizing transmission - i.e. TL=0.

If we place insulation under the footing and then bolt it to the floor, this would surely degrade the transmission efficiency as compared to bolting the bare footer to the floor without insulation. The overall surface area of the mechanical coupling between the wood and concrete would be reduced. The previous conditionals may in this case be ignored because the two example constructs are identically connected (bolted) apart from the aforesaid insulation (e.g. neoprene) in the one case. The insulation layer adds an additional set of material interfaces (wood to insulation, insulation to concrete) to the complete coupling mechanism which should reduce the overall efficiency of transmission.

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PostPosted: Fri Feb 24, 2012 10:55 pm 
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On a further note,
We understand the spring principle well enough, and there is no denying it's efficacy in isolation construction - being one of our primary and most effective techniques.
Another principle however is also involved but rarely acknowledged - the interface of dissimilar materials. I read a paper some time ago where this principle was demonstrated. Unfortunately I can't find it at the moment, but it explained how the propagation of a wave loses energy when bridging two differing materials primarily if I remember because the speed of sound is different in different materials. The juncture forces the wave to change speed and this results in energy being expended not to mention the losses incurred due to scattering effects at the microjunctures. This causes a degradation of the waveform - a sinus wave input became a sawtooth. With an appropriate selection of materials and due consideration given to their sequence, this can be a useful technique primarily for trapping but also for insulation.

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