acoustical sealant performance

[ngcost]

I have had a hard time finding info on actual acoustical sealant performance in isolation, just recommendations to 'seal gaps' or combination performance when applied between sheets, but not how it functions alone.

For instance, If you seal up a gap between two abutting sheets of sheetrock with acoustical sealant, would the acoustical sealant performance exceed e.g. two layers of 5/8" drywall (assume non staggered)? Is the drywall the weak link as long as the gaps are well sealed with the sealant?

e.g. something like a 1/16" gap doesn't look like much but over a length of lets say 20 feet, thats equivalent of about a 4" square hole filled with acoustical sealant.

Does acoustical sealant work better or worse at certain frequencies?

Thanks for any clarification on this detail.

[Soundman2020]

Acoustic sealant (and caulk) does several things at once:

1) It seals the gaps to make them air-tight. Any place that air an get through, so can sound.
2) It decouples: since it remains flexible after it cures, the seal also decouples the things that it is applied to. Sometimes this is very important, sometimes it isn't.
3) It replaces the lost mass. Since a gap is by definition an empty place with no mass in it, that's a big problem in an isolation wall. If there's a gap between the drywall and the floor, for example, then there's no mass there (in addition to no seal). That has to be replaced. And caulk is much more dense than drywall, so it can easily make up for the missing mass. It's about twice the density of drywall, so if you only need a bead of caulk about half the thickness of the drywall, to make up the mass. If you make it thicker, that's great, but as long as it is half as thick then you are OK.

e.g. something like a 1/16" gap doesn't look like much but over a length of lets say 20 feet, thats equivalent of about a 4" square hole filled with acoustical sealant.

Exactly. A tiny gap like that, if you left it unsealed, would trash the entire isolation of the wall. Sealing it with caulk or acoustic sealant will prevent that disaster. Most people don't realize just how serious a tiny gap is: you did the math, obviously, and proved to yourself that it's a VERY serious thing! Which is why you see the recommendation to "seal everything", repeated all over the forum. It really is VERY important.

Does acoustical sealant work better or worse at certain frequencies?

Acoustic sealant and caulk are basically mass, so Mass Law applies here, just as it does to the drywall itself:

TL(dB)= 20log(M) + 20log(f) -47.2

M is the surface density of the panel (mass per unit area (kg/m²) ), and
F is the center frequency of the third-octave measurement band

So yes, as with all mass, it works better at high frequencies, worse at low frequencies. And the equations for 2-leaf MSM isolation also apply to sealant / caulk, in the same way that they apply to drywall, bricks, concrete, plywood, OSB, etc.

Sealant / caulk is also resilient (rubbery, soft, pliable, flexible), so that's an additional factor to take into account if you really want to, but it improves the response, and the effect isn't large anyway, so just thinking of caulk as "mass" is a reasonable assumption.


- Stuart -

[ngcost]

Great information, and thoughtful reply. Especially the relative mass information compared to drywall. Thank you.