# Assess this detail



## erwinbel (Mar 23, 2010)

Hello,

I am building a new house with a HT. The house is wood framing to passive standard and is built on top of a concrete basement / garage. I plan to execute the HT as a box-in-box with decent decoupling. The drawing says it all, really. Even a floating floor can't be that hard to do: I will jsut ask thsupplier of the pads to specify the exact amount etc. I might do a third layer of drywall with Green Glue, but only between the HT and the bedroom.

I wonder what the double (or triple) drywall with Green Glue in between will do for the really low bass (sub 50 Hz)? Surely those frequencies must pass through? Will the concrete floor above the HT reflect the bass then? BTW, I got 2 LLT subs ready for action, tuned to 13 Hz....


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## bpape (Sep 14, 2006)

The green glue and extra mass helps to block sound and also to lower the resonant frequency of the structure. The more space in the cavities, the lower the resonance. Also, make sure you decouple the top plates of the walls from the joists with DC-04 or similar clips. On the ceiling, you'll need RSIC-1 or RSIC-V and hat channel again to decouple the mass from the shared joists.

Bryan


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## erwinbel (Mar 23, 2010)

bpape said:


> The green glue and extra mass helps to block sound and also to lower the resonant frequency of the structure. The more space in the cavities, the lower the resonance. Also, make sure you decouple the top plates of the walls from the joists with DC-04 or similar clips. On the ceiling, you'll need RSIC-1 or RSIC-V and hat channel again to decouple the mass from the shared joists.
> 
> Bryan


Bryan,

I didn't design all the little details, since these are obvious to me after reading a lot on the subject. But rest assured I will properly decouple the inside box (HT) from the outside box (house). I will even try to acheeve some decoupling where the tripple glass is joining the frame etc. In the (kids) bedroom, I will even use those light metal studs suited for drywall as ceiling frame since the smallish size of the room gives the opportunity to span the room without any (ceiling) hanger. Yes sir, I did my homework.

What is the aspected number of the resonant frequency for double drywall + GG? I know floated concrete floors are 5 to 10 Hz.

My concern/frustration is that sound damping and acoustics seem to be treated as two different subjects. Which may be correct for "normal" frequencies, but IMO certainly not for 50 Hz and lower. If we assume sub bass tones are reflected by concrete, but not by two sheets of drywall with Green Glue, what will happen at the ceiling of the HT I designed? Will the sub bass not pass through the said drywall + GG + drywall and the 2' cavity with fiberglass and the OSB underfloor before it hits the concrete of the floor above? Hence the sub tones will reflect back into the HT? Is the nature of the concrete above from influence, ie would a light "open cell" (with polystyrene in the mix) "screed" (?) be more absorbant to sub base? 

I read so much about this subject, yet I feel I know so little!


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## bpape (Sep 14, 2006)

Most of the bass will be reflected by the wall structure. A little gets through and the concrete will only flank a little of it through to the structure. Isolation and controlling acoustics inside the room are 2 different things - to a point. The place where they overlap is keeping sound OUT of the room to have a quieter starting noise floor, hence more dynamic range, more easily hearing low level dynamics and detail, etc.

Don't expect the concrete wall to do much of anything in terms of absorption. The only reason the drywall will help a bit is that it has a bit of flex to it which turns energy into motion as opposed to standard absorption which turns energy into heat.

Bryan


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## erwinbel (Mar 23, 2010)

Thanks for the swift reply!

Would you advise me to add a third layer of drywall (+GG?) to the ceiling structure (= lots of work) or maybe only to both sides of the dividing wall to the bedroom (easy peasy)? One side of the bedroom wall is structural and has OSB 5/8" for wind resistance. This needs at least 1 drywall layer with GG, but I could do 2 + GG (or 3). It's the most critical wall since it's the loudest room next to a bedroom.


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## bpape (Sep 14, 2006)

If the ceiling space is shared between the 2 rooms, then the isolation is going to be limited by the weakest portion of the isolation. IOW, if you do 3 on one wall, do them all. 

Bryan


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