There are a lot of discussions about building on concrete but it seems that all of them revolve around dealing with moisture issues or sound isolation. I have searched and searched and can't find a clear discussion on the implications of such a floor on the quality of sound inside the room (so please redirect me if there's a good thread).

So, forgetting about water and sound isolation, what is the best thing to build on top of concrete to get the best sound quality out of the room? I gather than concrete dampens low frequencies which is bad. So I need to build some sort of subfloor. If I put down something like dry-core and put plywood on top of that, is that good enough, or is it better to build up something even thicker? Perhaps a floating floor with dry-core, 2x4s and plywood?

If I do create air gaps, should I insulate them, fill them with sand or leave them empty?

Anyone? I'll take a link to another thread.

my room isnt complete, so i can verify the effects of the dricore (only) that i put down.

but i would think that the main goal is to decouple your sub/speakers from the concrete so minimize sound transmission directly into the concrete.

im sure a 2x4 subfloor with plywood on top would be much better than dricore, though i dont personally know how much. if you do go that route, definately fill the gap with insulation or sand to make sure there arent air cavities to resonate in the floor.

if its a large floor, it may be a royal pain to fill it all with sand, and may not make that much of a difference. i know that a lot of ppl do fill stages on the front with the speakers with sand though. so maybe just putting sand in the sections that are beneath the sub/speakers, then insulation in the rest of the room would be worthwhile.

im sure others will chime in on this.

Bare concrete is just about perfect. It absorbs about nothing, including low frequencies.

Andre

I believe that for typical movie soundtrack, the playback assumption is that the floor will be quite absorptive. The lower walls below ear level as well.

You're not going to create a bass trap with your floor framing, so really carpet and pad is the basis for absorption, plus whatever you feel you'd like under that such as Dricore.

Perhaps one of the room designers here will chime in.

Concrete does not damp low frequencies (it is not a diaphragmatic absorber). It is horrible, as would be any solid smooth floor surface, on the quality of sound inside a room. At a minimum, carpet over pad is a requirement to tame reverberation time and strong early reflections into the seating area. To isolate a concrete floor from the structure of the home, we willl typically have Acoustik Mat applied to the floor, 3/4" OSB or plywood over that (installed after drywall, btw), and all platforms, stages, etc. built over the new plywood subfloor.

One of the issues tossed around here has been the "tactile" difference of various floor structures.

One of the subtle sensory results of a built up sub-floor is the effects on vibration of your seat. A sub-floor can actually be set in motion by your sub-woofer. Most people prefer the See it, Hear it, Feel It experience in their HTs.

In my space I have pad and carpet over concrete for the front row but an 8 inch wood frame riser for the back. All have Aura Pro shakers. I recently discovered that you actually feel it more in the back row.

If I ever build another basement theater it will have a drycore (or other) sub-floor. I'm not sure of any benefit of putting additional layers on top other than pad and carpet.

If I may enter a question of my own. Would someone put dricore down before the walls or after?

Sorry if I'm hijacking!

Is Acoustik Mat a brand? I can't find any source for it. Google turns up many references to this forum and other acoustic forums but no manufacturer or supplier.

We carry it. I'll send you a PM

One of the subtle sensory results of a built up sub-floor is the effects on vibration of your seat. A sub-floor can actually be set in motion by your sub-woofer. Most people prefer the See it, Hear it, Feel It experience in their HTs.


The speed of sound is faster in structure than in air. If you like getting your feet or butt buzzed because the sufwoofer is vibrating the floor before you hear it - different strokes....

Personally I think a subwoofer slamming in your chest from sheer force is much more exciting when you don't have warning from tingly feet...

The add on chair devices are good substitutes if the WAF will not allow the subwoofer slamming - but it is not exactly the same as the real thing. Most acoustic designers go to effort to isolate the sub from the structure because it does make a difference.

At a minimum, carpet over pad is a requirement to tame reverberation time and strong early reflections into the seating area. To isolate a concrete floor from the structure of the home, we willl typically have Acoustik Mat applied to the floor, 3/4" OSB or plywood over that (installed after drywall, btw), and all platforms, stages, etc. built over the new plywood subfloor.

I don't believe I care about isolation. This will be a new addition to my house and there's no upper or lower floor. The foundation will be in contact with the existing brick wall on the outside of the existing house, but I can't imagine that the foundation is going to transmit sound into the house. I'll have a double wall to keep the walls isolated. In that case, it sounds like carpet and pad is all I need (which is what is in the rest of the house).

Would someone put dricore down before the walls or after?

I believe Dennis just said that you'd put it down after the drywall.

You may not care about isolation. It's your room, not mine. On the other hand, the purpose behind isolation is not really to prevent sound from leaving the room. The purpose is to bring the noise floor in the room down to an NC or NR of 20 (the average, very quiet residential home has a noise floor between 30 and 35). The softest sound on a sound track is 22dB. To hear those sounds (and understand whispers in dialog) you must raise the volume so the 22dB sound is louder than the background noise in the room. 22dB to 31dB is on the order of 8 times the power. If it takes 150 watts to reach reference at the seating position...well you do the math. (Not to mention, normal dialog and sounds are too loud for comfort and loud effects will (a) clip the amp; (b) fry a driver; and, (c) cause temporary hearing loss.)

Foundations transmit considerably more sound (particularily in the sub 500Hz range) than you'd think.

Dry core and Acoustik Mat are two entirely different products designed to meet entirely different requirements. Dry Core should be installed according to the manufacturer's instructions. Acoustik Mat and the OSB layer over it should be installed AFTER drywall and should not come in contact with the drywall.

Adding a room within a room really helps also. If I open the HT door I can hear there is a speedboat outside in front of our dock. If we had not isolated the room from that side of the foundation, we would have been listening to that speed boat reving up for hours at late times of the night up until around 11:00pm three times a week from the end of our cove.

The purpose is to bring the noise floor in the room down to an NC or NR of 20.
...
Foundations transmit considerably more sound (particularily in the sub 500Hz range) than you'd think.

Where does this noise come from? Is it noise in the ground, noise from outside that hits the side of the foundation or noise that hits the walls and travels into the foundation. Or is it just noise inside the room such as reflections or other sound you may want dampened?

When you say noise floor, does that mean the lowest level of noise heard in a typical room? With double walls and no external windows or doors, this won't be a typical room; if I understand you right, that would also lower the noise floor.

look at

http://ateam.lbl.gov/Design-Guide/DGHtm/noisecriteria.htm
http://www.hartandcooley.com/tech%20talk/2004%2007%20Tech%20Talk.pdf

It is noise in the room from all sources ... including internal sources (equipment, HVAC, plumbing, mechanical, etc. and those sounds entering the room from the outside including voices, traffic, foot steps, etc. There is a surprising amount of sound which penetrates the room from outside sources. A reasonably quick method of determining your ambient noise level in the room is to use an SPL meter. Use both "Flat" and "A-weighted" measurements. You would use an RTA to determine the frequencies of those sounds which are affecting your ability to achieve an NC20.

You may not care about isolation. It's your room, not mine. On the other hand, the purpose behind isolation is not really to prevent sound from leaving the room. The purpose is to bring the noise floor in the room down to an NC or NR of 20 (the average, very quiet residential home has a noise floor between 30 and 35).

. . . .

Foundations transmit considerably more sound (particularily in the sub 500Hz range) than you'd think.



Ok, I'll bite. How much sound transmits though the foundation floor?
I work in an environment with multiple concrete structures (i.e. all 6 sides poured concrete) and the only time I've heard sound being transmitted through the foundation was when we had a excavator with a breaker chipping away at a limestone vane that ran under the building.

My understanding is that 8" concrete walls have an STC of ~54. Presumably, a concrete slab on compacted earth would have a higher STC.

If I may enter a question of my own. Would someone put dricore down before the walls or after?

Sorry if I'm hijacking!

manufacturer recommends framing the exterior walls (right on the concrete), then playing all the dricore, leaving 5/8" (i think, you need to double check this dimension) gap around the edges, then framing the interiors walls on top of the dricore.
Te reason for doing it this way is two-fold. first, it makes it easier. wayy less cuts when installing, and way less edge pieces to install (which are the hardest). Secondly, having the interior walls ontop of the dricore kind of pins it down, and keeps it stable.

Ok, I'll bite. How much sound transmits though the foundation floor?
I work in an environment with multiple concrete structures (i.e. all 6 sides poured concrete) and the only time I've heard sound being transmitted through the foundation was when we had a excavator with a breaker chipping away at a limestone vane that ran under the building.

My understanding is that 8" concrete walls have an STC of ~54. Presumably, a concrete slab on compacted earth would have a higher STC.


Are there any other pieces of equipment coupled to the structure (HVAC, etc)? I know that for buildings they go to great lengths to decouple HVAC fans and cooling towers from the structure to prevent low-frequency tranmission.

There is a lobby next to my HT within the same concrete area that has a tile floor. This area is very boomy sounding when I open the door from the HT.

Are there any other pieces of equipment coupled to the structure (HVAC, etc)? I know that for buildings they go to great lengths to decouple HVAC fans and cooling towers from the structure to prevent low-frequency tranmission.

No. The rooms are not designed to be sound proof, and some noise enters through ducts.

Another personal example:
My house has two garages, one over the other. The downstairs garage is 10" poured concrete on two sides, and then back filled. One side is mostly cement block. The fourth is a garage door. The ceiling is 12" concrete on 12" I beams, which acts as the floor for the garage above. I dry walled and insulated the room and the ceiling. I use the garage as a shop. The only way noise enter or leaves the shop is through doors. I can't hear cars coming in above. You can't hear me working upstairs.
My point is that I truly doubt much noise enters a home theater from the foundation floor if you are on a slab, except undeer fairly unusual circumstances. That's why I asked for quantification, I'd like to know how much noise can be attributed to transmission through a concrete slab on earth, and how much a rubber mat and 3/4" plywood changes it.

I'd like to know how much noise can be attributed to transmission through a concrete slab on earth, and how much a rubber mat and 3/4" plywood changes it.

Ok, hurry up and answer this guy's questions (please!!!). They're pouring the slab for my HT next week, and framing the week after.

Not much at all according to my testing. When I place my ear against the other side of my HT screen wall I can hear voices well enough to make out dialogue. As far as the bass goes, I can feel that in the earth beneath me, and there is very faint audible frequencies. As far as the bass actually moving the concrete? That is very difficult and can be barely felt. Even when watching a movie in the HT there is very little bass moving the couch as a result to the floor itself, if any. If you have sheetrock and insulation between the concrete, your in pretty good shape. As for the 3/4" plywood and rubber mat, I'm not sure what your trying to accomplish as far as soundproofing is concerned. That might be good for some impact noise but it isn't going to contain much as far as sound in the room is concerned. If your interested in seeing some measurements of some before after testing I did of digging out hundreds of pounds of stone and gravel from my behind my screen wall, you can visit the link. You must know how to read them however. The stones have already began going into the garden one day after completion. It was a win/win situation, and mostly for testing purposes.

http://www.flickr.com/photos/tns29/sets/72157608159906311/detail/