concrete and sound

[jodyconcrete]

The short version is this- I am a music producer and engineer. Two years ago, I hired an acoustic designer and contractor to build a small music studio (approximately 600sq feet.) It was built from scratch; new footings, etc, with concrete block walls, and a 6" concrete slab floor.

The initial phase was to build the exterior and the pour the slab over 2" of rigid styrofoam and radiant-floor heating. Engineered drawings called for 6x6 6/6 wire mesh. It appears that they didn't use any rebar, but did use a smaller wire mesh (which was not lifted and is sitting directly on the styrofoam)

Once the structure and floor was built and poured, the interior was built according to the design of my acoustic consultant, with a high consideration of sound flanking, etc, including isolating different sections of the slab with floor-cuts.

Here is the dilemma: while the structure is aesthetically beautiful, and the general acoustic design is fantastic, sound is somehow passing through the concrete floor at an unacceptable level throughout the entire structure.

I understand that concrete can transfer sound, but if you lightly tap a pencil on the concrete at point "A", it can be heard up through the floor at point "B" 25 feet away. If you tap on the exterior block wall, you can hear the sound on the interior of the structure only through the concrete floor everywhere, while it doesn't transfer through the interior walls.

The sound that transfers in not muffled in any way.

My acoustic designer has confirmed that the slab is in direct contact with the exterior walls, but I am also wondering if the composition / engineering of the concrete slab is in someway deficient. We are at a loss over here. I suspect that the slab was not poured correctly.

Thoughts??

 

[MikeHalloran]

Well, since there's no steel _in_ the slab, it will be relatively easy to break up and remove.



Mike Halloran
Pembroke Pines, FL, USA

 

[jodyconcrete]

Hi Mike,
Thanks for your reply. I wish it were that simple… the space is already finished. glue-down engineered hardwood, double-layer walls, acoustic doors, acoustic panels, and all of my gear.. ugh. Any thoughts on how different quality could be at-fault here? It was specified at 32MPa, but I don't know if that's what's actually there…

 

[MikeHalloran]

Sorry, I was being a little sarcastic.

Have you got enough headroom to put in a false floor suspended from the double walls?

Mike Halloran
Pembroke Pines, FL, USA

 

[jodyconcrete]

Ha.. no problem. Yes, that's what we've actually been discussing. The space is divided into three rooms. One is a powder-room, one a small vocal booth, and the main room is for mixing music… If we put a floating floor in the powder room and booth, that would at-least help transmission problems. What still remains, is the problem from sound from the exterior of the building- it's obvious now that the slab is touching the block walls.. what I'm wondering is if the quality of the slab content itself is also at fault, or if it was more about the physical pour...

 

[MikeHalloran]

At my very first job, I swept floors for a Civil Engineer.
In his lobby was a brick-sized sample of MearlCrete(r), which looked like gray Styrofoam with rather large pores, and smelled like concrete. ... which indeed it was, foamed concrete.

I've always thought it would make a wonderful energy saving outside wall covering for our concrete block houses here in SoFla, with its relatively low thermal conductivity and resistance to insects. Last time I checked, Mearl Corp. was trying to license the process, but showed no apparent interest in producing the stuff in quantity.

... but it was still relatively stiff, so I wouldn't expect it to act as a sound break. I certainly wouldn't expect any solid-ish form of concrete to act as a sound break.

Given its ability to support a concrete slab while acting as a thermal break, I wouldn't expect Styrofoam to act as a sound break, either, but I might take a shot at using it around the perimeter of an interior poured slab surrounded by block walls. Was that your intention?

What's under the Styrofoam that's under the slab?



Mike Halloran
Pembroke Pines, FL, USA

 

[jodyconcrete]

Yes,it would have simply been used as an expansion joint at the wall, so no concrete flooring would be touching the block. Under the styrofoam should be 6" of stone, at least that's what was spec'd

 

[MikeHalloran]

While I have never had a masonry contractor _not_ screw up something, I doubt that there's anything wrong with your concrete that would affect its acoustic performance in any way.

I'd expect suspended floors to reduce transmission from one room to another. The room you leave without a suspended floor will still be subject to noise from outside. All I can think of for that is to damp the motion of the entire outside wall, all of them.

That heavy loaded self adhesive bitumen sheet sold for car doors was the first thing that came to mind, but it's expensive. It's also ugly, so you'd want to put siding over it. It works by greatly increasing the mass per unit area of a thin steel door, and by providing some viscoelastic damping of the sand or whatever is in the matrix. I don't think it will work as well on a concrete wall. Maybe your acoustics guy has some ideas about that.



Mike Halloran
Pembroke Pines, FL, USA

 

[Ron]

Jody...Mike is correct that the quality of the concrete has little or nothing to do with its acoustic properties. In fact, the lower the concrete strength, generally the slower sound will travel through it....it is one of the properties we use to check the integrity of hardened concrete.

Do you have a monolithic floor slab with the masonry sitting on the concrete or do you have footings, a stem wall and concrete placed inside the stem wall or inside the exterior wall extending from the footing? For acoustic isolation, monolithic slabs are not so good!

You might do so sound transmission testing to quantify the levels of transmission. This will help to see if your designer/contractor met existing code or industry requirements.

 

[jodyconcrete]

Hi Guys,
Some of what you're saying is beyond my level of understanding. The entire site was built from scratch. Footings were poured, and 10" CMU were build around the perimeter of the structure. Then, 8" walls were erected. After that, the slab was poured inside- the slab is definitely touching the block walls, which I believe has created a thermal and acoustic joining to the exterior walls… not sure if that info helps?

 

[TXStructural]

For next time, do not support a slab like this on rigid foam. Soil usually provides some damping but your construction "traps" vibration in the slab. Also, the irregular bottom on a placed-on-soil slab will reduce sound travel along the slab (think of a pipe with smooth walls versus one with rough walls.)

For this application, you might be able to get a company that does concrete repair and soil stabilization injection to drill through the floor and inject a bonding material (epoxy/foam)to make the slabs, foam, and subgrade more monolithic.