concrete sound problem

[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??

 

[Ron]

No. It has nothing to do with the quality of the concrete. In fact, poorer quality concrete has better acoustical properties!

It sounds as if your exposed surfaces are "hard". This allows both direct transmission of sound and significant reflection of sound.

You might consider a sound isolation membrane on top of the concrete and turning up the wall. Sound absorption material on the walls will help as well.

Hard materials will transmit sound, seemingly no matter what configuration you use. If you absorb the sound before it gets to the hard materials, most of the problem is solved. One inexpensive method of sound absorption is to use cotton blankets (furniture moving pads work great).

good luck

 

[jodyconcrete]

Thanks Ron,
I appreciate you message.
The problem doesn't have to do with exposed surfaces. That's the dilemma; the room is acoustically designed and built on the inside. Precise acoustic panelling, glued-down engineered flooring on-top of an acoustic underlay, and double-walls that are both isolated from each other, and from the floor.

If one taps on the EXTERIOR block wall of the building, you can hear it on the INTERIOR of the structure, but only through the flooring (not the walls). Nobody in the industry who has heard or seen this has ever experienced this dilemma before. Even if it were an apartment that was built, the sound travel would be unacceptable- and this was designed as a music studio…

The acoustic designer regularly works on studios which have concrete floors without this problem.

Sound is definitely travelling through the concrete, but we can't understand how it could be travelling so easily. There is a lot of mass, so a light 'pencil tap' shouldn't have enough energy to travel as far as it does…

 

[Ron]

Is there a chance that your contractor created a reverberation void between the concrete and the insulation, so your concrete is acting as a "drum head"?

 

[jodyconcrete]

At this point, anything is possible, though I'm not sure how there could be a void between the concrete and the insulation. That said, there is radiant pex tubing, and wire mesh in the slab. They were supposed to be raised, but I am guessing that they weren't..

 

[Ron]

The wire mesh would make no difference. The pex tubing might....is it filled with a viscous fluid?

A void could be created between the concrete and the insulation because of excessive shrinkage of the concrete. If the contractor had added a lot of water to the concrete during placement, it would cause a lot of drying shrinkage, thus separating the concrete from the insulation. Further, the insulation itself could have shrunk.

 

[jodyconcrete]

Hmm… the tubing is simply filled with water. My acoustic designer has done a lot of home theatres w radiant heat without this problem. Is this an issue of "too much" water added during the concrete? The concrete itself is very light grey- hard to see any aggregate at all, and is almost 'powder like' where there are cuts in the floor(I can see the full depth there)

 

[Ron]

Your description of "powder-like" concerns me. That is a possible indication of dusting, which is often caused by too much water addition. It sounds like they probably used a very fine pump mix (did they pump the concrete in place or did it drop from a transit mixer chute?).

Measure the width of the sawcut and measure the width of a similar blade that was used to make the cut. The shrinkage is approximately the difference (assuming little blade wobble). If your sawcuts go completely through the concrete, you might consider filling with an acoustic sealant.

Other than that, have an experienced local engineer who is well versed in concrete properties and technology take a look. You should consider a petrographic examination of the concrete to assess some of the hardened parameters and with a few additional lab tests, you can actually determine the water-cement ratio and whether water was added after initial mixing of the concrete. The last one is relatively easy to determine by someone experienced concrete petrography.

You don't give your location. Let us know your approximate location and perhaps one of us will be able to direct you to someone in your area who can help.

 

[jodyconcrete]

Thanks Ron,
I'm in Toronto- I've phoned a couple of local engineering companies, but it will cost about $2K to do a core sample, which I'm not certain that I'm prepared to spend yet.

Sadly, I wasn't on-site during the saw cut. My recollection is that the cement was mixed on-site, but had to be wheelbarrowed in, as access was an issue.

One more thing- 6" of 32MPa was specified, but within the saw cut, I measure 5.5" on one slab, and only a 4" depth on the other slab.

One of my concerns with starting to dig is that there is radiant heating within the slab, and it could be a huge can of worms...

 

[Ron]

Call exp in Brampton. They have all the capability you'll need.
Here is a link
exp

You can also call LVM in Toronto (formerly John Emery Geotechnical)

LVM

 

[jodyconcrete]

Ok, thanks for your help, Ron!