John Sayers' Recording Studio Design Forum Archive

We have a 5.1 mixing room that is currently working quite well. Unfortunately, the floor is concrete that was never cured and sealed, and I am concerned about the effect doing this will have.

We didn’t know that the concrete was not cured and sealed at the time (actually, it had been sealed, albeit unsuccessfully), so the mixing room was finished, tested, etc. a couple of years ago. I feel that the only way to handle this is to cure / seal the floor, have the room tested again afterward to find out what the effects / problems are, and then treat the problems that arise. I’m concerned, though, that the additional reflection from the finished floor will mean that we have to re-cover it, which we will be hard-pressed to do because of the expense.

More about the room: it’s about 3276 cubic feet (about 21 feet wide, 19.5 feet long, 8-9 feet high (the ceiling is sloped) (these dimensions aren’t exact, but I imagine their exactness isn’t crucial to this particular problem). There is a Roxul ceiling cloud covered with acoustic fabric above the mixing area and a three-foot-deep bass trap at the back of the room, meaning that the back wall is Roxul covered with acoustic fabric, beyond which is a three-foot air gap after which is a ragged stone wall (I did not include the area of the gap in the room dimensions). I’ve attached an image detailing the layout of the room and the construction of the walls (i.e. which ones are double drywall and which ones are fabric-covered Roxul).
There had been an issue with the concrete floor and drywall ceiling from earlier on that I think will be indicative of what may happen when the floor is cured and sealed: around the edges of the room, there was a flutter echo (by which I mean a hand-clap would buzz). We dealt with this by installing foam pieces along the top corner of the side walls (the drywall ones), as well as some additional foam on the ceiling (using some pieces that we had from an earlier room). I’ve included a photo of one of these walls to show the position of the foam. I imagine that making the floor more reflective will make that reflection problem worse (and that the ceiling cloud will help a lot), but it’s hard for me to imagine what “worse’ might mean. Maybe the existing foam will continue to dampen the reflections the same way it’s doing now?
I’m posting because I’m not sure what to expect. As I mentioned previously, I imagine the only way to really deal with this is through curing and sealing the floor, then testing the room again and fixing any problems that arise. Does anyone have experience with a similar situation - i.e. making a surface more reflective in a room that already works well? I realize the situation is a bit vague in that there isn’t a problem yet, but if anyone can offer any advice or encouragement / discouragement, I’d really appreciate it. Thanks!

Hi there "T Elliott", and Welcome!

the floor is concrete that was never cured and sealed,

I'm not sure that I understand what it is you are intending to do. While sealing concrete can be a good idea, there is nothing you can do to "cure" it: Concrete cures naturally, at it's own pace, through a process known as "hydration". That's a chemical reaction between the water and the various dry components in the original mix, that commences as soon as he mixing process starts, and continues forever. Concrete never actually "cures" completely, even after decades: it just continues to get gradually harder and harder. It reaches a very large percentage of its theoretical maximum strength after about 30 days, and can already take a heavy load and be built on after just a few days, but technically it carries on curing, slower mad slower, indefinitely.

There are several things you can go in the initial few hours and days to help it cure, such as keeping it damp, controlling the temperature, not subjecting it to loads or movement, etc., but after about 5 or 6 days, there's not much more you can do. I have never heard of anyone trying to "cure" concrete months ore years after it was poured: I have no idea what you would even do to improve the cure after such a long period of time has passed.

So sealing: Yes. Curing: ???

Maybe you could explain what it is you plan to do to "cure" your concrete! Are you planning to take a jackhammer to it, break it out, and pour a new slab, but taking care to cure it properly this time?

Also, what makes you think that your concrete needs additional curing anyway? What is wrong with it? Is it cracking excessively? Is it flaking? Blistering? Spalling? Is it exuding salts? Is it disintegrating into sand? Is your rebar corroding? In other words, what physical problem are you seeing that leads you to believe the concrete needs to be "cured"? From the photo you posted, none of those issues are evident. Maybe the problem is in a different section of the floor? Please post photos of the actual problem areas.

I imagine that making the floor more reflective will make that reflection problem worse

Why do you think that the concrete will become more reflective? Re-sealing the surface is not going to change the density, and neither is "curing" it. The density of the concrete was decided at the time the order was placed with the concrete supplier: that order would have specified the type and strength of the concrete, and any additives that might have been wanted for whatever reason. That's what set the density, and it cannot be changed now (unless you do plan to rip it out and re-pour it, in which case there could be a small change in density). But even then it is very unlikely that it would have any noticeable effect on the room acoustics. I'm not aware of any acoustic research that has shown anything to be concerned about with the density of concrete in studio floors. When I am designing a studio, I specify the concrete that I want and assume that it will have a density of roughly 2200 kg/m3. If it turns out to be 2000 kg/m3 or 2400 kg/m3, that would make no difference at all to the way I treat the room after it is completed

So I guess I'm just a little confused as to what you are concerned about here! Please explain in more detail why you think a change in concrete density or surface treatment is going to have large effect on your room acoustics.

For your own peace of mind, I would suggest that before you make any changes at all to the room, you should first accurately measure the acoustic response of the room as it is right now, using the REW acoustic software package (which is free!). Post the results of that test here on the forum (the actual MDAT file, not just some graphs) so we can look at it for you. Then after you resurface (or replace) your floor, do another test with the mic in the exact same place and all other conditions being identical, and post that MDAT file as well. Then we can compare the two, and see what changed (if anything). Based on the differences between the two tests, we can then suggest what type of treatment you will need, and where to put it.

(NOTE: For each test set in REW, you will need to do nine individual tests: one test with just each individual speaker on by itself and all the other speakers turned off, then one with the sub plus the left main, one with the sub plus the right main, and one with all of them on together. Do not change the calibration between tests! calibrate carefully to make sure you are getting 86 dBC with all speakers on and correctly balanced).


- Stuart -