Sand on floor

[davidm]

Hi all. I have a 20x30 space I am building out. Cinder block and metal roof as well as 60 year old concrete floor. I have 6 mil plastic, pressure treated sleepers and will cover with plywood for base floor. The question I have is: Is sand between the sleepers asking for moisture trouble? Trying to keep from getting that kinda hollow sound that may exist.
Thanks. I have searched this to no avail on the site.

[Soundman2020]

as well as 60 year old concrete floor. I have 6 mil plastic, pressure treated sleepers and will cover with plywood for base floor.

Why? If you already have a concrete floor, which is excellent both acoustically and for isolation, why would you want to put a raised, hollow floor on top of that? For what purpose?


- Stuart -

[davidm]

Thank you for responding.
The building had a very noticeable 5 inch fall from walls to center of room that had a drain for some purpose years ago. Used to be a shop or something. Very significant slope so I addressed it by scribing and ripping 2x4's to level floor. Just thought sand would help deaden the space below. Then I got to thinking that sand may be a moisture problem down the road. Any thoughts would be appreciated.
Thanks again.

[Soundman2020]

Before changing the basic structure of a building, I would look into WHY it was built that way. if the floor slopes, it is for a reason, and interfering with the design purpose of a building structure is never a good idea, unless you understand exactly why it was built that way, and exactly what the consequences of modifying it will be. A sloped floor usually indicates there was a need to force water to run off, which may due to the original purpose of the building involving wet floors, or it may be due to expected flooding, high water table, rain, etc. issues. Modifying the floor so water can no longer run off as originally designed, without first understanding the consequences, is not recommendable. I would suggest that you get an experienced architect to look into the reason why the floor was sloped by the original architect, and what would be the result from changing that. Very likely, you will need to include some mitigation in your new floor design to deal with the water problem if it happens to return. In which case, "wet sand" would certainly be a major headache!

If it turns out that there is no longer a valid reason to retain that slope, then the best way of dealing with that is to simply pour new concrete on top of the old, with a suitable bonding agent in between, and make sure the new one is level. 5 inches is a bit much for leveling cement, but ordinary concrete would do the trick.

From the acoustic side, I would never recommend a raised wooden floor for a studio, unless there are very, very good extenuating reasons for doing that, and if that turned out to be the case, I would want it designed properly, with both acoustic and structural issues taken into account.

Raised floors are resonant systems. They ring and vibrate at a certain set of frequencies, which are governed by the building materials themselves, and also the cavities and damping in the bays. That has implications for both isolation and also for room acoustics. The floor needs to be specifically designed to deal with those resonant issues, to prevent them from being a problem. Just laying out some 2x4s and nailing a wooden deck on top, without any design or calculation, is very probably not going to work out well.

Here's a link to a control room that I designed where the rear section of the floor is in fact a raised wooden platform: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 The entire rear floor (from a couple of feet behind the mix position, back to the rear wall) is tuned to complement the room acoustics. The spacing between joists is carefully calculated, holes are bored in the joists to interconnect the bays, some joists rest on the floor, some only support the deck, insulation is installed as needed in individual bays, and the deck sandwich was also calculated, as was the curved elliptical shape of the front edge.. All of that was done so that there are no unwanted resonances in the floor. It took weeks of work to design, and weeks of work to build. Here's what the framing looked like, part way through:

RDMIS-elepitcal-floor-framing.JPG

And here's what it looked like when mostly finished:

RDMIS-floor-done.JPG

The reason for the acoustic measurement mic on the stand, visible just in front of the raised floor, is to check that the results of all that work are as expected: That there were no unwanted resonances going on in that floor, and that it was doing what it was supposed to do.

So, to answer your question indirectly: Filling a floor with sand is a valid and useful tool in the studio designers toolbox. It can be done, if it turns out that this is what is needed, provided that suitable precautions are taken. The sand has to be of the right type, and perfectly dry, the slab has to be tested to make sure there are no damp issues, the background research has to be done to make sure that the floor is not subject to wetting from unsuspected sources, etc. You might be able to do that, but only after doing the background research to make sure it is a viable solution, and only after designing the floor correctly. A few forum members have in fact done that. If you use the search feature on the forum, you should be able to find references to their threads.


- Stuart -

[davidm]

Thank you very much for your reply.
The space is on "high ground" on our property. It was used for cleaning up old vehicles and farm equipment. The drain is located at the center of the building and the output is at the front of the building and there is no real concern for additional water infiltrating the space as I have filled the drain with concrete. I checked with concrete companies and it was going to be more money than I have available to do considering I would need a pump truck or a heck of a lot of labor with wheel barrels getting the material to the site. I appreciate the insight into the frequency issues surrounding my situation. I am glad to hear that sand is a possibility. I have Googled sand on floor, and many variations of the concept and have come up dry for information. What type of sand would be recommended? I will look at this site further to see if I can find the threads related to floor structure design. Thank you for the photos. I will add that I am a serious hobbyist that works as a Physical Therapist by day and wants to finally have a place to record at night. I have used every closet, garage, and small bedroom in all of the places I have lived for the past 40 years, so this space is a dream. I am trying not to screw it up.
David

[Soundman2020]

What type of sand would be recommended?

When I use sand for mass and / or damping, I use course river sand that has been thoroughly dried and is very clean, free from any organics. Even so, I would use pressure-treated lumber for your application, to help protect it from possible issues.

That said, if you live in a humid climate, I'd be vary wary of using sand. It is hygroscopic, and you certainly don't want to have mold growing under your floor. If you live in a very dry climate, it might be an option, but not in a humid area.

- Stuart -

[davidm]

Thank you so much for your time and expertise. Much appreciated. I am very fortunate I found this forum.
David