Floating floor over concrete
“I am going to build a wooden frame and put insulation in it and plywood on top for my home studio recording room.
Everyone is doing it…Isn’t this a real floating floor?”
I have seen this attempt to make a floating floor before. And some have wrestled sand into the frame as a means to add mass. The limp mass provided by the sand is effective in this case.
The sand is limp mass and certainly far more mass than insulation. You control the resonant frequency of the cavities and damp the flooring with either process.
These are important things to do.
There is a standard drum riser that uses owens corning 703 as a means to damp the vibrations from the drums to the rest of the home studio area. It simply decouples as best as possible, so it is not a true floating floor in this respect.
The main issue I have with the attempt to develop a “floating floor” made out of a wood frame is really two-fold.
First, an existing Earth damped concrete floor has very high isolation ability. Hell it has the World on the other side of it, so where the thinking came from that a floating floor could be developed on an existing slab with no ramifications in respect to the existing structure is far beyond me.
The second issue stems from the notion that acoustics is somehow the domain of a musical environment. It is not. It is the domain of vibration. Everything we know and everything that we are still learning have everything to do with someone in a room testing how vibration interacts with materials.
“Planes, trains, cars, windmills, span bridges, structures, roads, cellular antennas, high rise buildings and the list goes on and on and on, all have the same one thing in common. Vibration is the enemy.”
“Either by seismic or regional weather conditions that move the object, vibration will be a part of the life span of the item in question.”
So we stop vibration by moving the object farther away and when distance isn’t enough we add mass and when mass is not enough we decouple the object and when decoupling is not enough we damp the object.
The natural resonance of anything will allow vibration at that natural frequency to pass thru the object like it did not exist. It is a misunderstanding that sound is moving from one side of a wall to the other. Sound is vibration and vibration is what is moving from one side of a wall to the other.
If it were simply sound then a closed door would stop it. But it isn’t sound it is vibration and the natural resonance of every piece of material involved in anything will either let parts of the vibration pass or it will stop other parts of it.
Earth damped concrete floors have a very low center frequency meaning it will take a vibration at least at or lower than this center or natural frequency to excite/vibrate the concrete slab. The slab has mass and it is Earth damped.
Mass Law predicts that for ever doubling of mass you can expect to ceate a 5 to 6 point jump in the STC or sound transmission class value.
Normal weight concrete weighs about 4000 lb. per cubic yard. There are 27 cubic feet in a cubic yard.
A cubic foot of concrete weighs around 150 pounds. So in order to double that you would have to be able to support another 150 pounds per every cubic foot placed on the existing slab.
So how can it be that a lightweight wooden framed, insulation damped plywood sheathed membrane is even considered as a logical method to develop an even lower transmission loss capable, mass/spring/mass floating floor?
It is a high cost interior treatment that will most likely be a source of frustration when the natural frequency of the floor sheathing membrane is excited. Excited, by the way, at a higher frequency than the existing concrete floor would have produced on its’ on.