floating a concrete slab floor - 4 questions???

[brookster]

Hi All ,

I've decided to go for the concrete floor system in my new build. However I need to provide figures for my local environmental health officer to show him how such a floor can be pushed to a resonant frequency lower than 20 Hz, additionally I need to confirm my thoughts on how it should be built.

1. How do we determine the resonant frequency of a concrete floor in relation to it's cubic mass. If I have a room 5m x 3m and 10cm deep giving 1.5 cubic metres, what other factors come in to play.

2. I've read a lot about concrete slabs being mounted on 4" RW3 rockwool, and though this seems straight forward it seems that maybe a spring system would be more appropriate. If using springs what should be the desired amount of compression when loaded with the floor and its contents (i.e. 50%, 75% etc.) and is it possible to use car springs?

3. Should the walls be mounted on the slab or independently. This seems to be a lot of subjective opinion on this but I'm simply looking for the best solution.

4. Should the ceiling be rested on top of the walls atop neoprene, or is better to suspend it from the existing structure so it remains independent of the new room and simply slots in there surrounded by a minimal gap to be filled with caulk.

Thanks in advance guys, this site has been an immense help and a true treasure trove of info

[z60611]

Kinetics makes both pad (RIM) and spring (
http://www.kineticsnoise.com/arch/noise ... crete.html
http://www.kineticsnoise.com/arch/noise/lsm.html

You should check out what Paul Woodlock is doing with 50mm Sylomer Elastomer under a concrete pad
http://forum.studiotips.com/viewtopic.php?t=107

[Eric_Desart]

Brookster,

If you use helical steel springs the calculation is easy. (Helical springs are linear springs).

Then the formula becomes:

Metric
fo = 15.8/sqrt(d)
fo = resonance frequency
d = deflection (compression in mm)
d = (15.8/fo)^2

Imperial
fo = 3.13502/sqrt(d)
fo = resonance frequency
d = deflection (compression in inches)
d = (3.13502/fo)^2

If you use synthetic pads, you must know the properties because they work non-linear. Therefore you need the data from the producer.

Your airgap (cavity) however is also a spring.
On my site you find an Excel mass-spring file.

For stability reasons is always easier to put the whole room as one unit on the springs or pads.
It's not easy to decouple walls from the floor and keep them stable.
So then you get a floating room rather than a floating floor.
Depending on required TL and decoupling (very low fo) such springs can start looking as springs used in trains rather than cars.

Linear spring without side measures will act as very undamped springs.
Undamped springs have the advantage that they give higher decoupling in the decoupling region > (fo x sqrt(2)) but the disadvantage that the mass-spring becomes much more explicite (very strong amplification at fo).

Synthetic pads or rubber/neoprene are mostly more damped (in varying degrees depending on type of material).
If using helical springs one must be carefull for the critical frequency of the spring itself. This are standing waves within the spring wire itself (is higher frequent).

However there can be numerous reasons to follow other paths.
You can use any spring as long as you are aware of the properties and stability. You don't want springs (and your floor/studio with it) to knick sideways.

For a floor alone, the rockwool solution is or can be a perfect one.
Normally producers have selected boards for those purposes, knowing exactly the dynamic stifness versus thickness allowing to calculate the fo yourself.

Cost comparisons, loads and whatever are for you.

Best regards
Eric

[brookster]

Thanks guys - great stuff!