Super Soundproofing Community Forum

Soundproofing Forum Topics => Acoustical Questions? Ask here about Sound absorbsion, Reflection, ETC => Topic started by: MixMan on May 15, 2015, 01:49:23 AM

Title: Tripple leaf effect in a floor ?
Post by: MixMan on May 15, 2015, 01:49:23 AM

I am in the process of soundproofing the floor in my flat.

I am roughly following  this model:

However, I have substituted the cement impregnated boards with 18mm plywood screwed (with as few screws as possible) and the top layer will be 14mm engineered wood.

The effect is generally disappointing....

After having added the Rockwool and plasterboard the very low end (under 100hz) has quietened and so has the highend...a bit.
But, it seems that the lower end of my neighbours speaking voices i.e frequencies around 100-200 have been slightly enhanced (!) and I'm wondering if I have created a tripple leaf effect that has created a resonans in the cavities between the top of the plasterboard Over the battens) and the bottom of the 18mm ply.

The gap between the joists are approximately 300 mm's wide and the cavities are approximately 4-7mm deep though they are all slightly different ffom each other as I had to fit them around noggins, supporting joists etc.

My questions are:

Can such a small space (3000x300x4mm's) potentially have created any substancial resonans?

If so:
Should I fill out the cavities ? I have thought of using thin plwood or Green Fiberboard underlay.

Also. Would it help if I decoupled the plywood floor with 5mm joist strips (or would that just create a bigger cavity) ?

- or should I just fill the cavities and screw the 18mm ply down, and put down a heavy 4-5mm rubber underlay like the

-another alternative could be to screw down the ply as is and put down Phonestar Db boards down before adding the underlay and ply ?

Any comments and suggestions are greatly appreciated.

Cheers, Andreas
Title: Re: Tripple leaf effect in a floor ?
Post by: Randy S on May 15, 2015, 04:32:31 PM
Yes, you have created the triple leaf effect.

You need to fill the cavity with an absorption product (insulation)


move the center leaf making contact to the subfloor or the final sheet below.

Randy S.
Title: Re: Tripple leaf effect in a floor ?
Post by: MixMan on May 16, 2015, 09:18:48 AM
Thanks a lot for your comment Randy.

It is quite a big job to move the plasterboard/center leaf at this stage.

1. Do you think using absorption like the green fiberboard underlay may do the trick or should the insulation be heavier ?

2. Using a heavy rubber matt could be another option...?


3. Do you think it would be better to try breaking up the waveforms by putting down varying lengths of material (I.E. filling out the gap with thinner plasterboard, thin plywood etc) ? In theory that should create diffusion and break up the soundwaves randomly.
Title: Re: Tripple leaf effect in a floor ?
Post by: Randy S on May 18, 2015, 04:12:16 PM
Changing the mass of one or more of the leafs might work just fine, but anytime you leave a hollow cavity the results may differ and results are unknown...
I have applied that method in baffle boxes to stop a standing mid range and it has worked.
The problem is you dont know until you do it.

Randy S.