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[Travis Engler]

When placing fiberglass over a cavity is there a point of diminishing returns when it comes to the fiberglass thickness and density in relation to the cavity depth?

I have a large alcove in my drum room that is about 4' deep, 6' high x 12' width.  I was thinking of making it into one big slat resonator. Is it worth the expense of making the front absorption panels as thick as I can afford or will a couple inches of 703 provide me with as much bass trapping as is reasonably needed?  Would there be any benefit of adding absorption materials anywhere inside the cavity or should I just put everything on the front face?

I was thinking of making it into one big slat resonator.

[Travis Engler]

I was playing around with the Porous Absorber calculator but I don't see how to input the difference between 703 and 705.  What numbers should I use for Flow Resistivity for those materials?

The cavity size creates some weird dips in the response according to the graph.  Having that much depth allows me to absorb lower but it creates peaks in other areas.  Do I need to treat the cavity as if it were it's own room, (corner traps?) in order to help flatten out the response of the trap as a whole?

Based on the default resistivity it looks like 200mm panel thickness is where the cavity and no air graphs are about the same.

It also looks like there is no real benefit, in terms of a smooth response, to an air cavity when using perforated panel.  Is this correct or am I reading something wrong?

[franman]

I'm not sure what calculator you're using, but If I had a 4' deep alcove to use as a trap, I would be pleased... I would line all the perimeter of the alcove with 4" of 705 (area of max pressure and min velocity) and frame out the opening. I would suggest looking at a Helmholtz Slat absorber, especially if the depth of the alcove is varied. You could create a great broadband bass trap with awesome extension.. look at using 1x5 wood slats and play with the 'slot' (space) width.. remember that the 'effective' thickness of the slat is 1.2x the thickness of the 1x wood (.75 x 1.2).. that's the way I do it.

Otherwise, you could create a broadband trap by hanging some 'baffles' of unfaced batt insulation inside the space and framing the opening then fitting approx 4" thickness of 703 into the front area. Cover the whole shebang with stretched fabric... You really can't go wrong.

Do worry so much about the 'peaks' in the response, as it is an absorber and it will definitely take the boom out of your room!!

Just do it!!

[Travis Engler]

Thanks Franman.  I roughed in a basic absorber trap on the face and it made a huge difference.  I'll try the slat absorber as budget allows.

In slat absorbers do you only place the insulation on the back walls?  You don't put anything on the face?

I have a few corners in other rooms where the walls slope in towards the ceiling, it's a dome house.  If I put in some slat absorbers there do I put the larger openings at the bottom or does it matter?

[Travis Engler]

How low do you worry about absorbing in a drum room?  Do I need to go down to 30hz or should I concentrate on the 80-200hz range?

[franman]

Contrary to some texts, we place absorption throughout the depth of Helmholtz slat absorbers. I thing of varying depth slats as a 'combination' trap that makes use of both the Helmholtz slat effect as well as deeper porous absoption effect.

If you actually effectively trap 80-200 in your drumroom you'll be way setup!!