Cover superchunks with poly = Instant Bass traps?

[scoring4films]

Just chunked 3 corners of my room, using the large Roxul "Safe 'n' Sound" batts. A total of about 100 stacked triangles (24"X24"X34"), floor to ceiling.
The room is VERY dead now, but sadly the bass issues seem to be worse, not better. Went from having one ~4dB peak/trough at 120Hz, to having a couple of ~3dB peaks at 120 and 200, and other smaller peaks/troughs in the low-mids that didn't seem to be there before.

So now I have an ultra dry room with a woofy bottom end.
What now? I'm wondering what would happen if I covered one or more of the chunks with poly. From what I've read, the poly will act like an LP filter and the superchunks will eat up more lows and highs. Would this work? Should I rip open the superchunks or just move on to making bass traps? If the poly is worth a try, is there a particular thickness I should try? Any suggestions greatly appreciated....


Thanks!

[Glenn Kuras]

You could put a poly in front of them but diffusion in the corner is not needed. I would just cover them with FRK or thin cardboard.
BTW did you test the room to make sure you really are having "more" low end problems? Where is the mix spot located??

[DanDan]

I reckon it is unlikely that your room is worse. Before and After tests need to be done very precisely. Mic in the exactly the same spots.
I suggest you take a look at the Waterfalls rather than Frequency response.
The improvement there should be dramatic.
Corner SSC's are wonderful. As Glenn says, thick paper, thin card, a plastic membrane, on the front will bounce back some HF at you, in a nicely angled way, and should considerably increase LF action.
Modes have homes, and not all of them are in the Corners.
From your Waterfalls I would take note of the problem long modes, and the nulls. Play sine waves at these frequencies in your room. You will have to tweak a little, but full resonance is very obvious. With a particular frequency/mode fully driven, move about and find the hot spots and nulls.
This should suggest places to put Bass Traps, best or dangerous listening spots, and best or dangerous places to put speakers.

You could also virtually see what and where using this hunecke.de | Room Eigenmodes Calculator

DD

[scoring4films]

Thanks for the feedback. Silly me, I did not carefully mark the test positions, so there's no way to know if the SSCs caused more problems. Though I did do many tests prior to the installation and the response never looked quite as lumpy and it does now.

I haven't figured out how to do waterfalls yet, but I can hear a VERY, VERY dramatic improvement in the RT60. Man, I've never heard reverb tails go for so long (coming out of my Genelecs, of course). It's like discovering a whole new world and I'm heaven. That said, the bottom end is a mess. I just moved into this room and this level of looseness in the low end is an unwelcomed change and it's keeping me awake at night.

I picked the current locations for the monitors with some trial and error. This is the least lumpiest spot I've found so far. I'm willing to forego imaging (i.e. place my monitors closer/farther than optimum) to get this bass stuff under control.


So here are the options I'm considering...

1. Cover all 3 SSCs with LP barriers (i.e. poly, paper, card, etc.). But will this actually help "flatten out" the low end or will it just "turn it down", with the peaks/nulls still intact?


2. Put LP barriers on SSC 1 and 2 and turn SSC 3 into some sort of "tuned" bass trap, that targets what appears to be my main room tone (that's ~120Hz -- it shows up as a peak/null in every test). I was thinking one or more triangular plywood enclosures, lined with Roxul, covered with poly, with height dimensions set to the quarter wavelength of 120Hz. Crazy?

3. Same as option 2, but forego the LP barriers on SSC 1 and 2 ('cause man do I love the RT60 where it is right now)


I would really appreciate any suggestions you guys might have. It would be nice to sleep at night again.


cheers and many thanks,
t

[Glenn Kuras]

Covering the front will help absorb a bit more low end but will also reflect some of the highs. For testing I would check out he following program to test the room.
Room EQ Wizard - REW Home Page
For your lay out it looks like you are in the center of the room. That generally is the worst spot. I would try moving forward a bit to see if you get a better responds. Also I would put some THICK absorption on the back wall as you have strong nulls and peaks coming from that area.
See the following for a layout of acoustics.
GIK Acoustics: Room Setup

[DanDan]

I think it is reasonable to presume that SSC's will effect a improvement in the horizontal axial modes. Or did we all get it wrong?

As Glenn said, centre of the room? With respect, it might be a good idea to read the basics on room setup and treatment. There is little point in us repeating the basics question by question. This is not the first square room!

Where is the Cloud and RFZ? Where are the measurements?

The diagram is very helpful. I would rotate and have the window behind me, or in front if that is not possible. Move those shelves and install a third SSC.
Membrane on the front will improve the LF performance. This is however not a frequency response thing, which is what I can only assume lumpy means.

The Room Analysis Primer was written to help get simple measurement done before the first coffee of the day. The Waterfalls arae a click away, they are in your current measurments, if you saved them.

DD

[ciro]

Originally Posted by DanDan

Where is the Cloud and RFZ? Where are the measurement?
DD

Cloud! (120 is height related).

Ciro

[ciro]

Originally Posted by scoring4films

Went from having one ~4dB peak/trough at 120Hz, to having a couple of ~3dB peaks at 120 and 200, and other smaller peaks/troughs in the low-mids that didn't seem to be there before.

+1 for the measurements, because a (only) 3db peak in the lows is not bad...

Ciro

[Glenn Kuras]

Originally Posted by ciro

+1 for the measurements, because a (only) 3db peak in the lows is not bad...

Ciro

Not bad?? Heck most would kill for that.

[scoring4films]

I guess I was spoiled by my last room. Thanks to all for the suggestions.

[ciro]

Originally Posted by Glenn Kuras

Not bad?? Heck most would kill for that.

YES (+/- 1,5) I think I wrote "excellent" first time ...

Serious, the problem obviouly are the nulls, coming from ceiling, rear wall, corners etc and not these small peaks.Room just needs much more traps (and L and W dimension are pratically the same...)

Ciro

[Glenn Kuras]

Originally Posted by ciro

YES (+/- 1,5) I think I wrote "excellent" first time ...

Serious, the problem obviouly are the nulls, coming from ceiling, rear wall, corners etc and not these small peaks.Room just needs much more traps (and L and W dimension are pratically the same...)

Ciro

I agree, but covering those peaks is not a "bad thing".