[SoCalWJS]

I'm trying to figure out what to do, but don't quite have enough info to decide which course of action to take.

I see that both the Tri traps and OC 703 are for sale on the GIK website. They have posted info for the absorption rate/frequency of the Tri traps, but I can't find data on the OC 703.

If I cut 2" sheets of 703 into 2' triangles (so it works out to 4 triangles per sheet and would stack 8" high) and stack them, do they have a higher, lower, or similar characteristics of the Tri Traps? I have a nasty "suckout"  (almost 20 db) at around 50-65 Hz at the primary listening position which I am guessing is a result of cancelation from standing waves.

Which solution would be more effective?

[bpape]

If you cut the 703 into 4 triangles per piece, they'll be considerably larger than the Tri Traps at 24x24x34".  The Tri Traps are 17x17x24". 

The larger thicker ones will reach lower. The big question is whether or not the problem you're having is going to be addressed by corner treatment or not.  If it's a boundary interaction or a cancellation off the rear wall, we can treat every corner in the room and the problem may still largely remain.  I would want to see if we can identify the source of the problem before throwing random treatment at it.

Bryan

[SoCalWJS]

Thanks for the prompt reply Bryan!

I'm pretty much stuck with the current setup and current room as it is not a dedicated space. I have farted around with speaker placement and a few treatments (GIK 242 at first side reflection points). I recently purchased an Omnimic and spent about a week playing around with placemant and treatments, but I can't get rid of the dropout, regardless of speaker placement. Further away from the the front wall only reduces the dropout by a couple of db. I placed solid sheets of the OC703 in the corners behind the speakers, 6 behind each (3 on top of 3 so that it is 6" thick, 8' tall). It reduces the dropout a couple more db, but that is all. I tried adding the 242's as well (both behind same speaker and measured it by itself), but there was no further improvement.
The room measures 186" along the wall with the speakers. The other dimension is 212", but it is only a partial wall - it opens up to a lower floor which is a large space. The ceiling is a cathedral type with the low end behind the speakers, but still 8' tall.
Thinking about cutting up the OC703 into wedges, which would only be 4' tall behind each speaker, and adding either more 703 or tritraps on top.
I'm at my wits end on this one.

[bpape]

What happens when you move the mic front to back?

Bryan

[SoCalWJS]

What happens when you move the mic front to back?

Bryan

The dropout reduces further, but does not disappear. I can get it to reduce to a point where it's an acceptable level, but then I am in a nearfield position and I have linesource speakers. Would love to have a slightly larger room, but...........

[bpape]

Then treatment on the wall behind you would be your best option to offset the wavefront off of that area which is causing the cancellation most likley.  Does it simply reduce or does it also change in frequency center?

Bryan

[SoCalWJS]

Tough to describe this....

The wall behind me as I am listening is a half wall - but it comes down from the top of the cathedral ceiling at a height af almost 12' to a height of about 5'. The lowest 5' opens onto the lower rooms with a railing separating. I have curtains across that opening to control  light - hope that made sense.

As far as the frequencies affected by the dropout changing, it is only in amplitude. The frequency stays at the 50-65 Hz range (really begins at 45, but only a couple of db doen at 50, which I could live with).

[bpape]

OK. If the center isn't changing, then likely it's not purely rear wall. It's likely 2 different things going on and you're getting more favorable interaction when you move forward.  Is anything else around there changing in frequency?

Bryan

[SoCalWJS]

Do you mean boost/drop in a nearby frequency?

I focused so much on the main problem, I'm having trouble remembering the other things I saw. I think I remember there are some problems around 120-150 Hz, but it seemed to change pretty drastically with slight changes in the position of the microphone. Most everything else is pretty good - in the +/- a couple of db range.

Really weird things that you observe using the Omnimic, and I'm not sure which ones take precedence.

You can vary the sensitivity of the frequncy response graphs. A 1/3 octave graph looks pretty flat.  A 1/48 or 1/96 octave graph looks scary in places.