Axiom Home Page Forums General Discussion Home Theater Fred's Excellent (hopefully) Calibration Adventure

Here are two pics of the panels I built.


Side view of the two panels stacked behind the spl meter. They are about 2.5 feet from the back wall.


At the front wall about 2 feet out from the wall. I have room here to almost double the treatment size if I fiddle with my aqm. lighting a little.

Fred, in your first picture, it looks like your SPL meter is horizontal facing your mains. I could be wrong, but I'm pretty sure I read it should be vertical for measurements. (I have no idea if it makes a difference, but do remember reading that somewhere).

Jason

The panels are made using 1/8" ply with corner braces to hold them together and hold the fiberglass panels in place. I used thin ply to keep costs and weight down as these are strictly testing panels. It worked quite well and I will probably keep these two the way they are.

Charts:
The purple trace in all plots is the untreated response and is put in for comparison.


This one is the rear wall location you see in the first photo. I was quite surprised at how low the panel effected things. You can see some smoothing in the 22-33Hz. range.

The other big surprise was the elimination of the 40Hz hole. You can also see a reduction at 80 and 120Hz suggesting that this is a room length mode.

The big dissapointment: it got worse at 52 Hz.


Front Wall placement (2nd pic). Slight improvement at 52Hz and the 75Hz null narrows quite a bit.


Corner Placement. Interestingly here the 75Hz null is reduced a little, but not narrowed.


I was curious to know how the dining area affected the room overall. A lot of the effects are the same as other placements, but this one shows the best improvement in the 75Hz null. I wonder if this is a room mode somehow related to the width at this part of the room?

I still want to put the panels over the spl meter to see how that effects things.

Jason. It is omni-directional mics that are supposed to face strait up. The RS meter is directional and should be facing the speakers you are measuring. Since bass frequencies are omni-directional it should really make no difference which way the meter points (I think??).

When I was doing the leveling, I tried various positions with pink noise playing from one speaker and it made absolutely no difference in the spl measured.

I should try doing sweeps horizontal and vertical to see what happens.

Fred:
I hope I'm wrong here, because as a fellow member of the "Rooms that Sucketh" Clan, I'm hoping you're finding some solutions.... but.....

I don't think your panels are reducing that 40Hz hole. My understanding is that any absorbers that would make a difference in bass frequencies would need to be much thicker...and ideally corner-mounted.

Again, I could be wrong...

A question: Was the "before" plot done immediately before adding the panels, and with the mic in exactly the same place, or was it taken from previous tests? Because a mic position just a few inches one way or another over two different testing days might be responsible for that change you see at 40Hz....

Mark. Yes, that is exactly what I did. I moved the panels into the bathroom so they would not interfere with measurements. I then set up, calibrated and measured to get the untreated graph.

From there, I put the treatments in position and measured again: no moving the mike, no level adjustments. I have measured at this position on at least 4 occasions, so with slightly different mic positions, and always get the 40Hz hole

FWIW, the reason that corners are good (as I understand it anyway) is that you can treat multiple modes (length & width, axial & tangential) in one place. It has to do with the way modes accumulate in corners.

So, I to hope you are wrong this time. I am contemplating making a set of thinner pannels to see what the difference is between the two.

Or you can try moving the panels away from the wall. I have two panels, one behind each speaker, and they are about 6-8 inches away from the wall. I think the idea is to "trap" the lower frequencies in the created gap.

That reminds me, I need to run some frequency sweeps and graph them again. I forgot what difference they make.

In the front rear and dining area positions the traps were between 2 and 3 feet from the wall. I did this on purpose to get the maximum absorbtion from the panel (closest to 1/4 wave points, and at maximum sound wave velocity).

I can permenantly install the traps at the front location with the front of the trap at 22" from the wall, pretty much where they were for the test.

I am specifically interested in how much of a difference thickness will make at a given location.

It looks like I am, and I hoped so too! This means there might be hope for my room as well without having to put in absorbers that reduce my (already small) square footage another 25%!

Huh. Mark with "hope". This should be interesting.

Pass the popcorn.