u/bigsink22

https://preview.redd.it/dija62t0asjh1.png?width=2574&format=png&auto=webp&s=a5984e03398769d5a0cc0db4657797310cf3ca7a

[SOLVED] Can anyone make sense of why there is a large impulse after the first one at t=0, about 840 microseconds apart? This measurement used 2.1 channels with 2 subwoofers powered by an AVR. After running EVO A1 Audyssey, there are still two peaks like this, but the second one is at t=0 and the first one arrives before t=0:

https://preview.redd.it/y2q1woxjasjh1.png?width=2570&format=png&auto=webp&s=7239f32b74bd3f6a10593e5cfe34021cd0d5523e

1. Before

2. After

3. Spectrum app initial testing to identify room mode.

4. amhroc Room Mode Calculator results

5. amroc Pro results

6. DIYed Acoustic Cloud

This is my first time using REW and first time targeting a specific room mode with treatment, but it seemed to be pretty straight forward when taking a scientific approach. I have a very annoying 120 HZ room mode in this new large room with a cathedral ceiling being used as a workspace. The resonance was so bad that it would give me a headache from my own voice just talking. Here's how I tackled it:

Devices:

• KEF LSX ii LT on 9" stands on a 6' desk.
• UMIK-1 to measure with REW.
• I unplugged the 8" Kanto Sub8 to fine-tune the speakers first, sub placement will come later.

Identifying the room mode:

1. I heard a loud resonance in the room when listening to music and when talking, so I used the "Spectrum" mobile app for a quick way to identify which frequencies were a problem (image 3). I played some pink noise and the app was pretty accurate at identifying 120 HZ and 180 HZ peaks.
2. Played a 120 HZ tone through speakers and moved myself around the room to figure out which direction the mode was in. I noticed moving forward/back and left/right did not affect the amplitude I heard much, so I tried squatting down and stepping up on a stool which did have a major effect on the volume of the tone, so I got a hunch that it was a vertical room mode.
3. I used amroc's free room mode simulator (image 4) which confirmed that it was likely a vertical room mode at 120 HZ. Since the room has a cathedral vaulted ceiling, I had to play around with the room height a bit till the chart showed room modes at ~120 HZ.
4. I also payed $5 to use amroc Pro to be able to use a non-rectangular room shape (image 5), but this didn't really give me much more information, nor was it as accurate at identifying the mode frequencies.
5. I decided a large acoustic cloud suspended right above the desk and listening position would be the best move given the amroc graphic visuals. I needed to determine how far from the ceiling to hang it. For this, I did 1/4 of the 120 HZ wavelength of 9.38' which gave me a distance of 28" from the ceiling. This would be the distance where the sound pressure is least but sound velocity is greatest, for this frequency. You want to avoid the blue and red areas in the amroc graphics for treatment because those are pressure maximas and velcoity minimas.

I built the single 40" x 60" panel using the following:

1. (1" x 6") common pine boards for the frame (I picked the lightest weight wood at Home Depot). Wood glued and screwed butt joints to make a 40" x 60" frame that was 5.5" (6") thick.
2. 1" x 2" pine for 2 supports running parallel to the 40" members against the front face so that the cotton batting doesn't slump down much from gravity.
3. 100% cotton muslin fabric to wrap the wooden frame.
4. 12 yards of 27" wide 100% cotton upholstery batting which weighed 13 pounds from the "onlinefabricstore.com". Sound is never as important as avoiding the health risks from the industrial chemicals and materials used as insulation in homes. I folded the insulation over itself until it was the right height of ~5.5 inches and had to make a cut length wise to get the width to stack up to the 40" frame dimension I was planning for.
5. Stapled fabric while pulling very taught.
6. To hang, used 1/16" wire rope from Home Depot to make four connection points with some screw hook hardware and added adjustable hook and loop turnbuckles to each of the four cables for leveling. Made the wire rope ends into loops using just ferrules I crimped with electrical pliers, no need for a swaging tool for this, I did double up on the ferrules at each loop just in case. Made sure to screw hooks into ceiling joists.

The online resources on YouTube, this subreddit, and r/audioengineering were very helpful, so I made this post in case it helps anyone else.

The 180 HZ room mode was not quite as loud, so I didn't bother targeting it, but it was also an axial vertical mode stemming from the 60 HZ base mode (multiply by 2 to get the 120 HZ main problem and by 3 to get the 180 HZ problem). Although I didn't mount the panel at the perfect height to target the 180 HZ node, it was good enough to also fix that issue for me.

I do have a question in case any experts are reading, why might now the loudest room mode resonance be 113 HZ? Image 2 shows this at the only red tear drop area I now have. In the before spectrogram graph, 113 HZ was not very high, but now it is the most excited frequency. Thanks.

Cons:

now I have a big shadow over the desk, off to r/lightingphile I go