Filling a closet as a corner bass trap? Advice welcome!

[musicwins]

Hi all. I'm working on acoustic treatment for my home studio and, because I'm in a small, almost-square room, I'm considering using an existing closet as a DIY corner bass trap. I'll attach my REW measurements from my untreated listening position, some acousticmodelling.com simulations I've been running, and a few photos including a floorplan.

The room: 322cm x 346cm with a little nook where the door enters (73cm 97cm). Floor plan attached with color coded walls. Desk is on the red wall. The closet is on the orange wall -- two side-by-side folding door closets, roughly 247cm tall x 73cm deep, sitting essentially in a corner.

REW: Bass is a mess below 200Hz, which is what you'd expect from an untreated room this size. I'm trying to address that before I do anything else.

The plan: I have two closets next to each other, both with bi-fold doors. I'm going to remove one of the doors and fill the closet with insulation to use it as a deep corner bass trap. My acousticmodelling.com simulations suggests that 500mm of porous absorber with a 100mm air gap performs meaningfully better than 500mm flush against the wall, and better still than shallower options. Since the closet is ~730mm deep, I have room to actually do this properly if I leave an air gap behind the fill.

My questions:

1. Is this a bad idea for any reason I'm not seeing? I'm very new at this. I've been drinking from a fire hose for a week, so I have lots of blind spots. Assuming this isn't a ridiculous idea...
2. Is the air gap actually worth preserving, or is filling the entire closet and covering it with acoustic fabric just as effective at these depths? The modeling suggests the air gap matters, but I want a reality check.
3. I was going to build a shallow faux bookcase inside the door frame to make this look like a built-in bookshelf -- but if the air gap matters, I probably need that depth. So the bookcase idea might be out. What's a cleaner aesthetic alternative? My current thought is to remove the folding doors entirely and build a simple frame with acoustic fabric stretched over it, making it look like one large intentional panel. Anyone done this well?
4. Cost-effective fill options: I don't care at all how it looks inside the closet. Can I just hang standard pink batt insulation from the closet rod/ceiling? Creative ideas for this? I still have to read up on how loose/tight to pack things. I briefly considered blown-in cellulose but I'm not sure that makes sense here because idk how to measure it and prevent gravity from doing it's thing. Long story short, what's the most effective and best product to fill this space?
5. How deep do I actually need to go? Is there a point of diminishing returns where I'm wasting money going from, say, 350mm to 500mm of fill?

If you have time to help me out, thanks in advance!

P.S. I did my due diligence searching for similar posts, but there's a chance I missed it.

 

[thcdru2k]

If it was me I would say not worth losing closet space. Money better spent on front wall treatment and than re-evaluate

 

[Keith_W]

1. Your graphs are way too zoomed out. This has the effect of artificially flattening the curve.
2. I suggest you try equalisation first. Reducing a peak also reduces the decay time. Very likely it will be all that you need.
3. Please read this thread about audibility thresholds for bass decay. In short: the thresholds are very long.
4. The problem with broadband absorbers is that they are broadband. It will likely kill all the high frequencies whilst providing less absorption for long wavelengths, leaving you with a dead top end and boomy bottom end.

In short: think hard about what you want to do before you do it. The effect of these things is profound, and if you simply chuck thick absorbers in there you will change the sound in a really bad way.

 

[musicwins]

@Keith_W thanks.

I updated my original post without smoothing. I'll attach here as well. Your thoughts would be welcome!
I'm checking out that thread (#3). It's an intimidating read for a newbie! I'll read it a few times to see if it make sense. Thanks for sharing.

Everything I've read so far is "treat first" and "the thicker the better in a room your size" - including from some reputable sources. My learnings so far suggested I shouldn't bother EQing until I have the basics of treatment done. I'm just starting this journey, so thankfully I haven't committed to anything yet beyond a measurement mic.

I'm not sure if it helps to know what my set up is...but:

Kali LP-4's. I have them fairly close to the front wall at the moment. I don't have the low cut dipswitch activated.
They're on monitor stands that are on the desk.
I have no treatment anywhere.

 

[Keith_W]

Please upload an MDAT and not a TXT file. You will need to zip your MDAT first. Also, could you please supply your room dimensions. And what is the purpose of the system? Studio mixing, or recreational listening?

You should think of room treatment as akin to loudspeaker design. There are all sorts of choices you can make, and most of the choices will lead to bad outcomes if you don't know what you are doing.

 

[Flaesh]

Yesterday, there was a room plan here with dimensions. But the microphone location isn't indicated.
The 50 and 100 Hz axial modes are longitudinal (relative to the location of the speakers and the listener); wool in a cabinet on the side won't help much. When placing speakers against a red wall, absorption cabinets can be useful.

 

[musicwins]

@Keith_W

1. Purpose: Home studio for music recording and mixing. The only live instrument I play in there is guitar, and I also sing. A few more photos attached for more context. Right now I do all my mixing on headphones; but I'd like to get my room closer so I can mix a bit with monitors. As you can imagine, the room is very boomy and muddy.
2. REW: I'll upload all my REW results. Here's what I did:
   1. I put one speaker in the corner on the ground
   2. I put a mark at the exact center of the room and then measured there as well as a few other placements. I did the 38% spot, I did a few others back/forward left/right from the center access.
   3. The least crappy listening position to my ears is where I have my desk at the moment. I'm not confident in my ears yet, so I'm open to an experienced people telling me the data is saying something else. I do have some real constraints (See my comment below to Flaesh).
      1. R Desk Front Right Sweep.mdat
         1. This is what I posted originally. I don't have the exact location denoted, but I can measure that when I'm home if that would helpful.
   4. I then set my desk back up and put my monitors on the stand in that position.
      1. R R Desk 8x9 - Monitors on Desk Apr 26.mdat
3. The room dimensions are in the original posting.

Thanks for chiming in @Flaesh - I was originally thinking the red wall as well, but that window + sun exposure has made it untenable. One of my constraints is that I can't cover the window (it's a space used for other things as well), so the desk facing the window = blinding sun. I can do the green wall or the blue wall.

Thanks all! Lmk if anything else would be helpful.

 

[Keith_W]

1. Room dimensions please.
2. See the eBook in my signature. Go to p.55 onwards and read the section on RT60 and how to take the measurement. Report back.

FYI if it's for a studio, then the room needs to be more dead than for recreational listening.

 

[musicwins]

1. Room dimensions please.
2. See the eBook in my signature. Go to p.55 onwards and read the section on RT60 and how to take the measurement. Report back.

FYI if it's for a studio, then the room needs to be more dead than for recreational listening.

1. Room dimensions are in the original posting. I'll post again here.

 

[musicwins]

Hopefully I did this right. I didn't do all six positions as you described in the eBook, but I have eight measurements taken at various positions across my room. I've overlaid them in the RT60 tab using Topt as you recommended. I also included a few of the sepctrograms with normalized peaks.

It seems like every position sits between 380-500ms Topt in the 50-100 Hz range. The EBU studio target for a room this size would be around 150-300ms if I'm understanding things correctly, so I'm consistently 100-200ms over across all positions. At my current listening position, I got Topt of 397ms.

I'm not sure if the Zoom level is what you were looking for...but you do have all of the native REW files should you care to dive in.

 

[Keith_W]

Room dimensions have to include the ceiling height. And there is no MDAT, only screen shots.

This picture tells me that your room is full of specular reflections. The measurements converge pretty high up, indicating you have a reverberant field above 5-6kHz. I don't think that stuffing one closet full of sound absorbers is going to provide the treatment that you need. They will need to go on walls and maybe even the ceiling. Of course, if I had the MDAT, I could SHOW you where the specular reflections are coming from. I suggest you read the section on the ETC and how to determine the source of the reflection by calculating the distance from the time-lag.

That dip in the RT60 at about 120Hz likely corresponds to an SBIR cancellation since it's about the same wherever it is in the room. It is better to observe this directly in the frequency response since REW calculates the RT60 in chunks. And indeed your FR does show a nasty cancellation at about 150Hz, which I can't see properly because it's hidden in a forest of measurements. Or if you prefer, you could look at RT60 decay for a better view. But once again ... you did not upload an MDAT.

I have drawn red lines indicating the likely source of the SBIR cancellation. 120Hz has a wavelength of 2.85m, and the speakers need to be spaced 1/4 wavelength to produce the SBIR cancellation. In this case, about 71cm. If you want to confirm, push the speakers close to the wall and repeat the measurement. You only need to take one sweep.

You might note that you have a rather high RT60 at 50Hz. Compare that to your freq response curve. The reason the RT60 is so high is because the measurement was contaminated by noise.

You are correct that you need to get the overall RT60 down to below 200ms.

 

[musicwins]

And there is no MDAT, only screen shots.

Shoot. I see what happened. The .zip file is too large, so it's unable to upload; I missed the error message the first time. The docs said 10MB is the max, and it's 7.8mb, so not sure what's up. You can retrieve them here: https://www.dropbox.com/scl/fi/03du...only.zip?rlkey=gzg49j1qnx9pcv0f592fi1l0h&dl=0

I have drawn red lines indicating the likely source of the SBIR cancellation. 120Hz has a wavelength of 2.85m, and the speakers need to be spaced 1/4 wavelength to produce the SBIR cancellation. In this case, about 71cm. If you want to confirm, push the speakers close to the wall and repeat the measurement. You only need to take one sweep.

Some of my original measurements were with the speaker pushed into the corner you highlighted. Does that suffice?

The reason the RT60 is so high is because the measurement was contaminated by noise.

That might be as good as I can get in my space. No one was home, but I'll try again and double check no HVAC is running.

Ceiling height is 244cm.

 

[Keith_W]

This is your Energy-Time Curve (ETC) with arrows indicating specular reflections. The aim is to have no specular reflection exceeding -15dB in the first 20ms.

The reflection from the rear wall is as indicated. This was calculated by converting the room length to time using the speed of sound, then looking for a peak that corresponds to the expected time lag. Your room length is 3.46m, so t = 1000d/c (d in m, c = speed of sound 343m/s) = 10.1ms. Follow the same exercise in reverse for each wall.

Since I don't know exactly where the mic was placed, I can't tell you which surface was responsible for which specular reflection. But there are a heck of a lot of them, and I would guess that every surface is involved.

Anyway, to answer your question:

1. You won't benefit from stuffing bass absorbers into your closet. This is because you do not have a bass reverberation issue.
2. Your main problem is a highly reverberant room with many early and loud specular reflections.
3. All frequencies above 200Hz are excessively reverberant. As a rule of thumb, acoustic foam needs to be 1/8 the wavelength of whatever lowest frequency you want to absorb. 200Hz has a wavelength of 1.72m, so the foam should be about 21cm thick as a minimum (thicker foam = lower wavelength, denser foam = more attenutation).

As a start, I would suggest getting 4 panels. Place them at the first reflection points (including the ceiling) and then repeat the sweep. If it still doesn't reach the target, get more panels.

 

[musicwins]

Thanks. I got sick so I haven’t been able to respond thoroughly. I plan to do some follow up measurements and calculations based on this.

 

[Keith_W]

BTW a quick reality check. You aren't using a mini tripod and putting it on your chair, are you? That will make the reflections much worse than reality.

 

[Bjorn]

Of course filling an open closet against a surface will be beneficial for the lows. It's just a matter of using the right material for the depth. An airgap is never the optimal way.

It will lower the decay, minimize resonances, and potentially even out some areas in frequency response.

It's not this vs treating secular energy but both are highly needed in all rooms for a good result. Of course, if you can treat other surfaces for the lows as well, that's even better. But we do what we can and some is always better than nothing.

 

[Truth Ness]

Don't know if i missed something, but what about a curtain in front of the window? And placing your desk in front of it? That way you don't get blinded by the sun and you get other benefits like better symmetry and the ability to properly treat the first reflection points on the sides. And thick (better VERY thick as in your case) absorption on the back wall (closet) is one of the most beneficial things one can do in terms of room acoustics, especially in a music studio.
That would be the best compromise in my opinion.

 

[Flaesh]

Don't know if i missed something, but what about a curtain in front of the window? And placing your desk in front of it? That way

already discussed:

One of my constraints is that I can't cover the window (it's a space used for other things as well), so the desk facing the window = blinding sun. I can do the green wall or the blue wall.

 

[Truth Ness]

Ah okay, seems i've overlooked that.

 

[Truth Ness]

You will need absorption on the first reflection points nonetheless.
But i'd definitely fill the closet, remember room modes don't just create peaks and longer decay times at specific frequencies and places, but also cancellations (dips), which are also reduced (filled) using absorptive material of appropriate depth (or other methods like pressure absorber etc), which results in more even decay times and frequency response. The more low frequency absorption the better if you can sacrifice the space. Period.
At such a thickness it gets very effective, 650 mm using the lowest gas flow resistivity material you can get (most probably ~ 5 kPa*s/m2 glass wool, else it gets expensive) will give you significant absorption down to 35 Hz.
A layer of 25 mm or so polyester batting (dacron) in front of it to keep fibers in and a breathable fabric on top gives a nice finish.
And don't waste your money on those overpriced ready-made panels / clouds / diffusers etc. Rather diy them, it's plain simple and as efficient if not more than bought ones for a fraction of the price. Plus you can customize them, their size, depth and looks etc, suitable for your specific needs and circumstances.
There are many myths / misinformation / snake oil / ripoffs out there so watch out and in case you need help or information just ask.

Cheers