Room EQ & treatment feedback--Oddly shaped room

thegnu

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Hi guys!

I'm setting up a mixing room, and I was wondering if I could get any feedback on my plans. I'm attaching a zip file with all the files (mdat file, CAD drawing, photos), but I'm embedding a few pics for easy review.

Basically, it's a 12'x10'2" room with a slanting ceiling along the length. The back of the room has a closet, and above the closet, there's a 24" high x 16" deep cubby that runs the width of the room. Walls are sheetrock with some plaster texture on them that seems to make them slightly more reflective on the high end.

Side.jpg Top.jpg

Interface: Steinberg UR22
Mic: Electro-Voice PL9 (48" high, 64" back from front wall, centered)
Speaker: Jamo sub200 subwoofer (center of the front wall on the floor)


waterfall.png


I'll have my monitors in the next few weeks, and I can do some work on the high end then.

Here are things I have been considering:

1. Definitely treat front corners of the room from floor to ceiling with either panels with dead space behind them or chunks. Any advice on dimensions?

2. It's more convenient to keep the doors off the closet, but I imagine if I lined the doors with acoustic material, it could function as a bass trap since the door panels are hard and reflective. Thoughts?

3. What is the best way to deal with the cubby above the closet? For now, I've been storing some cheap bed foam up there to eat up some flutter echo, but should I just treat the corners or the whole thing?

Any help would be greatly appreciated. I know the general advice is to just treat all corners, but I'm not sure the best way to approach the closet and cubby, and I'm trying to not waste money on doing the wrong thing, since my taxes went differently than I expected this year.

Thanks!

Here's a Dropbox link to all the files, if you prefer that: https://www.dropbox.com/sh/e6ee5xghn6ehbcj/AAAlt_T-Vj7v5uxAcScpIje2a?dl=0
 

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thegnu

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Here's the frequency response of the EV PL9 mic:
fig3.gif


And the Jamo Sub200 frequency response is 40 - 150Hz
 

Matthew J Poes

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Hi Thegnu,

It's a little hard to make suggestions based solely on the information you have provided. The Subwoofer waterfall appears to show strong modal ringing at 40hz, but I need more information to go by. What signal did you feed the sub in this measurement? What was the range you set when doing the impulse response?

You will need a different microphone as well, that microphone is not designed for acoustic measurement and whiles its response is flat within its operating range, it rolls off significantly below 70hz and gives no data by 40hz. I know very little about that mic beyond what I could find online and your response plot, but it doesn't appear adequate for the job. A true measurement microphone can be had for under $100 that will work better. A Dayton EMM-6 would be my suggestion in your scenario. You have the interface and know how to use it, plus you could always use it to record if need be.

Small spaces like yours are always going to need more than usual absorption, but less absorption goes farther in small rooms too. As such, its important to go by the data and not abstract advice. I have seen too many folks suggest basically covering every surface in a room like yours with absorbers without regard to the effect on the room's acoustics. A room like yours will become too dead with an uneven decay rate if you aren't careful.

A couple generic thoughts that I find helpful in small mixing room acoustics:
  1. Consider using acoustic panels that have a membrane over the surface to reduce high and mid range absorption. This should not be MLV or other heavy materials, just a low density membrane that helps increase LF absorption. Gik sells such products and I certainly recommend them, but if you want to go the DIY route, I can also share some thoughts on how to achieve similar results.
  2. The front wall in front of (behind) the mixing speakers should generally be absorptive and you want good LF absorption here. The goal is to absorb the bounce off this wall that causes what are known as SBIR effects
  3. A different subwoofer location is likely better than dead center of the wall, but a little trial and error is helpful
  4. Consider a Cloud over your mixing desk. The Angled ceiling actually helps address some issues here, but I suggest a cloud for LF absorption
  5. Consider more than one subwoofer, the bass can be made smoother and better balanced
  6. Consider moving your desk around
  7. Pay attention to diffraction sources. I see a lot of people place their monitors between their speakers in a manner that exacerbates diffraction from the monitors. This is not good for sound. You want to get the monitors up and away from the monitors as much as possible. Use of separate speaker stands is always good where possible. As with any speaker, you want the speaker at ear height though, so really what you want is the desk and monitor a little low. I've seen some great setups with monitors mounted at a steep angle low on the desk and the speakers just about ear height ensuring everything is moved far apart.
  8. I agree with your bass traps idea, but try to use the best/most efficient bass traps you have room for. Triangular traps are popular, but they are less efficient absorbs of LF's than are square profile traps. While they use up more floor space, this is often floor space people aren't using. I suggest these over triangular shapes where possible.
  9. I'll take a more careful look at your room to see, but my guess is your room is too small for traditional diffusers. If your RT60 values are too low and you find the room overly dry, a hybrid product using a MLS approach might be ok, as you can sit much closer. A low depth (under 8") QRD or PRD diffuser would probably be ok for a ceiling location or rear location.
  10. Use of a constant directivity monitor is, in general, a better option. Most studio monitors I'm aware of do not provide controlled or constant directivity. Notable exceptions are JBL, Genelec, and Neumann. Behringer used to make an affordable and accurate monitor with CD performance, but unfortunately their current offerings don't appear to meet that criteria. Outside of JBL, I don't have first hand experience with other affordable brands that have good CD behavior. The major benefit, acoustically, will be that you can focus entirely on LF absorption as trying to absorb the high frequencies from first reflections will be not only somewhat unimportant but potentially invaluable. I do tend to diverge from the opinions of Geddes and Toole a bit here in that I still find that some rooms benefit from absorption, but agree with Toole's conclusion that if you are going to do it, you need to absorb it carefully. A non-flat absorption is a bad idea and will deteriorate sound. I use MLS absorber diffusers on my ceiling and a membrane absorber on my sidewalls, with a curved front surface.

Overall I think your focus on bass is a smart one. A lot of home studio's mix the bass inaccurately do to an inaccurate reproduction of LF's in the room. Be careful to balance the response and get it as smooth as possible. You will unconsciously compensate for this in your mixing otherwise. This is part of the "circle of confusion" as Toole calls it. There is no standard for what a studio should measure like. We don't really know what is "correct" for the response of a studio system. I think we can at least agree that a smooth response is preferable. I think that a flat response is not desirable, and that like the final listening room response curve, a rise in the bass and drop in the highs is preferable. Still, it's too bad this can't be standardized. My opinion is that the engineer should be listening in an idealized version of what he/she intends the consumer to listen. Since there is very good research to show that a flat response is not desirable in a home system, I think the same would make sense in a studio (where no such research exists to my knowledge). To evaluate the response shape, I would suggest using material that has been professionally engineered, is notable for its engineering, and listening to it in your setup trying out different curves until you find a balance that sounds right. I also make comparisons between headphones and speakers when I'm not sure, but...that assumes the headphones are well balanced. I'm starting to find that some of the headphones I previously thought were so neutral didn't have a very good balance (or extension for that matter!).
 

thegnu

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Thank you for your detailed response! I continued my research after this post, and your response helped orient me as well.

I ran the test sweep in REW from 20Hz to 1000Hz, but the window is set to 30-400Hz.

I will be pursuing a DIY solution. I have Yamaha MSP10 Studios that are in the shop for torn tweeters, and they will be on speaker stands I built. My ear is right about at tweeter height, and I can adjust my chair if need be.

My best headphones are Beyerdynamic DT770 PRO's, so I can use those to A/B.

I will also be able to borrow my boss's measurement mic, which I will do when I get my speakers back this coming week.

If you want, I can come back with those measurements, and we can continue then, but I have had some further thoughts.

Step 1


I'm currently thinking I'll begin with the superchunks and cloud, and then go from there. Here's a sloppy mock up:

Top.jpg


My speakers most likely wouldn't be directly in front of the superchunks. Would I need to put absorption behind them as well? I assume I don't want my speakers spread too wide, due to early reflections. I have a window with a heavy curtain behind one, which I understand is better than nothing, but I could put up small panels if that's ideal.

What membrane would you recommend for the bass traps? I was going to face the chunks with Roxul Comfortboard 80, but I considered faced batt for the very reason you stated. Contact paper is very affordable.

For the cloud, I was going to frame it in 1x6 and probably use faced 6" batt, unless you have another recommendation.

Step 2
I have a loft cubby along the top of my rear wall. It runs the full width of the room (122"), and is 16" deep, and an average of 22" high. It looks like this:

isometric.jpg



I was thinking of three options:

1. Run a superchunk the whole width of the room.

2. Put superchunks in the corner, and make a Helmholtz resonator to address the 40Hz mode.

3. Put superchunks in the corner, and put diffusion in the center.

I was also thinking of running diffusion along the green line. That area is between 80"-100". Is that too high? Where would you mount it on the ceiling?

I have access to a router, and was planning on making the Arquen Fractal Diffuser, since it's reasonably low profile.

Finally, do you have a recommendation on treatment for the left and right walls to filter early reflections? I've considered slatted/slotted panels as a good starting point, but would be interested to hear your opinion.

Thanks again for your input. It has been invaluable already!
 

Matthew J Poes

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Oct 18, 2017
Messages
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Thank you for your detailed response! I continued my research after this post, and your response helped orient me as well.

I ran the test sweep in REW from 20Hz to 1000Hz, but the window is set to 30-400Hz.

I will be pursuing a DIY solution. I have Yamaha MSP10 Studios that are in the shop for torn tweeters, and they will be on speaker stands I built. My ear is right about at tweeter height, and I can adjust my chair if need be.

My best headphones are Beyerdynamic DT770 PRO's, so I can use those to A/B.

I will also be able to borrow my boss's measurement mic, which I will do when I get my speakers back this coming week.

If you want, I can come back with those measurements, and we can continue then, but I have had some further thoughts.

Step 1


I'm currently thinking I'll begin with the superchunks and cloud, and then go from there. Here's a sloppy mock up:

View attachment 6366

My speakers most likely wouldn't be directly in front of the superchunks. Would I need to put absorption behind them as well? I assume I don't want my speakers spread too wide, due to early reflections. I have a window with a heavy curtain behind one, which I understand is better than nothing, but I could put up small panels if that's ideal.

What membrane would you recommend for the bass traps? I was going to face the chunks with Roxul Comfortboard 80, but I considered faced batt for the very reason you stated. Contact paper is very affordable.

For the cloud, I was going to frame it in 1x6 and probably use faced 6" batt, unless you have another recommendation.

Step 2
I have a loft cubby along the top of my rear wall. It runs the full width of the room (122"), and is 16" deep, and an average of 22" high. It looks like this:

View attachment 6368


I was thinking of three options:

1. Run a superchunk the whole width of the room.

2. Put superchunks in the corner, and make a Helmholtz resonator to address the 40Hz mode.

3. Put superchunks in the corner, and put diffusion in the center.

I was also thinking of running diffusion along the green line. That area is between 80"-100". Is that too high? Where would you mount it on the ceiling?

I have access to a router, and was planning on making the Arquen Fractal Diffuser, since it's reasonably low profile.

Finally, do you have a recommendation on treatment for the left and right walls to filter early reflections? I've considered slatted/slotted panels as a good starting point, but would be interested to hear your opinion.

Thanks again for your input. It has been invaluable already!

For the area behind you, I would fill that cubby with a low density fiberglass or rockwool insulation. You don't want high density because one you exceed about 12" thick, the high density stuff starts to become reflective at low frequencies. There is an online modeling tool that actually shows this fairly accurately.

http://www.acousticmodelling.com/porous.php

The Flow resistivity of rockwool at like 6lb density is pretty high, like 40,000 if I recall. Put that in and make it 300mm thick and see what you get. Now reduce that value to 10,000 or even 5000 and see what you get. You will see the lower flow resistivity (lower density in this case) has much better absorption. That is good because it means you can use normal wall insulation up there. Don't pack it in tight either.

Over the front of that I would put a membrane. Contact paper is fine, but I usually use either a plastic PVC drop cloth (which you can buy big rolls of) or a roll of adhesive carpet protector. I bought a 500ft roll for like $40 I believe.

In front of the membrane you can put an MLS pattern of wood if you like. You could also do a proper diffuser like you talked about but I'm not sure it will be valuable the way you are showing. A diffuser works best when it is on the ear axis. If it is very high such that it mostly functions on the oblique reflections, they aren't very useful. I've seen people do that, but I can only guess they just didn't know what they were doing, or they know something I don't. In general I only think they should go at ear height roughly (say within 12" of the floor up to about 12" above your head) and only in positions considered either early reflections or directly behind you. Placing them on side walls but well behind you or placing them on the front wall where the speakers are makes no sense to me. What are they diffusing?

The diffusers you are talking about making would work best at your first reflection points on the side walls, the ceiling over the speakers, or between the speakers. The best location is directly behind you where the closet is. If you don't mind, you can put them on stands with wheels and move them out of the way, but have them at that location most of the time. I wouldn't actually put them on the ceiling or sidewall because I think your room is too small. You can always try it, but I suspect it will create a weird sound.

As for what I would do with the sidewall, I would probably experiment. I would not do a 2" or thinner panel, it won't absorb enough and could create some imaging problems. Instead, I would either put a 4" high density panel, or another MLS diffuser like I mentioned earlier. I like these in scenarios where you sit close. They also improve LF absorption some.

I would stay away from Helmholtz absorbers for now. They work when constructed right, but they are very easy to construct wrong. You also need a VERY large Helmholtz absorber to be effective. If your room was being used for recording then acoustic absorption solutions would certainly be needed. Since your room is being used more for home mastering where it is your speakers producing the sound, preconditioning the signal (i.e. use EQ) is fine. I would use an EQ with your system below 100hz to flatten the bass and run that all the time. I'm not sure what software you are using, but most home mixing/recording software lets you apply EQ to the monitor output or use a VST plugin to do so. It is your best option. It will work better too. If that isn't enough, again, I would just add more subs, that will still work better than the Helmholtz. To get a sense of how big a Helmholtz absorber needs to be to effectively tame a 40hz mode, imagine filling your entire closet and having it stick out another foot or so. When you see those in big professional studios, remember that the wall treatments are often many feet thick and encompass huge area. In addition, if you get the tuning wrong, then it was all for nothing (and note, a bunch of online calculators are incorrect).
 

thegnu

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OK, excellent. This is all coming into focus for me, thank you so much. Helmholtz resonator is off the table.

I was thinking about eventually having gobos behind my listening position to mitigate the closet, so I think I will try those stand-mounted diffusers.

A few more questions, and I think I'm good to go for the next couple weeks at least.

1. On my superchunks, I was thinking of doing 2ft square pillars. Would you space those out from the wall at all?

2. If they're out from the wall, would you face all 4 sides with membrane, or just towards the sides away from the wall?

3. When you say high density, what do you mean? My densest options locally available are 1.5" Roxul Comfortboard (8pcf), 3" Roxul Safe n' Sound (2.5pcf), and 3" OC Thermafiber SAFB (also 2.5pcf). 6" wouldn't be a problem at all, and I plan on experimenting with and without a panel facing with MLS cuts in it.

4. For the cloud, would you use low density material (fiberglass batt), or would I be better off using Roxul Safe n' Sound, or even Comfortboard?

5. If i mounted small (maybe 2ft square) absorbers behind my monitors, what density/thickness would you recommend?

Thanks again! Conceptualizing this has been stressful, and the stress is largely gone thanks to your help. I really do appreciate it.
 
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