Small mixing room acoustic treatment

All about acoustics. This is your new home if you already have a studio or other acoustic space, but it isn't working out for you, sounds bad, and you need to fix it...
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geo63
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Small mixing room acoustic treatment

Postby geo63 » Sun, 2019-Oct-27, 10:34

Hi everybody!
My name is Geoffroy and this is my first post on this forum.
I started to do some acoustic treatment in a bedroom in my house (3.67meters long * 2.99 meters wide*2.47 meters high) in order to do some mixing at home.
The room is in a quiet part of the house, where I do not disturb anyone (tested and approved!) and there is no neighborhood around, so it is not soundproof at all, and does not need to be.
I know the room is small, so I do not expect it to be perfect, but I still would like it to be decent.

Here is a sketchup plan of the room :
from above.jpg

front wall.jpg


Here is what the room looks like right now :
Back wall.JPG

Ceiling.JPG


As you can see, I made 60cm deep hangers panels on the back wall (thank you Stuart for helping with that ;) ), 60cm deep superchunk bass trap above the door, and I have filled the ceiling with 20cm deep glasswool with a density of 17kg/m3 and an air flow resistivity of 7kPa.s/m². I have covered it with polyester batt in order to avoid fibers from falling of the ceiling.

Here are some REW measurements of the room at different stages :
First, empty room :
empty room measurements.mdat

plus backwall treatment (hangers and superchunk above the door) :
Door Superchunk.mdat

plus the ceiling :
ceiling no fabric.mdat

and finally, plus 4 acoustic panels, on the side walls, I made a long time ago. This panels are not intended to stay in the finished version of the room, because I think they are too thin (only 8cm deep), but I found this measurement to be interesting nonetheless to try to anticipate future treatments :
ceiling+4panels.mdat

Here is a picture to show the set up of the panels :
Panels.JPG


The next step is to frame the ceiling with fabric and to implant the light in the framing.
Next, I was thinking of adding 2 corner superchunks to the front corners of the room with some light glasswool (4kPa.s/m²), and covering it with a thin plastic film to avoid making the room too dead (which will probably happen in the process I imagine?)

-Do you think the superchunks will be effective at cleaning up further the low end?
-Concerning the measurements, I am a little bit concerned with the big null in the 90-130Hz region and the uneveness in all the high end of the frequency spectrum starting above 2Khz, and also with the RT60 which looks like, to me, it is already a bit too dead in the low-mid range. Do you guys have any inputs on what is happening in this spots?
I know there is still room for improvement, but are this results fine at this stage of the process?

Thanks a lot!
Geoffroy
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DanDan
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Re: Small mixing room acoustic treatment

Postby DanDan » Mon, 2019-Oct-28, 22:17

The results look pretty good to me. Even your Topt is fairly close to EBU recommendations. Those pesky LF dips are always there to try our patience. Part of it is our desire for symmetry, plus the speaker is often half height, so many central or equal distance null whammies there all complicit. I would encourage really playing around with speaker and listener positions. For sure try the speakers almost touching the Front Wall ( I often use masking tape to prevent scratching, that close!) Play with width. Let's see if the frequency of that null changes with distance to the side wall. Would it be practical to place your speakers IN that window, sort of half flush? With each new speaker position you do need to keep checking your optimal listening position. It may move. I often use Pink Noise and the Spectrum/RTA zoomed to LF to get an immediate result for such multiple moves. If you do indeed have side wall BIR contributing or causing those dips, Superchunks would help, the big ones. But Square or rectangular 'SoffiTraps' would be better. You would need quite substantial plastic to deliver any useful bounce. Jeff Hedback posted a very substantial increase in LF absorption when he clad such SoffiTraps in simple Pegboard! I will have a look for the link in a mo. Those side panels I would probably butt together or just much closer, and include a proper 1:1 airgap behind them. Width modes...... You definitely need more bass. Those speaker moves may help, but don't be afraid to just turn it up. ADAMs can be pushed usefully. I had +6dB below 150Hz on my S3As and a -4dB on the tweeter.
DD

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endorka
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Re: Small mixing room acoustic treatment

Postby endorka » Tue, 2019-Oct-29, 06:05

I only have time for a short comment just now sorry! Some of the bad stuff you notice might be from reflections from your desk and/or rack. There is a peak in the impulse plot for both left and right speakers about 1 millisecond delayed from the direct sound. This should be under -20 dBFS and it is not. In your shoes I'd remove the desk and rack and measure again.

If moving the desk is impractical you could put a big lump of insulation on the desk between speaker and mic and see if it makes a difference.

Cheers,
Jennifer

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geo63
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Re: Small mixing room acoustic treatment

Postby geo63 » Tue, 2019-Oct-29, 15:37

Hi!
Thanks to both of you for your answers!
Today I took out my desk and my chair of the room, and moved my rack to the back left corner of my room and took some measurements.
Here is what the setup looked like :
no desk no chair.JPG

And here are the corresponding measurements :
no desk no chair.mdat

So I noticed a big improvement in the high end which are much "smoother" above 2Khz, but I also noticed a new huge dip at 425Hz with a very narrow Q.
The Low end dips seems to get worst too.

Next, I took some left over glasswool and cut it to do a rough and quick superchunk in the left front corner of the room and then measured the left speaker to see what effect it would have on the low end. It looked like this :
left corner superchunk .JPG


SPL left corner superchunk.jpg

As you can see the 2 dips are much clearer and much deeper with higher Q, but between the two there is an improvement.
So I don't know if it's a good or a bad thing?
So the mystery remains intact concerning this 2 low end dips.
What do you think of it?
Cheers!

Geoffroy
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endorka
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Re: Small mixing room acoustic treatment

Postby endorka » Wed, 2019-Oct-30, 09:12

I'm a bit rushed at the moment, no time to prepare screen captures, so will have to comment with text only at the moment sorry. I'll try to be clear in my words :-)

geo63 wrote:I noticed a big improvement in the high end which are much "smoother" above 2Khz,


Good, that's exactly what one can expect from removing the desk. The "loud" peak in the impulse graph or the left speaker with desk at about 1ms has gone. Instead you have new "quieter" ones at about 4.4ms and 8.3ms. Likely from the floor, or some other surface that the desk was obscuring. They are both below -20 dBFS, so that's good news.

but I also noticed a new huge dip at 425Hz with a very narrow Q.


If you look at the phase plot on the SPL & Phase graph you'll see a phase rotation of 360 degrees at that point. Use "unwrap phase" in the controls if it's not clear in the default display. My understanding is likely to indicate a reflection.

At this point it would be useful to do the "string thing" to determine the exact source of these reflections you are getting. I know Stuart has posted a detailed method for this elsewhere, I don't think it is in the reference section here yet though. I'll have a look.

Next, I took some left over glasswool and cut it to do a rough and quick superchunk in the left front corner of the room and then measured the left speaker to see what effect it would have on the low end. As you can see the 2 dips are much clearer and much deeper with higher Q, but between the two there is an improvement.
So I don't know if it's a good or a bad thing?


Woah! That is fascinating, and beyond my layman's understanding I'm afraid. Perhaps one of the more experienced acoustics people will be able to explain it, I for sure would be interested to know why too.

At this point, my approach would be to get the Wall_Bounce_Calculator_2D.xls , it will help figure out if any of these are SBIR related. I'd also try Stuart's walking mic test, there is a post about this on the reference section here. It will help establish predictable patterns and causes about the acoustics in your room.

Cheers,
Jennifer

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geo63
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Re: Small mixing room acoustic treatment

Postby geo63 » Wed, 2019-Oct-30, 12:31

Hi Jennifer!
Thanks a lot for your answer.
I will research how to do the « string » test, and how to use the bounce calculator. I will also play with the speakers and listening positions as suggested by Dan. I will share here if I find something interesting.
At least I know how to keep myself busy this week end.
Cheers!

Geoffroy

DanDan
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Location Location Location

Postby DanDan » Wed, 2019-Oct-30, 13:28

I would encourage really playing around with speaker and listener positions. For sure try the speakers almost touching the Front Wall ( I often use masking tape to prevent scratching, that close!) Play with width. Let's see if the frequency of that null changes with distance to the side wall. Would it be practical to place your speakers IN that window, sort of half flush? With each new speaker position you do need to keep checking your optimal listening position. It may move. I often use Pink Noise and the Spectrum/RTA zoomed to LF to get an immediate result for such multiple moves.

Somebody in my memory wrote or said, regarding acoustics, optimising what we are dealt, Location, Location, Location, is 70% of the job. Perhaps it was Bob Hodas.

DD

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endorka
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Re: Small mixing room acoustic treatment

Postby endorka » Wed, 2019-Oct-30, 14:48

What Dan says!

Also remember that certain positions can improve the SPL plot (frequency domain) while making the waterfall and spectrogram (time domain) worse, and vice versa. So keep an eye on both when evaluating the different positions.


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