Small mixing room acoustic treatment

#1

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 :

Here is what the room looks like right now :

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: (10.32 MiB) Downloaded 1458 times

empty room measurements.mdat: (10.32 MiB) Downloaded 1458 times

plus backwall treatment (hangers and superchunk above the door) :

Door Superchunk.mdat: (9.99 MiB) Downloaded 1427 times

Door Superchunk.mdat: (9.99 MiB) Downloaded 1427 times

plus the ceiling :

ceiling no fabric.mdat: (10.02 MiB) Downloaded 1459 times

ceiling no fabric.mdat: (10.02 MiB) Downloaded 1459 times

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: (9.83 MiB) Downloaded 1445 times

ceiling+4panels.mdat: (9.83 MiB) Downloaded 1445 times

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

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

#2

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

#3

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

#4

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 :

And here are the corresponding measurements :

no desk no chair.mdat: (9.82 MiB) Downloaded 1460 times

no desk no chair.mdat: (9.82 MiB) Downloaded 1460 times

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 :

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

#5

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

#6

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

#7

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

#8

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.

#9

Hi!
It 's been a long time since I updated this thread, so I wanted to share the changes in the room.
I played with the speakers position and the listening position, and while the 2 dips in the low end slightly moved, I didn't find a better position than the one I already had, so I kept things like that.
In my understanding, the lower dip (90hz) seems to be side wall related, and the higher one (120hz) seems to be floor related.
For the side wall dip, I added another panel in front of the ones I already had at the first reflection points. It is now twice as thick and it helped a bit.
I put some furnitures on each side of my desk and it helped a little with the floor bounce, and putting some cushions on the top of the furnitures helped to smooth out a bit the high-mid frequencies. The only problem with that, is the rack on my right hand side creates a ringing at 100hz on the right speaker (you can see it on the REW measurements in the RT60 tab). This rack is made of quite thin wood panels and I think I need to buy something sturdier.
I added an EQ on the output of my soundcard (with the software of the souncard) to smooth out further the low end, and to add a bit more low end.
Here are some REW measurments :

12-12-20.mdat: (20.18 MiB) Downloaded 1156 times

12-12-20.mdat: (20.18 MiB) Downloaded 1156 times

You can see the measurements with and without EQ, and I also included some measurements called "open shutters" :
I had a ringing at 160Hz in my measurements that seemed to appear and disappear randomly, and I couldn't find the cause of this, because I was searching the problem inside the room. Well, I happened to find the solution outside of the room. This ringing only takes place when the rolling shutters of the room are open; when it is closed, no more ringing! I think it may have something to do with the ringing caused by the double glass window which is altered or canceled by the addition of a third layer (the shutters in this case). But honestly, I don't know, and if anybody has an explanation about that, I would gladly hear it! But for now, I always mix with the shutters closed!
Here are some pictures of the setup right now :

: listening position

: side wall

: rear wall

So as you can see, the room is far from being perfect, the RT60 is a little short for exemple, BUT it is a huge improvement from my last room which was a nightmare to work in. This room allow me to work much more quickly and with much more accuracy, and I do not have the usual troubles with bass translation in the real world anymore.
Nonetheless, I am always trying to improve things (and I enjoy tweaking things as well), so I would be glad to hear any suggestions for improvements, and any comments are welcome!
Take care of you!
Geoffroy

#10

Hi Geoffroy,

I think, as far as my knowledge goes, looking at the waterfall, you have come a long way.

I did notice a few things, although based only on what you have published of course:

- In the first picture of post #9, I can see the microphone on a stand, which I would suggest to change. How can you make sure, as the microphone is supposed to represent your listening position, that it is always in exact that position, given you do NOT use just the straight part of the stand but added an arm to the stand. Not having the microphone on exact the same position most likely makes the outcome of the measurements inconsistent. Just us the microphone stand without the arm, have the pole of the stand almost touch the ground and put a cross on the ground using a strong tape and leave the tape on the floor. The pole should be on the center of the cross. Of course the tape should be right under the middle of your seat to represent the listening position. Using more positions to check, just move you mike around and measure every new position, tape the ones you like and number them. Decide on the best. Also position the Mike straight upwards, so not on a angle, so it forms 1 line with the strand pole. Now you have a setup that is easy to keep consistent, 1 exact location, 1 hight (your ears while sitting on the chair, 1 angle of the mike.

- In your measurements from 12-12-20.mdat it looks like the measurements are not always at the same SPL level. Do you know why that is.

- Although you have a very nice rear wall bass strap, the door opening is a completely untreated part of this wall. Maybe this is causing the dips in the LF at 90 and 120 Hz? As a suggestion, have a Gobo (moveable panel with damping material) at the same position as the start of your rear wall bass trap and do a measurement to see if those dips change. The Gobo will leave the door accessible and have it in place while you are working, even put wheels under the Gobo for ease of use. See if that will improve / lessen the dips.

- If you find RT to short, maybe add some reflective material in front of the rear wall bass trap. Rooms that are heavy damped tend to sound dull and fatiguing, our brains are constantly working on looking for natural reverb conditions, which will never be there.

- Last but not least. From experience on problem solving, may be good to have in the back of your mind. When working on solving issues and problems, the main challenge we all face is the following. Suppose your want to improve the acoustics of your room, which you do, there are a lot of topics & parameters within those topics that you need to understand, including the relations between those topics. Let's assume we have 4 issues that prevent you from being happy with your room of which one is inconsistent low end, some low frequencies are hard to hear / handle. So you start measuring and looking at those measurements. You find 2 dips in the SPL plot and you start to think at first, WHY these dips show up in my measurements. You come up with an answer, for instance "because we lack low frequency damping", so you apply more damping and measure again. Looking at the measurements NOTHING changed. Conclusion, it is not the damping that is solving the issue so we ditch working on damping. THIS is the standard reaction but also potentially biggest mistake we all make. There may be other artifacts in your room that prevent you from SEING results of damping because those artifacts simply have a much bigger impact. So when you have improved the damping, just leave it there and look for the next most likely cause. Going over and over, at some point in time it all works out.
Decide for yourself when it is good enough, keep in mind that we can measure an issue which we are incapable of hearing. Is it then an issue you want to solve?

Basics of this, there will always be more than one issue, but the effect of the biggest issue is masking all others and most likely the chance that you start with the biggest one is smaller when you have less knowledge. So, when you apply a chance which is very likely to be correct, leave it in place and continue with the next. When you start a change and have no clue on what can be expected, don't apply the change.

But on the late side, but with all good intentions.

Frans

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