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Help with bass trap design and placement in small control room Audio Interfaces
Old 17th April 2019
  #1
Here for the gear
 

Help with bass trap design and placement in small control room

Hello everybody,
I've been a (mostly live) sound engineer for a few years and now I need a little (big) help (and general clarifications) on my studio setup. I'm an avid Gearslutz reader and lurker, but I haven't posted much, so I hope this thread complies to all the regulations of the forum. Please forgive me otherwise.
Also, English is not my mother language so please forgive my syntax and my (very likely) grammar errors!

I'm building a small project studio (main uses are mixing, pre-production and occasional recording of acoustic instruments) composed of two rooms: a "chill out"/conference room and a production/mixing room. In the latter, I'm focusing my efforts on getting the most professional acoustic setup I can with my limited DIY resources. I'm attaching room shape and size (two pdfs, a basic one and a detailed one)
All the mixing will happen in the box (ie no mixing desk) and the speakers are currently Adam A77X. I'm not using a subwoofer and I think I'll keep doing so in the foreseeable future.

[TL;DR] this post is long and filled with information you probably don't need or just can't bother reading. There's a tag like the one at the beginning of this line to reach the actual question. Everything else is in the attachments at this link: Dropbox - Resources.zip

STRUCTURE
The room is roughly 6m x 4.6m x 2.6m. It has a trapezoidal shape, so the longest room size is not evenly 6m (it goes from 6m on the long side to 5.4m to the short side). The front wall, which will host the mixing position, has two right angles.

I tried providing sizes in both metric and imperial units to suit every taste, but I have absolutely no familiarity with the latter so please forgive me if there are mistakes.

VOLUME: 60m^3 - 2118.6 cu ft
TOTAL SURFACE: 99.75m^2 - 1073.21 sq ft

of which
FLOOR (wood surface): 25.76m^2 - 277.18 sq ft
CEILING (wood surface): 25.76m^2 - 277.18 sq ft
BACK WALL (concrete): 9.16m^2 - 98.56 sq ft
FRONT WALL (drywall): 10.72m^2 - 115.35 sq ft
RIGHT WALL (plaster surface): 12.26m^2 - 131.92 sq ft
LEFT WALL (plaster surface): 12.30m^2 - 132.35 sq ft
DOOR (birch wood surface): 1.79m^2 (125Kg) - 19.26 sq ft
WINDOW BACK (birch plywood surface): 1.65m^2 (35Kg) - 17.75 sq ft
WINDOW LEFT (birch plywood surface): 0.35m^2 (7.53Kg) - 1.32 sq ft

Door and window areas have already been subtracted from the wall surfaces.
As can be seen in the attached drawings, every surface except the floor is actually a counter-wall with different layering. The front wall and the ceiling have been constructed to be decoupled and acoustically isolating, the side walls are just bricks with a plaster finishing. This configuration is the unfortunate consequence of inexperience on my part when I first started working on the studio (more than 4 years ago) and preexisting building conditions. Right now it would not be practical to undo this. Of course this will influence the outcome of room predictions, but room room modes calculation using the actual room size (ignoring the counter-walls) seem consistent enough with the actual measurements.

Here's what I got so far.

MEASUREMENT/PREDICTIONS
I took acoustic measurements in the empty room. I'm attaching both images and the measurement data files. I know some of the conditions I took the tests in aren't ideal, including volume, but I still hope they can be of use. I could retake them if needed, but it would be a bit hard right now.

SPEAKERS: Adam A77x (I know this is not ideal, but I don't really have access to any other options)
MICROPHONE: Sonarworks XREF 20 (is that what it's called? Calibration file attached). I couldn't find any specific information on the microphone (except for the calibration file), but the Sonarworks software instructs to point the microphone directly at the speakers, so this is what I did)
INTERFACE: Focusrite Scarlett 18i20 (first issue) (calibration file attached)
SPEAKER POSITIONS: roughly 150cm from the side walls and 190 from each other; roughly 110cm from the wall behind the speakers (sketched in the attached floor plan)
MICROPHONE POSITION: roughly 265cm from the wall behind the speakers and 230cm from the side walls
SOFTWARE: REW
RT60 CALCULATION: REW's Topt

Instead of predicting the reverberation time (since I'm not even sure if the surfaces I have can be accurately predicted), I used Sabine's formula to estimate the current total absorption in the room based on my measured reverberation time. The goal would be to use these calculated coefficients in combination with the predicted ones for the acoustic panels I'm building to have an estimate of reverberation time after mounting them. Formula used: RT60 = 0.161V/A (V volume, A absorption in metric sabins). The resulting absorption coefficients (Frequency-Measured RT60-Metric Sabins-Est. Absorption Coefficient) are:
63Hz: 1,08s - 9,12 - 0,09
125: 1,49s - 6,61 - 0,07
250: 1,11s - 8,88 - 0,09
500: 0,86s - 11,46 - 0,11
1000: 0,78s - 12,63 - 0,13
2000: 0,83s - 11,87 - 0,12
4000: 0,77s - 12,80 - 0,13
8000: 0,68s - 14,49 - 0,14

ANALYSIS
As far as I can see, the most "gross" peaks and dips in my measurement, as well as the low frequency ringing, are more or less consistent with the results I got from different room mode calculators (the one in REW being one of these). I also noticed a significant difference in response between the two speakers in the 40-60Hz range and around 130Hz, but I haven't had the time to check whether the speakers actually have a different response or it's an acoustic effect. I also noticed a weird dip at about 16KHz which doesn't appear in either the L or R speaker response, so I'm guessing this is an issue with the room.
RT30 is well over 500ms in all 1/3 octave bands and peaks to about 1.5s near 150Hz. Topt RT60 shows similar results.
I'm also attaching the ETC, but not the filtered bands. I'll post them if needed and, of course, they're available in the attached measurement file. I have a sense of how to interpret the ETC, but probably not enough to draw any valuable conclusion from it.

CORRECTION
I would like to tackle this in steps. I get the sense that the main issues that need addressing are: on one side of the transition frequency, room modes and SBIR; on te other side, more or less uniform absorption and diffusion (for the latter, my guess is that the sensible locations would be reflection points). Plus, generally, the early reflection energy. I'm not including the specific issues discussed earlier (difference in response etc) because I don't really know what could cause them and I think I could get a clearer picture after reducing the impact of the main problems.

[TL;DR]
So my first and currently main concern is bass treatment. This is where I feel most lost, as I can't really get a sense of how effective things are. Angular bass traps seem to be the most effective way of addressing room modes and I see most people recommend using broadband absorbers (as opposed to tuned ones). I spent a bit of time fiddling with one multilayer absorber calculator (Multi-layer Absorber Calculator) and came up with something, but I REALLY don't know how accurate the calculator is or if some combinations of layers are more accurately predicted than others etc etc. The resulting panels' absorption curve and construction is in the attachments and here is a link to the configured calculator: Multi-layer Absorber Calculator

I haven't decided on a precise configuration yet, because I still can't understand if my reasoning is at least in the right ballpark. My idea is to start with 3 types of panels.
A) triangular, limp membrane angular bass traps (red curve in the attached image). These should be in all the four vertical angles. I assume that a triangular trap is equivalent to half a rectangular/square one with the same base and height, as the resonant frequency should be proportional to the volume. This should create an area of approximately 8m^2. These should contribute to reducing ringing between 60Hz and 150Hz.
B) 40cm by 40cm rectangular porous absorbent traps, to be placed in the angles around the ceiling. These should generate a surface of about 8m^2. Do both exposed faces contribute the same amount towards absorption? These should cover the whole spectrum above 100 Hz but still contribute to reducing ringing at 150Hz
C) A few broadband slotted panel absorbers (I assume these are Helmholtz resonators?) centered on the "crossover" frequency between the other two. These should be mounted on the walls, in 1m by 1m by 0.15m modules, to provide additional sabins of absorption (is this expression correct?). There should be around 10 of these around the room.

Again, I'm trying to focus on the low end of the spectrum right now (below the transition frequency).

I have a simple spreadsheet doing the math for this and it suggests this should bring the reverberation time at about 0.5ms more or less evenly across the spectrum (less at very high frequencies). Of course I don't expect it to be this simple or accurate. I'm more concerned about whether I'm in the right ballpark, if this approach is valid (I'm basically calculating how many sabins I need at every 1/3 band and simply counting how many absorbers I need to reach the desired quantity) and if the absorbers I calculated are "valid" or if there's an (or many) inherent mistake somewhere.

ON TO THE QUESTIONS
1. I described what I did in annoying (sorry) detail, but of course I'm not under the assumption that my reasoning is correct. So I'd like to ask how much I am off course and if there's any meaning in what I did up until now
2. Are the assumptions I made in designing the traps correct or at least negligible?
3. Can the traps I designed actually do what the calculator estimates, even if with less efficiency? or is the design broken?
4. Assuming they do, are they effective in my case?
5. Could you please point me in the right direction and instruct me on how to effectively tackle this situation?

Thank you SO much for taking your time reading this. Any help will be greatly appreciated.

Have a nice day!
Attached Thumbnails
Help with bass trap design and placement in small control room-lr-20-20k-frequency-response.jpg   Help with bass trap design and placement in small control room-lr-20-500-spectrogram-1500ms.jpg   Help with bass trap design and placement in small control room-lr-t60_t30.jpg   Help with bass trap design and placement in small control room-l-etc-250ms.jpg   Help with bass trap design and placement in small control room-l-etc-250ms.jpg  

Help with bass trap design and placement in small control room-lr-20-500-frequency-response.jpg   Help with bass trap design and placement in small control room-absorbers.jpg  
Attached Files
File Type: pdf Control Room Basic A4.pdf (1.00 MB, 11 views)
Old 18th April 2019
  #2
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akebrake's Avatar
 

Quote:
Originally Posted by Azazello View Post
... this post is long and filled with information you probably don't need or just can't bother reading. There's a tag like the one at the beginning of this line to reach the actual question. Everything else is in the attachments at this link: Dropbox - Resources.zip
Interesting project and thanks for sharing!

RE: The mdats in your dropbox ”Resources” folder. As measured response will change a lot depending on position of woofer and microphone I have some questions.

1. Are the speakers stationary in all measurements? Like in the blueprint?

2. What hight are the woofers and microphone in the mdats? Same or different?

3. E.g. Measurement named ”FR corner” means microphone in front corner? At what Hight?

4. Measurement named ”FR corner” means which speaker used? L, R or L+ R?

Best

Ake

BTW Your measurements looks valid but there are newer (updated plus more features) in later versions of REW. Now 5.20 beta 8
Old 18th April 2019
  #3
Here for the gear
 

Thank you for you interest!

1) both microphone and speakers are stationary in all measurements, except for the ones labeled with "XY corner"

2) the microphone "simulates" the listener's head, so it's pointed towards the center of the two speakers (I realized just now this has an effect on the single speaker measurement ), vertically aligned with the middle of the woofers and the tweeter (A77x have a horizontal design), at 130cm from the ground. Sorry I missed it

3) the corner measurements refer to the corresponding corners (front right etc), in which the microphone is at the same height and pointed towards the corner, at about 40cm from the corner. My reasoning was to get a sense of what the frequency content was at about the position where membranes or perforated panels would be

4) all corner measurements have been done with both speakers active

Yes, I think I took the measurements on version 4.something (some time ago) and updated REW after that.

I will be in the studio on Tuesday, so if different measurements are needed I think I can provide them

Also: there's no technical or valid criteria on the speaker placement I used, aside from "furniture" reasons and the fact that I (lazily) tried to avoid quarter/half wavelengths

Last edited by Azazello; 18th April 2019 at 12:07 PM..
Old 4 weeks ago
  #4
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akebrake's Avatar
 

More testing

Quote:
Originally Posted by Azazello View Post
Thank you for you interest!
You're welcome

I suggest you start investigating your room a little more.
Make a couple of new (bare room) measurements.
Save for future reference.

1. Check the modes. (Diagonal test like the pic below)
”Drive” the room modes strongly by placing one loudspeaker on the floor, front Left (seen from the engineer) Woofer close to the floor corner.
The mic in the opposite diagonal rear Right Celing corner. Measure!

2: Swap corners. (R spkr / Left ceiling.) Looks similar?

3. Map your room. As your room is not a square ”shoebox” you don’t know where the modal nulls and peaks are situated...
Long time GS member Jens Eklund wrote this nice explanation.
Nulls in listening room, help!

Quote:
Also: there's no technical or valid criteria on the speaker placement I used, aside from "furniture" reasons and the fact that I (lazily) tried to avoid quarter/half wavelengths
4. That can be improved by moving the whole listening triangle forward.
Speakers close to the front wall (nearly touching). Measure L & R separately/ full sweep.

Best
Attached Thumbnails
Help with bass trap design and placement in small control room-modal-test1.jpg  
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