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PostPosted: Fri Jul 05, 2019 1:31 am 
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I recently soundproof'ed a 36 cubic meter room to be used as an experimental control room. I develop DSP and I'm also in the process of producing an electronic music album. So, this room ideally needs to help with my sound designing and mixing.

Currently, the room has no treatment.
Side walls and the front wall are brick, ceiling and floor concrete, and the back has a 2-leaf dual wall (drywall, 4 inches of rockwool, air gap, 4" rockwool, dry wall). There were two windows which were sealed using the same insulation and drywall at the front and on one side. Extremely satisfied with the sound-proofing part which was a much needed aspect of this project due to the noise in the environment.

*Please see the attached image for a summary of dimensions. Notice that the equilateral triangle between speakers and the listening position is slightly behind the listening position, contrary to what is shown in the sketch.
Room dimensions (all in meters):
W = 3.24
L = 3.94
H = 2.83
Volume = 36.13 m3

I followed Soundman2020's REW guide to get the results in the link below (exactly in the format he recommended).
Dropbox Link

Equipment and measurements:
The room is mostly empty (just the speakers, a large electric piano, small table with computer screen). However, it has two 6” ventilation pipes running on the left wall and on the ceiling near the back, and collectively there’s around 5 meters of large pipe running by the walls.

Speakers: Genelec 8050Bs; no subwoofer
Mic: Personus PRM1
No EQ added in the speakers.

Speaker position: I followed Genelec’s own guide to place these monitors in the room. They are 1m from the side walls, 0.5 meter from the front wall and creates a 1.07 meter equilateral triangle with the listening position which is at a little over 1/3 of the room length. Tweeters are 1.2 meters from the floor. Ideally, I prefer these speakers to be a bit further away from the listening position, but due to the room dimensions and all factors considered, this is the best placement I found for them so far.
Mic position/listening position: 1.47 from the front wall; 1.62 from side walls; height is 1.085 from the floor. Since the tweeters are slightly higher, the speaker is angled down a little.


I’m not sure if a detailed treatment can be drawn for this room at this time before adding some basic treatment first. But, this is what I’m planning to do:
1. Fill the 4 corners with rock wool and seal with drywall. Side length of the isosceles right triangle is to be decided.
2. Two early reflection absorbers on the sides, 4” 50kg/m3 rock wool
3. Horizontal absorber on the front wall, i.e. in front of the listening position, same thickness and density as above
4. Add a 1m long diffuser on the back wall (after another test)

Finding material in my location is a bit difficult. So, if you have ideas for certain membranes, please provide some alternatives too (maybe less effective). At the moment, I’m mainly concerned with the corner traps, in terms of how large they should be (I'm thinking of 12" depth from the middle point to the corner, so it takes about 1.5 feet of wall from the corners).

Ideally, I would like to avoid unsealed rock wool because this stuff easily affects me. So, diffusers are preferred. Since they can be made with wood, it also helps with avoiding the hassle of finding and shipping materials to my location. I also like the idea of the room sounding larger.

Thank you for your advice.


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PostPosted: Fri Jul 05, 2019 3:37 am 
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Hi there "timeguy". Welcome.

Please read the forum rules for posting (click here). You seem to be missing a couple of things! :)

Quote:
I followed Soundman2020's REW guide to get the results in the link below (exactly in the format he recommended).
... and you did it very well! Here's a comparison between the predicted response for your room, and the actual response that you measured:

Attachment:
Timeguy--REW--FR--Predicted-vs-measured.jpg


The top graph, with the frequency response curve in black, is predicted. The bottom one, with the curve in orange, is what you actually measured.

Don't you just love it when the real world matches theory so well? :)

OK, but I had to change things just a little to get that perfect match: The dimensions of your room. The room seems to be a bit smaller than what your tape measure told you. Acoustically, it seems to be 3.77 long x 3.2 wide x 2.76 high. So it seems to be about 3% to 4% smaller. There's several possible explanations for that, such as the effect of the pipes and things in the room, air temperature, humidity, inaccurate measuring, building materials, inaccurate clock in your computer, latency in the system, etc. It's no big deal, as long as you are aware of it.

Quote:
Speaker position: I followed Genelec’s own guide to place these monitors in the room. They are 1m from the side walls, 0.5 meter from the front wall and creates a 1.07 meter equilateral triangle with the listening position which is at a little over 1/3 of the room length. Tweeters are 1.2 meters from the floor. Ideally, I prefer these speakers to be a bit further away from the listening position, but due to the room dimensions and all factors considered, this is the best placement I found for them so far.
Mic position/listening position: 1.47 from the front wall; 1.62 from side walls; height is 1.085 from the floor. Since the tweeters are slightly higher, the speaker is angled down a little.
There's a few issues with that. I would suggest something better, closer to the theoretical best, which would give you this result:
Attachment:
Timeguy--REW--FR--Predicted-V2-better-layout.jpg

Notice the much smaller dips in the nulls and at the cancellation frequencies, and the overall smoother response. You would get that like this:

Attachment:
Timeguy--New-layout-A.jpg


Attachment:
Timeguy--New-layout-B.jpg


With typical basic treatment, you could then expect results something like this:

Attachment:
Timeguy--REW--FR--Predicted-V2-better-layou-with-treatmentt.jpg


It's a very small room, so it would be hard to get much better than that. Not impossible: just hard.

The reason why my response is so much better than yours, is because I'm not using the infamous equilateral triangle, which is a myth anyway! :) I mean, you do see it everywhere, and it does work for the vast majority of rooms (which is why you see it everywhere...), but just because it works doesn't mean that it is OPTIMAL for every room! I optimized that suggested layout, based on ignoring the equilateral triangle, and going with empirical "rules" and experience.

Some of the things I did:

1) move the speakers right up against the front wall, leaving only a 10cm gap where you will insert a 10cm thick porous insulation panel.

2) Spread the speakers further apart, so they are at 27% of room width, 87 cm from the side walls, and not a meter from the side walls, where you had them.

3) I also raised the speakers slightly. By the way, it is NOT the tweeter that you want at 1.2m above the floor! It's the acoustic axis of the speaker. Genelec publishes a document on their website that shows the location of the acoustic axis for every single model they make. Find out where yours is, and use that for locating and aiming the speakers. It's another common myth that you locate the speaker using the tweeter: not true! So now I have the acoustic axis of your speaker at 123 cm above the floor.

4) I moved the listening position further forward, so now it is at 127 cm from the front wall (about 34%). You often see the famous "38%" rule quoted all over, but that's not a rule: it's just a guideline. And in fact most engineers prefer to be a bit forward from that location by a few cm, because it is usually a better position.

5) I raised the listening position to a mote reasonable height. Unless you are very short, your ears will not be at 1.08m above the floor when seated. For most people, they will be about 1.20-1.25 , above the floor. So now the listening position is at the same height as the speaker axis, at 1.23 Your ears might be a little lower than that, which is fine. Do not adjust the speaker height! Keep them at 1.23 m, regardless of your actual ear height. See below...

Another error: do not tilt the speakers down. Keep them flat and level. Tilting them down will increase the reflections from your desk / console surface, and also has other undesirable psycho-acoustic effects. There's not a huge difference in height here, and you will only be off-axis by one or two degrees, max, which just isn't a problem for fine speakers such as your 8050's. They have very good dispersion characteristics in both vertical and horizontal, so being one or two degrees off axis is no big deal at all.

So that would be a better layout, with smoother response, to start with. Now for the treatment.

Quote:
1. Fill the 4 corners with rock wool and seal with drywall.
Yes, certainly, build superchunks in all four corners, but then why do you want to remove them from the room by paneling over them with drywall? Peak absorption for such a wall would be around 250 Hz or so, and the coefficient would only be very low, maybe 0.1 or so. That's not going to do much.

Rather, just build standard superchunks in all four vertical corners. It's a small room so they will have to be large: at least 60cm along the front wall, 90 down the sides, and for the rear wall, 90 across and60 down the sides. Or even 90 down the sides too, if you have the space for that.

You will likely also need another superchunk across the top of the front wall, and perhaps additional ones along the tops of the side walls. Those can be a bit smaller, but certainly not less than about 40cm on each side.

Do NOT put drywall over those! That would exclude them from the room. Just put a layer of cheap black breathable fabric over them, then a layer of nice looking finish fabric over that (also breathable), and add wood slats later to return some of the lost highs to the room, but without losing the low absorption.

Quote:
2. Two early reflection absorbers on the sides, 4” 50kg/m3 rock wool
Yes, but I would go for 6" if you can, and perhaps lower density: maybe something like 30 to 40. You need good low end absorption for dealing with your modal issues, and likely flutter echo too.

Quote:
3. Horizontal absorber on the front wall, i.e. in front of the listening position, same thickness and density as above
Perhaps, but only after you have the absorbers in place between the speakers and the front wall. Large panels, 4" thick, low density. The rear corner of each speaker should be just touching the front face of those panels.

Quote:
4. Add a 1m long diffuser on the back wall (after another test)
Nope! No way. Not in that room. It's way, way too small for any type of numeric diffuser, and it wouldn't do anything for your low end modal and SBIR issues anyway: it's near impossible to tune a diffuser low enough for that (it would have to be enormous! Bigger than the entire room...).

This is yet another myth: you can't use numeric diffusers in a small room, for a very simple reason:
Attachment:
QRD-Diffusion-lobing--pattern-graph-SML-ENH-2.PNG

Self explanatory! That's the lobing patterns generated by a typical numeric-sequence diffuser. You have to be at least ten feet (3m) away from it to ensure that you are not in that chaotic zone, where timing shifts, phase shifts, and intensity levels are all over the place, such that each ear would be hearing a different pattern, with different response, and even a slight move of your head would put in in a completely different zone. Ten feet is the minimum distance: it might be more than that, depending on what frequency range you tune the diffuser for. So, despite what you see in some home studios, numeric diffusion is not an option for small rooms. You can only use it rooms that are large enough for the mix position to be at least ten feet from the diffuser. That means rooms that are at least 20 feet long. There's a LOT of ill-informed folks out there, with very pretty diffusers on their rear walls that cost a lot of money, and they are really proud of them! Yet they don't realize that they are doing more harm than good...

So, your rear wall will need the standard minimal treatment for small rooms: superchunks in the corners, then 6" of porous absorption across the entire rest of of the rear wall, floor to ceiling. That won't be enough, of course, (it's a small room!), but it will do a decent job of getting some low-end control. If you really wanted to get good rear-wall control, then you could do 4" of insulation instead of 6", then 18" wide hangers in front of that, angled 40° or so, then a frame with fabric on it to hide all that ugly stuff, perhaps with more insulation just behind it, perhaps cover (partially) with thin plastic, perhaps with slats on the front. (those "perhapses" are for tuning).

And you are forgetting one of the most important aspects of your room treatment: the ceiling cloud! You will need one: make it large, hard-backed, and angled properly.

Quote:
So, if you have ideas for certain membranes,
You didn't mentioned membrane traps! Where did you plan to put those? What frequency would you tune them too? It is a possibility, yes, but they do need to be tuned properly, and placed at the pressure peak for the problem they are tuned for.

Quote:
At the moment, I’m mainly concerned with the corner traps, in terms of how large they should be (I'm thinking of 12" depth from the middle point to the corner, so it takes about 1.5 feet of wall from the corners).
That's much too small for your room. It's a small room, so it needs a LOT of low end treatment: the deeper the better. That's why I suggested 90cm (+/-32") along at least one wall and 60cm (+/-24") along the other, for each trap. The front ones should extend 90cm down the side walls, while the rear traps should extend 90cm across the rear wall. The ones on the wall/ceiling corners can be smaller, yes, but the ones in the vertical corners need to be bigger.

Quote:
Ideally, I would like to avoid unsealed rock wool because this stuff easily affects me.
Then you are probably buying the wrong type of mineral wool! Some cheap versions use binders that off-gas ugly smelly chemicals, such as formaldehyde and other VOC's The better ones (such as the real Rockwool brand) do not use those chemicals, and have developed more innocuous binders that don't off-gas.

If you can't get mineral wool made properly, then try fiberglass instead.

You can also cover much of the insulation with plastic, to prevent it from off-gassing and also prevent it from shedding fibers over time. You can do that with MOST of the insulation, but not all of it: you can't do it with the panels on the side walls, for example. But using the two-layer fabric method I outlines above also helps.

Quote:
So, diffusers are preferred.
Except that you cannot use diffusers in your room, as it is way too small for that.

Quote:
I also like the idea of the room sounding larger.
You cannot make a small room sound larger! That's yet another myth. The overall sound of the room (the frequency response curve) is set by the modal pattern, and the modal pattern is set by the dimensions. It is impossible to make a small room sound large. And even if it were possible, diffusion would not do that anyway! Diffusion simply eliminates specular reflections at some frequencies and some locations, and spreads the energy around more evenly. Technically, it attempts to simulate an perfect acoustic reverberant field, meaning that no matter where you are in the room the energy seems to come from all around you, evenly. But that does not make the room sound larger: it just makes it sound less ugly. What governs how big a room sounds, is something called the "mean free path length". That's the average time that a sound wave has to travel in the room, before it hits a surface and bounces off. The only way to make the mean free path longer, is to make the room bigger. Sorry. You cannot make a small room sound like a big one. The laws of physics prevent it. At best, you can make a small room sound less ugly, by damping the low end modal response as much as possible, and evening out the decay times across the entire spectrum, so that each frequency band decays at roughly the same rate. That's what makes a room sound decent. In a large room, you can do that with a combination of diffusion, absorption, and reflection. In a small room, you can only use absorption and reflection.


- Stuart -


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PostPosted: Sat Jul 06, 2019 12:46 am 
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Hi Stuart,

A big thanks for your detailed response.

I did the changes you recommended to the speaker & listening positions. Some calculations you made there were different from the percentages (for example, the distance from the front wall to the listening position calculates to 1.34), so I went with the percentages you mentioned. Also, the speakers are leveled now and the height of the acoustic axis is at 1.23 m. The dimensions of the room you predicted by analyzing the data is not exactly the room size, so I assume some other factors contribute to aberrations in the data. Just to be thorough, I cross-checked my measurements with the measurements done using a laser measure by a company that did the ventilation in this room, and they are exactly the same. However, there are small issues in one brick wall where it sort of curves in the middle that adds slightly more to the volume.

I certainly like the effect of having the speakers further away from me as I described earlier. I updated the Dropbox link above with the new tests. For anyone interested, I juxtaposed the graphs below as 1. old speaker placement (equilateral placement described in my first post) 2. Stuart's recommendation (see the above message) 3. exactly the same as Stuart's recommendation, but the listening position is 8cm below the acoustic axis (this is the highest I can sit due to a problem described below).

As you mentioned, this placement did narrow the range of the level response and also smoothened it. Unfortunately, I cannot sit taller than 1.15 meters (from the floor) due to the height of my electric piano table. Otherwise, I will have to buy a new keyboard altogether. But, from the 3rd graph below, I didn't hear a significant difference from sitting below the height of the acoustic axis.

About the basic treatment:

1. I will go with the 90 across X 60 down on sides front bass traps as you described. I had trouble finding a popular brand of rock wool here (location updated), so I went with an Indian brand called "Rockinsul". I'm not sure if I'm allergic to the gases (formaldehyde) like some other people complained online, but the fibers annoy me quite easily with an immediate cough. The data sheets on their website doesn't clarify if it's non-formaldehyde. But, it does have a unique smell. When we worked with this product, we wore gas masks and gloves, but when I walked in the room after an hour or so and desettle even a small amount of fibers left on surfaces we couldn't clean with a wet cloth/mop, I still get a bad cough. So, I'd like to avoid staying in a room full of these since I don't know how long it'll take for these to settle. I hope covering the front of these bass traps with a thin layer of polythene won't affect their effectiveness as the frequencies these are intended for will pass through. Over it, I can put a breathable fabric for a better finish.

2. The room does have flutter echo which I hope will go away once the basic treatment is added. 6" early reflection absorbers it is.

3. I forgot to mention the cloud which I had in my notes. I planned this to cover about 2/3's of the width of the room and made square shaped, again 4", centered above me. I will review your suggestions on this. More fiber around me and above me.

4. Because of the placement of the door in the back wall (which was a necessary placement due to another wall outside of the room blocking a decent entrance to this room), on one corner, the furthest I can go is 45cm along the back wall. So, I guess, despite being less effective, I must make it symmetrical and go 45 cm on both corners. Or do you suggest otherwise?- perhaps by extending the length along the side walls, or making one corner trap larger (in the lengths you mentioned) and the other smaller under this limit.

Again, thanks for your hard work. Lot's of helpful information for me to review.


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PostPosted: Tue Jul 09, 2019 12:22 am 
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I have decided on these treatments based on Stuart's recommendations above. I'd be grateful if someone or Stuart himself could review these and guide me before I order some expensive glass wool (mainly due to shipping).

1. Front corner traps (superchunks, floor to ceiling): 0.60 m along the front wall and 0.90 along the side walls. 48kg/m3 density.

2. Back corner traps (superchunks, floor to ceiling): 0.45 X 0.45 m. 48kg/m3 density. Near one corner, there's a door that limits the distance which can be achieved on the back wall. To make this corner trap larger, I can either go more distance along the side wall or make only a single corner in the dimensions recommended (0.60 and 0.90 m). I'm not sure what is more important in this case under this limitation-- i.e. whether to make both traps larger by covering more length along the side walls or make one larger and create asymmetry in the stereo field.

3. Front and side, upper corner traps along the edges (superchunks): 48 density again, 0.60 X 0.60 m.

4. Front wall panels (2 behind the speakers and 1 in the middle): 30-40 kg/m3 (is there a particular density recommended?), 4" thickness, 2 X 4 ft panels. I assume these are large enough, and also hope that the middle one could be shorter - or is it better to make all 3 the same size?

5. Side panels / early reflection absorption: 30-40 density, 6", 2X4 ft panels.

6. Cloud: 30-40 density, 4" , 4X4 ft panel. I'm uncertain about the size of this, whether it should be larger.

7. Back wall: I'm getting 30-40 density insulation to cover the whole (excluding the corner traps) back wall 4". I'll decide the treatment after finishing all of the above and keep this area more for tuning. Or, is covering the ceiling corner with superchunks better at this point? Again, the intended length along the sides is 0.60 and 0.60 meters for superchunks if they are needed up at the ceiling and back wall.

Thank you for any helpful information.


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PostPosted: Tue Jul 30, 2019 12:40 am 
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Soundman2020 wrote:
2) Spread the speakers further apart, so they are at 27% of room width, 87 cm from the side walls, and not a meter from the side walls, where you had them.


Why 27% in specific? I have seen that you often give specific numbers for speakers width so is this some kind of a "rule"? I mean there is a possibility that you always find the best distance to be around 27% on all your designs so from your experience you now trust this 27% for most situations?


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PostPosted: Tue Jul 30, 2019 3:41 am 
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almaelectronix wrote:
Why 27% in specific? I have seen that you often give specific numbers for speakers width so is this some kind of a "rule"? I mean there is a possibility that you always find the best distance to be around 27% on all your designs so from your experience you now trust this 27% for most situations?
It's not a rule: that's just for this specific room. All rooms are different, speakers are different, so layouts will be different. Sometimes you can get a a benefit from having a speaker at exactly 25% to help with modal issues. Sometimes that's not a good idea. I wish there was a simple rule that would work for all rooms and all situations, but unfortunately it's not that easy. That said, you'll generally end up with your speakers somewhere in the region of 20% to 30% room width. But that a rather large ball-park!

- Stuart -

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PostPosted: Wed Sep 18, 2019 5:51 pm 
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I have completed adding broadbrand treatment (listed below) to this room. It sounds much better than I expected. To be honest, fantastic clarity wise!

There are some issues still. Noticeable flutter echo between the sidewalls which I will fix in the next phase while also tuning this room if necessary.

REW test can be found here: https://www.dropbox.com/sh/xlp2s4l9cz6f25h/AADdIwn7GYS4-ONlZ4B3lm6Ba?dl=0

My concern is about the massive dip at 85 Hz. What might be causing this and would I be able to fix this?

Also, I am wondering whether adding a large slanted board (with stands, like an idea board) near one wall might be a solution to treating flutter echo. Please let me know if I'm off on this prediction.

Thank you in advance to any helpful info, guidance.

Treatment added: 4 corner traps from floor to ceiling; two 4" absorbers behind speakers, 6" early reflection panels, a 4'X4' absorber at the back wall, front and back ceiling corner traps, two small absorbers (2.5'X2') at the sides near ceiling (centered at the listening position), 4'X4' cloud.


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PostPosted: Thu Dec 05, 2019 1:07 am 
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Hi, Time Guy!

Nice, isn't it, to see and hear the results of your efforts? :D

In my room (still under time consuming construction) I have a similar dip at aroud 85Hz. I suspect that it has to with floor- or ceiling bounce. In my room I plan to hang a cloud which should solve that, at least to a degree. The same may apply to your situation. Doesn't hurt to try!


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PostPosted: Thu Dec 05, 2019 1:50 am 
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The Sound Guy wrote:
Hi, Time Guy!

Nice, isn't it, to see and hear the results of your efforts? :D

In my room (still under time consuming construction) I have a similar dip at aroud 85Hz. I suspect that it has to with floor- or ceiling bounce. In my room I plan to hang a cloud which should solve that, at least to a degree. The same may apply to your situation. Doesn't hurt to try!


Glad to see you followed this thread. Yeah, this project turned out better than I imagined was possible with the resources and time I had.

This issue with the dip around 85Hz can be solved easily by changing the speaker placement. I asked the same question at a different forum, and everyone unanimously mentioned that the speakers need to be closer to the front wall. Before, I couldn't move the speakers further back because of the tripod stands I used. After new sand-filled stands, I managed to move them around 1.5" towards the front wall, and now, as someone recommended above, they are almost touching the front panels. Currently, that dip is around 10dB, which is significantly less. In my design, I left a 2" gap behind the front wall panel, which needs to be removed for better treating this issue. So, I guess, the correct way to do this is to hang the front absorbers right on the wall without leaving a gap, and place the speakers backs almost touching those panels.


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PostPosted: Thu Dec 05, 2019 10:05 am 
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Hi there,

With monitors on speaker stands, unless your room is extremely big, it's better to have your speakers almost touching the front wall.

You are struggling with SBIR here. By putting the speakers closer to the front wall, you are increasing the frequency to one that you can manage with absorption. You can't use absorption to treat an 85Hz frequency practically.

Place a slab of 4"/100mm of fibreglass/rockwool directly on the wall behind each speaker, and push the speakers back so they are touching the insulation.

This should improve the SBIR.

Dan


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PostPosted: Thu Dec 05, 2019 12:05 pm 
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Waka wrote:
Hi there,

With monitors on speaker stands, unless your room is extremely big, it's better to have your speakers almost touching the front wall.

You are struggling with SBIR here. By putting the speakers closer to the front wall, you are increasing the frequency to one that you can manage with absorption. You can't use absorption to treat an 85Hz frequency practically.

Place a slab of 4"/100mm of fibreglass/rockwool directly on the wall behind each speaker, and push the speakers back so they are touching the insulation.

This should improve the SBIR.

Dan


Thanks for mentioning the correct term for this Dan. This is exactly the solution I've found.


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PostPosted: Thu Dec 05, 2019 6:28 pm 
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Hopefully this helps you and others. Here are some of my SBIR notes:

Speaker Boundary Interference Response

If the distance = >3m (10 feet) or more like 15 feet, the SBIR frequency will be lower than we can hear

If the distance = between 3m and about 60cm, then the frequency will be in the range of 30Hz to 120Hz where the wavelength is too long to treat. So, these distances must always be avoided.

If the distance = less than about 60cm, the frequency will be above 180Hz, where the wavelength is short enough that it can be treated by a thick, porous absorber.

SBIR is a comb filter

To calculate the first and most prominent frequency cancellation, convert the wavelength (distance from cone to wall → include insulation depth), to frequency.
F = Speed of sound / Wavelength
F = 1130 ft/sec / Wavelength (feet)
Then the cancellation will obviously happen at the quarter wavelength, so divide the frequency by 4 and THAT will be the SBIR dip.

Greg

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