Absorption all round to begin with... or not?

[callaghan]

Hi! I posted my introductory plan quite a while ago and finally the isolation phase of John's inside-out wall is completed, walls and ceiling are up, with a 15° sloping ceiling over my listening position.
Luckily the isolation goal was met, traffic noise from the street is not audible any more. Now I'm on to treatment phase.

Goal: multipurpose room for composing/mixing tv and movie stuff mainly, but wouldn't mind the odd acoustic instrument or vocals recording.

Size: the room is in my apartment at the bottom floor of the building. Internal leaf room size is 5,24 x 3,91 x 2,59 meters which places it quite in the middle of the Bolt area.

Existing features: just starting from my listening position on my right is a big window totaling 4 square meters, guess I'll need a heavy curtain for it.
Wooden tiles on the floor. Door is on the rear left side (see snapshot), will be turned into Rod's superdoor design. Also near the door is an old radiator which is now closed out behind the gypsum wall.

Pat studio plan july 2014.zip

Pat-Rome-studio-july-2014-sketchup.jpg

Pat-Rome-studio-july-2014.jpg

And now as soon as I finish caulking I'm on to treatment phase and that's where I'll need your advice again..

1) I was thinking of going on with the absorption rockwool+fabric approach all round with superchunks in the corners, then correcting it according to REW tests in order to liven it in case it's too dead.
That said, here is the first question if I may:
Is there an area that I should keep reflective or diffusive right from the beginning, or should I directly start with covering all the 4 walls and ceiling with rockwool?

2) And this takes me to the 2nd question. I've done a lot of search in the forum on plastic sheeting over rockwool and found lots of clues but I'm still unsure on this matter...
Stuart once said: "...I would use 1 or 2 mil on the cloud and first reflection points, 4 mil on the back, and 6 mil everywhere else...".
Would this apply in my case too? I also saw some prefer to AVOID plastic in first reflection points, is that mandatory or does it depend on the room characteristics?

Thanks again guys!!

Patrick

[Soundman2020]

the isolation goal was met, traffic noise from the street is not audible any more.

Excellent! Glad to hear that it worked out.

is a big window totaling 4 square meters, guess I'll need a heavy curtain for it.

Probably not necessary, as it seems to be behind the mix position. There won't be any first-order reflections from that getting to your ears if it is behind the mix position.

Door is on the rear left side (see snapshot)

That's a bit of a problem: that zone is where you need a large bass trap.

Is there an area that I should keep reflective or diffusive right from the beginning, or should I directly start with covering all the 4 walls and ceiling with rockwool?

I would start by doing a REW test with the room exactly like it is now: empty. Then put in superchunk bass traps in the four vertical corners and some of the horizontal corners too, and do another REW test. Based on that, you can decide how to proceed with the next round of treatment.

Would this apply in my case too? I also saw some prefer to AVOID plastic in first reflection points, is that mandatory or does it depend on the room characteristics?

You should definitely use plastic on any insulation that is above you, such as on the ceiling, a cloud, etc. For vertical insulation on the walls, it depends on what you are trying to do. It might be needed, or might not.

- Stuart -

[callaghan]

Hi Stuart ! I finally got shipped from the states a Dayton EMM-6 microphone (with it's own individual calibration file provided by Cross Spectrum Labs) and made REW tests before and after building superchunks and bass traps as you suggested!

The standard 60° monitoring with a 150cm sided equilateral triangle left me with considerable unevenness on the bass freq, while moving the speakers wider (234cm apart) with a 90° monitoring setup made some improvement in the lows.

Here is the bare room spectrogram:

mono bare room test03 spectrogram 24-20k.jpg

Here is with the 4 vertical rockwool SUPERCHUNKS added:

l+r 6cm-wall 45°- 33 from superchunk - mic 153 from wall.jpg

And here it's with the 4 vertical rockwool superchunks + 8 (8 just to begin with) foam bass traps, will move them on the ceiling corner later on of course (2 on each side, 60x38x10 cm, same material and density as the Auralex LENRDS). I know It's not as effective as good ol' 40Kg/m3 rockwool but I wanted to be able to move them around easily and make some experiments in order to decide later on where to place the permanent horizontal corner bass traps.

l+r 6cm-wall 45°- 33 from superchunk - mic 153 from wall+traps all.jpg

L+R 4 superchunks + 8 foam traps.jpg

Here is the MDAT file:
http://web.tiscali.it/webspace7/Callagh ... s.mdat.zip

And here are a few questions if I may Stuart :

1) Please tell me if I'm wrong. I feel kinda stuck now... my idea of the final look of the room was to get the entire studio covered with the classic acoustic fabric finish (stapled from a pro guy) and incorporating the absorbing part of the treatment within a wooden frame under the fabric (including the vertical superchunks I've already built), quite a classic approach I guess, right?
Since I gather that the final placement of the horizontal corner bass traps (on the ceiling corners) should go over and overlap the wood frames containing the rockwool+fabric+plastic layer, and I guess the same goes for slat resonators, diffusers etc. would it make sense to proceed now on the construction of the absorption layer where it will be needed, and then add all the ceiling corner bass traps, slat resonators, diffusers etc. according to a new set of REW tests? I feel I can't build any definitive horizontal bass trapping and spend money on it until I don't finish the framed absorption/fabric layer... And could I eventually add wood surfaces to liven up the room in case it's too dead?

2) If yes, where do you think I should absorb? From what I've seen in these years reading the forum I gather that deep bass absorption on the rear is mandatory, but I've seen that all the other sides and ceiling differ depending on the situation for mid-highs absorption. Do you think that we have enough data to determine where I should start building the wood frames for the absorption? That would allow me to get all the fabric cover job done, with all the mid-highs "shallow" rockwool absorption underneath (including the deeper bass absorption on the rear), and once done that I could move on to build the extra bass trapping, resonators, diffusers according to your knowledge?

3) Of course I guess we'll need to address those bass peaks and nulls in some way, but according to my REW file, do you think that this I a good or a bad starting point? I always wondered how the ideal spectrogram would look like... is there somewhere a mdat file of a properly treated control room so that I know what I should strive for?

the isolation goal was met, traffic noise from the street is not audible any more.

Excellent! Glad to hear that it worked out.

Yep, your advice of detaching the internal window from the outer leaf proved effective

as always, thanks for your help Stuart,

P

[Soundman2020]

Oops! sorry for the looooooong delay in getting back to you! Lot's of things going on, and not enough hours in the day!

I finally got shipped from the states a Dayton EMM-6 microphone (with it's own individual calibration file provided by Cross Spectrum Labs) and made REW tests before and after building superchunks and bass traps as you suggested!

Excellent! I downloaded your MDAT file, and I've been looking at it a bit, but there are some issues with it. The data is valid, but... :

1) There's only one measurement in that file! I was hoping to see the tree sets of measurements in there, that you already did, to compare them directly and see what is working and what is not working.

2) There's only one measurement, and you don't way how you got it! Which speaker did you use for that? The left one or the right one? Or both? What you should do is, for each measurement set, take one with just the left speaker, one with just the right speaker, and one with both speakers. So there should be nine measurements in your MDAT: an L, an R and a Both for each of the three situations you measured...

3) You didn't calibrate REW correctly, or if you did then you measured with the level too low. You measured with an average level of a bit less than 70 dB, but in reality it should be about 86 dB for the "both" reading, and 80 dB for the individual speakers.

So I'd suggest repeating those measurements after re-calibrating REW with your sound level meter, and doing tests for L, R and Both for each scenario.

The standard 60° monitoring with a 150cm sided equilateral triangle left me with considerable unevenness on the bass freq, while moving the speakers wider (234cm apart) with a 90° monitoring setup made some improvement in the lows.

You shouldn't really be fiddling around with speaker positions yet, until you have more of the treatment in place. If you had your speakers 150 cm apart, then they were too close for that room. And at 234 cm, they are too far apart! In fact, from the photos it's clear that they are nearly in the corners, which is not good at all.

With a room width of 391 cm, each speaker should be about 108cm from the side wall, which means they will be about 175cm apart. Your head should be about 200cm from the front wall in your case, which would mean that your ears are also about the correct distance from the speakers (somewhere around 160-180cm, depending on the size of the speakers and how much insulation will be behind them. The angle of each speaker will be a bit less than 30° in this case, if you want to get the speaker intercept point a good distance behind your head.

So please set up the speakers and mic like that, and do anther REW test (in addition to re-testing with the way you have it set up now!) to see of there's a good difference when set up like that, or not.


OK, about the treatment you have in the room right now: the bass traps don't look big enough to me. What is the distance across the front face of those? I think you might need to make them bigger. The basic issue that you are way short on bass trapping in that room. Surprisingly, all of your axial modes are visible in the REW data... but at higher than normal harmonics! For example, there's a big one about 99 Hz, which ir your 3,0,0 mode (third-order front-back axial mode), another big one at about 142 Hz (forth-order front-back axial mode), and another at about 177 Hz (4th order side-to-side axial mode). It's unusual for the higher order modes to be so much more prominent than the fundamentals and lower order (although those are visible too, in the data).

So overall, you need much more bass trapping in there.

my idea of the final look of the room was to get the entire studio covered with the classic acoustic fabric finish (stapled from a pro guy) and incorporating the absorbing part of the treatment within a wooden frame under the fabric (including the vertical superchunks I've already built), quite a classic approach I guess, right?

That's a good plan, yes, but don't forget that this will make your room too "dead" in the highs, so you'll need to either prevent that with some type of reflective membrane in front of some of the absorption (under the cloth), or perhaps with slots walls (slats over the cloth).

I gather that deep bass absorption on the rear is mandatory,

Absolutely! especially if you are doing an RFZ style of design. I'd suggest making that about 6" thick (about 15cm), but also enlarging your rear superchunks. Perhaps just put a full panel of 703 in front of each one.

I'd also suggest that you put in a large, angled, hard-backed ceiling cloud over the front of the room.

One other thing I'd do, is to put absorption between the speakers and the front wall (at least 2" of 703, preferably 4"), and at the first reflection points on the side walls (at least 4").

I'd hold off about doing the side walls until you have the rear and ceiling cloud in place, to see what actually needs to be done to those.

Do you think that we have enough data to determine where I should start building the wood frames for the absorption? That would allow me to get all the fabric cover job done,

I would take it one step at a time: first get your bass trapping and other modal issues resolved, then the initial panels mentioned above (front wall, side walls), then measure with REW and see what still needs doing.

is there somewhere a mdat file of a properly treated control room so that I know what I should strive for?

I can't give you the actual MDTA file, since this is from one of my paying customers, but I can show you something about how it should look, compared to hoe yours looks at the same scale.

First, SPL graph below 500 Hz:

How it should look:

d9-spl-15-500.jpg

How yours looks:

callaghan-spl-15-500.jpg

Next, the waterfall plots for the same region (below 500 Hz):

How it should look:

d9-waterfall-15-500.jpg

How yours looks:

callaghan-waterfall-01-15-500.jpg

Then the Spectrogram plots, for two different ranges. First 15--500 Hz:

How it should look:

d9-spectro-15-500.jpg

How yours looks:

callaghan-spectrogram-01-15-500.jpg

Now the same, but focusing on the area below 200 hz:


How it should look:

d9-spectro-15-200.jpg

How yours looks:

callaghan-spectrogram-01-15-200.jpg

And finally, the RT-60 plots (more correctly: energy decay plots):


How it should look:

d9-rt60-single.jpg

How yours looks:

callaghan-rt60-01.jpg

OK, those comparisons aren't really fair, since the ones I'm showing you for "how it should look", are from a room that is quite a bit bigger than yours, and that has been very carefully treated and tuned over many months. You wont be able to get the curves as smooth and balanced as that in your room. But at least it gives you an idea of what you should be aiming for.

Yep, your advice of detaching the internal window from the outer leaf proved effective

Great! Glad it worked out for you!

- Stuart -

[callaghan]

Oops! sorry for the looooooong delay in getting back to you! Lot's of things going on, and not enough hours in the day!

Dear Stuart, absolutely no prob, you're helping me out for free, it is I who should apologize for pestering Thanks for answering!
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So I'd suggest repeating those measurements after re-calibrating REW with your sound level meter, and doing tests for L, R and Both for each scenario.

Ok, will check the level again with my slm, and will do more detailed measurements from now on!
Just for your info the MDAT file I posted was taken with both speakers playing (I know about the comb filtering effect on mids and highs with a L+R measurement but I was mainly interested in the bass for now…)
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With a room width of 391 cm, each speaker should be about 108cm from the side wall, which means they will be about 175cm apart.

Got it, as a matter of fact I've started taking measurements with nearly exactly that standard setup you are suggesting, equilateral triangle (60° each corner) between speakers and listening position.
Unfortunately with this initial classic setup (beginnning tests with speakers at 117cm from the side wall) there was a severe bass null at around 80Hz:

Callaghan-60-degreees-monitoring.jpg

Left speaker - 117cm from side wall.jpg

.....then, as I moved the speakers further apart (at 98cm from side wall) the null got better but still had dips at 80Hz and 130Hz:

Left speaker - 98cm from side wall.jpg

.....then finally the bass got more even when I widened the speakers even more at 75cm each from the side walls (moving them even further apart than this, bass started gettin worse again). This was the smoothest response I got from bass freqs, according to speaker placement:

Callaghan-90-degreees-monitoring.jpg

Left speaker - 75cm from side wall.jpg

But since they were now at 234cm apart from each other I needed to go for a 90° monitoring setup, which as you pointed out is too wide apart for my room, mainly I guess, because it could lead me to under-pan while I mix.
I've also done some 20 measurements while trying to move the speakers away from the front wall towards the middle of the room with 5cm increases for each test, and the best bass response was with the speakers right against the front wall, of course leaving a gap for future treatment.
So...

Q1 - SPEAKER PLACEMENT - ...if I go for the initial standard equilateral triangle setup that you suggest, how will I deal with that big null at 80Hz (in the first spectrogram pic)? Can I fix it later on, or should I stick with the wide apart 90° monitoring setup from the beginning?
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OK, about the treatment you have in the room right now: the bass traps don't look big enough to me. What is the distance across the front face of those?

errrr .... 60cm (24") I saw a few guys doing them this way on the forum... Is that not enough in my case?

Callaghan-rockwool.jpg

Q2 - REAR - Also is there an alternative to 6" bass trapping on the rear? Would MEMBRANE bass absorbers have the same absorption capacity in a smaller space than 6"? I guess the problem with these is that they're hard to build right isn't it? Could/should I give it a try?

Q3 - REAR - Something I've always wanted to know: which is the relation between 40Km3 mineral wool (which as you often say is ideal for bass absorption) DEPTH and the bass frequency absorbed? Talking about simple bass traps with no membrane, for example a 6" thick trap down to which frequency will it manage to absorb?
Considering my 3.0.0 axial mode at 99Hz front-back wall, 1/4 wavelength of that is roughly 2,5 feet of absorption needed, how much will that 6" of mineral wool affect that?
Are these sound absorber calculators reliable?
http://www.acousticmodelling.com/porous.php
http://whealy.com/acoustics/Porous.html
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I gather that deep bass absorption on the rear is mandatory,

Absolutely! especially if you are doing an RFZ style of design.

And that takes us to another big subject, the LAYOUT I should go for, as a matter of fact as you can see in the pics above I decided not to splay my walls, I just angled my ceiling 15° (as suggested by you) a while ago so I guess that that forces me to go towards an absorption on front, side walls and ceiling approach right?
Or, since you suggest at least 4" of absorption on the side walls...

Q4 - SIDES - ... could I try and make an in between approach (a partly RFZ design with free standing speakers) by placing 6° or 12° angled slat walls on the side with Helmholtz resonators or simple absorption in them? I know that from a RFZ point of view real splayed solid side walls are not the same as splayed slat applied on non-splayed side walls, but do you think I could go for that approach, just as Jbassino was planning to in the pic below? That would mean that closer to the front corners I would get much more than the 4" absorption you suggest for the sides, maybe starting from 6" (plus slats) and then decreasing to 0" as it gets angling down towards the middle of the room… What do you think?

Q5 - SIDES - And if yes, which would be the advantage of those angled slat/absorbers side walls? I guess that, since the wall behind is not angled, they divert early reflections only on the high freqs right?

Jbassino's pic:

Jbassino.jpg

http://www.johnlsayers.com/phpBB2/viewt ... e+standing
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I'd also suggest that you put in a large, angled, hard-backed ceiling cloud over the front of the room.

Q6 - CEILING - I see , I really had wished I could avoid the cloud in some way , since as suggested by you I built a 15° angled ceiling over the listening position and that has already took quite a lot of space above, is there any other possible alternative to the cloud? Already having an angled ceiling could allow me to get away with it with plain absorption directly on the angled ceiling itself?
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I can't give you the actual MDTA file, since this is from one of my paying customers, but I can show you something about how it should look, compared to hoe yours looks at the same scale.

Wow thanks Stuart, that at least gives me and idea of what to strive for, veeeery useful thanks!!
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Let me know what you think about the above,
thanks again for you time ,
Pat

[Soundman2020]

Unfortunately with this initial classic setup (beginnning tests with speakers at 117cm from the side wall) there was a severe bass null at around 80Hz:

Yep... but did you try them at 108cm from the side walls / which means they will be about 175cm apart? And also not angled exactly at 30°, but rather whatever angle is necessary to have them both aimed a the same point on the center-line of the room, and about 230c, from the front wall, with the mic set up at about 200 cm from the front wall?

With the speakers in that position, take a few readings with the mic in seven different locations: At exactly 200 cm from the front wall (the theoretical ideal position), then also at 215, 230 and 245 (moving the mic backwards, towards the rear wall), and also at 185, 170 and 155 (moving the mic forward, towards the front wall). That will show you a good set of data around the "ideal" listening point, and from that we can determine where the best position really is, as we'll be able to see which problems are modal, and which are SBIR.

.if I go for the initial standard equilateral triangle setup that you suggest, how will I deal with that big null at 80Hz (in the first spectrogram pic)?

Have you considered soffit-mounting your speakers? That would change a lot of things, for the better...

errrr .... 60cm (24") I saw a few guys doing them this way on the forum... Is that not enough in my case?

It's a start! 24" is pretty much the minimum, but 26" would be better. Do you still have some of those insulation panels left over? If so, then try putting a full panel just in front of each of your superchunks, and do another measurement like that.

Also is there an alternative to 6" bass trapping on the rear? Would MEMBRANE bass absorbers have the same absorption capacity in a smaller space than 6"?

Probably not. I recently did some limp-membrane traps for a studio a bit bigger than yours, and they ended up being nearly 16" deep... Besides, membrane traps are tuned devices, and tuning them is NOT as easy as it sounds by reading a book or a web site! In any case, you need broad-band absorption, not tuned absorption. You have several issues below about 400 Hz, and there's bot enough space on your rear wall to build individual devices for each of them!

Something I've always wanted to know: which is the relation between 40Km3 mineral wool (which as you often say is ideal for bass absorption) DEPTH and the bass frequency absorbed?

The density doesn't affect the frequency much, just the efficiency at each frequency. It's the depth that defines how low it goes, and it's based on the quarter-wavelength of the frequency. For normally incident sound, 560 Hz has a wavelength of about 24", so you would get good absorption from a trap that is 6" deep. For 280 Hz, the wavelength is about 48", so the trap would need to be 12" deep. For 140 Hz, wavelength is 96", depth is 24". BUT! And this is a big "but". That's OPTIMUM absorption for NORMALLY INCIDENT sound (sound that arrives at 90°, head-on to the front of the absorber). Most of the sound in a room is not normally incident: it arrives at lower angles, so it "sees" a greater thickness of the absorber. At 30°, for example, it sees twice the depth, so a 70Hz wave approaching at 30° would still see optimum absorption. And I did say "optimum"! It's not that the 70Hz wave gets 100% absorption while a 69Hz wave gets 0% absorption: Rather, the absorption tails off slowly for lower frequencies, and there is good evidence that even an absorber that is only 3.5% of the wavelength can still provide usable absorption for sound that is not normally-incident to the surface, or 7% for sound that is normally-incident.

The density of the absorber controls how well it absorbs, and more dense materials are worse at low frequencies, better for high frequencies. If you compare the specifications for OC-701, 703, and 705 you'll see the differences there clearly.

Are these sound absorber calculators reliable?

Reasonably yes, if you understand the principles and set them correctly. But both of those are for normally incident sound, as far as I recall, and don't fully take into account randomly incident sound, which is what most sound waves are doing in a room. I know they have buttons for that, but I'm not convinced the buttons are implemented properly, since the graphs don't seem to change as they should. But those are both still useful to give you an idea of how a porous absorber will behave.

I just angled my ceiling 15° (as suggested by you) a while ago

Great!

so I guess that that forces me to go towards an absorption on front, side walls and ceiling approach right?

Not if you go for a full RFZ design, with soffit-mounted speakers...

could I try and make an in between approach (a partly RFZ design with free standing speakers) by placing 6° or 12° angled slat walls on the side with Helmholtz resonators or simple absorption in them?

That would help a bit, yes, but as you already know, that isn't enough to be RFZ, so you would be getting reflections from the slats back to your ears. You would still need to have treatment on your first reflection points.

That would mean that closer to the front corners I would get much more than the 4" absorption you suggest for the sides, maybe starting from 6" (plus slats) and then decreasing to 0" as it gets angling down towards the middle of the room… What do you think?

Slot walls are tuned devices: the absorb a specific frequency range, set by the cavity depth, slat dimensions, and slot dimensions. They aren't too hard to tune, since normally you want them to be set up as broadband, and one method of doing that is as you suggest: making them shallow at one end and deep at the other. That will work, but you do need to calculate, to make sure you are absorbing a useful range!

I guess that, since the wall behind is not angled, they divert early reflections only on the high freqs right?

Right, but not enough! If you ray-trace, you'll see that there are still reflections that get to your ears. RFZ needs bigger angles.

I really had wished I could avoid the cloud in some way, since as suggested by you I built a 15° angled ceiling over the listening position and that has already took quite a lot of space above,

I had forgotten that you did the angled ceiling! So you are probably OK there, and very likely will not need a cloud. Worst case, just some thin absorption for the high frequencies, but if you angled it enough then you probably don't even need that. However, you could put that hard-backed cloud at the rear of the room, where the ceiling is still flat...

- Stuart -

[callaghan]

Wow Stuart! That's a treasure trove of precious info, that cleared many of my long standing doubts, and the "random incidence" absorption theory greatly boosted my faith in absorbers!!
That said, If I may, just 2 little last question before I proceed with the next phase of construction:

Quote:
could I try and make an in between approach (a partly RFZ design with free standing speakers) by placing 6° or 12° angled slat walls on the side with Helmholtz resonators or simple absorption in them?

That would help a bit, yes, but as you already know, that isn't enough to be RFZ, so you would be getting reflections from the slats back to your ears. You would still need to have treatment on your first reflection points.

Quote:
That would mean that closer to the front corners I would get much more than the 4" absorption you suggest for the sides, maybe starting from 6" (plus slats) and then decreasing to 0" as it gets angling down towards the middle of the room… What do you think?

Slot walls are tuned devices: the absorb a specific frequency range, set by the cavity depth, slat dimensions, and slot dimensions. They aren't too hard to tune, since normally you want them to be set up as broadband, and one method of doing that is as you suggest: making them shallow at one end and deep at the other. That will work, but you do need to calculate, to make sure you are absorbing a useful range!

Q1 - Unfortunately, for the time being, I cannot go for a proper soffit design and will have to live with a free standing approach... In this case, which broadband absorption range would you suggest for these side slat walls, once I've angled them enough to be ok for this hybrid RFZ? (at least lateral RFZ for free standing monitors approach, that is)

Quote:
I guess that, since the wall behind is not angled, they divert early reflections only on the high freqs right?

Right, but not enough! If you ray-trace, you'll see that there are still reflections that get to your ears. RFZ needs bigger angles.

Q2 - How can I calculate which angle I would need in my case, is there some way to add a ray-tracing feature to sketch-up? Or can you recommend some ray tracing software that you know of?


Will proceed with construction of the rear wall 6" absorption layer construction and will do tests with the speakers in all the positions that you suggested and will get back with some REW tests, thaaaanks!

[Soundman2020]

In this case, which broadband absorption range would you suggest for these side slat walls, once I've angled them enough to be ok for this hybrid RFZ?

I would suggest that you use John's "standard" spacing for slot walls, but also first check with REW when you get to that point, to make sure that you are treating the right range. It's the mid range that is most problematic, but the actual range depends on each room, and the treatment you have in it, so it's best to measure with REW just before you start cutting the wood for the slats.

Q2 - How can I calculate which angle I would need in my case, is there some way to add a ray-tracing feature to sketch-up? Or can you recommend some ray tracing software that you know of?

The way I do it, is manually. I start from the acoustic center of the speaker, and draw a line to represent the acoustic axis, extending out from the front panel at 90°. Then I draw lines every 10° from there, going each way, up to about 60°. Then I follow each line to see what surface it hits, figure out where the surface normal is on that surface, and create a second line to represent the reflection, starting from the point where the first one hit the surface, and angled exactly in the "mirror image" direction from the first one. Then I check to make sure that all of those "second" rays ("bounced" rays) do not get close to the head of the engineer. I usually draw a few circles around the head, and distances of 20cm, 40cm, 60cm etc. so I can see how close the rays are getting, and if there are any rays inside the inner circles, then I angle the reflecting surface a bit more until there are no more rays.

It's a slow, boring process, but it works very well to give you a clear understanding of how the room is working for RFZ.

Don't forget that you have to do this both horizontally, and also vertically.

Will proceed with construction of the rear wall 6" absorption layer construction and will do tests with the speakers in all the positions that you suggested and will get back with some REW tests

- Stuart -

[callaghan]

Thanks Stuart! Ok I'll proceed to the next step!

[callaghan]

Hi Stuart! I've built the 6" deep mineral wool bass trap all over the rear wall, and beefed up the superchunks with another layer of mineral wool.

Callaghan-rear-wall-bass-trap-construction.jpg

Callaghan-full2.jpg

I've also moved the speakers at 108cm from the sides and took REW measurements at different locations starting from 200cm from the front wall and then backwards and forwards at 15cm increases. I found a particularly good response at 140cm from the front wall where the dreaded bass nulls were quite smoothened out.
Here are all the measurements before and after the treatment with both and single left and right speaker testing, big file due to all the triple measurements at 9 different locations:

http://web.tiscali.it/webspace7/Callagh ... _tests.zip
(edit: link fixed!!!)

And here, as usual , are a few questions if I may:

Q1 - REAR WALL SO FAR - Do you think that considering the available space in my room the 6" deep rear wall bass trap that I've built has made enough improvement that's worthy of sacrificing all that space? So far, according to theREW measurements I've uploaded, what do you think of the outcome of the rear wall 6" trap I've built?

Q2 - FOAM BASS TRAPS - If, after bass trapping the left and right wall in the next stage, you will still find that I need to do more bass trapping, I was also planning to buy more of those foam bass traps (same material properties of the Auralex LERNDS) to place on most of the ceiling horizontal angles, which are surprisingly quite effective considering their small size, at the moment I've put just 6 on the floor corners just as a test. Do you think that that might give us enough bass trapping for the whole room?

Q3 - FRONT SIDES - I guess that now I should move on to treat the front part of the side walls right?
Considering this:

Quote:
That would mean that closer to the front corners I would get much more than the 4" absorption you suggest for the sides, maybe starting from 6" (plus slats) and then decreasing to 0" as it gets angling down towards the middle of the room… What do you think?

Slot walls are tuned devices: the absorb a specific frequency range, set by the cavity depth, slat dimensions, and slot dimensions. They aren't too hard to tune, since normally you want them to be set up as broadband, and one method of doing that is as you suggest: making them shallow at one end and deep at the other. That will work, but you do need to calculate, to make sure you are absorbing a useful range!

...for the sides should I go for:

1) a simple broadband absorption angled trap with fabric and no slats, maybe starting at 6" deep (or more?) from the middle of the 2 front superchunks and gradually going towards 0 at the end of the listening position and finally getting flush with the untreated side wall (left) and the window (right),

...or:

2) the same but as a tuned device with slats? If the latter is what you advice, how can I determine which frequency range I should absorb?
One of the reasons for which I was thinking of doing an angled trap on the sides of the listening position is also an aesthetic one, so it gets flush with the window on the right side, otherwise if we make it straight it would form a big drop where it meets the window… Can it be done that way?


Q4 - REAR SIDES - And for the portion of the side walls that are behind the listening position, should those be left untreated (and reflective) just with fabric directly on top of the gypsum board? Or maybe just 1" of mineral wool underneath?

Q5 - FRONT WALL - I wish I could get away with just 2" absorption on the front wall to save space, do you think I could?

Q6 - CEILING - The ceiling above the listening position is angled 15° so you suggested to treat that later on only if necessary. But in that case will it need to be all absorptive all the way to the rear, or just in the section that's behind the listening position, considering that is not angled at all there?

Thanks Stuart , I really wish you might find some tome to answer to this, I'd like to take advantage of the Xmas holidays to work on the studio:))

Thanks again,
Pat


P.S.: Found this ray tracing plugin for Sketchup 7, maybe someone might find it useful?
http://rhin.crai.archi.fr/rld/plugin_details.php?id=601

[Soundman2020]

Great set of REW data! But it's easier to put it all in one MDAT file, instead of several files. That way, it's very easy to compare different readings just by flipping back and forth between them. In consolidated your data into two files: one with all the incremental mic moves, which clearly shows which problems are SBIR and which are modal, and the other with the sequence of treatment steps, which clearly shows the improvement with each step.

From the first file, your modal issues are very obviously at roughly 44 Hz, 68 Hz, 82 Hz, 98 Hz, 139 Hz, 161 Hz, 181 Hz, and 225 Hz. If you compare those to your room dimensions, you'll be able to figure out which walls need more treatment.

There also appears to be some flutter echo going on towards the front of the room.

You can also clearly see the SBIR issues going on at various locations between around 60 Hz and 90 Hz, as well as 100 to 120 Hz, and up around 190 - 250 Hz.

It looks like the smoothest response is a bit further back than normal in your room, at about 220 cm from the front wall. SBIR is worst at about 180 cm. Where you are right now (at 140 cm), there's a big SBIR dip between about 50 Hz and 85 Hz, and SBIR can't be fixed easily with treatment at such low frequencies.

So far, according to the REW measurements I've uploaded, what do you think of the outcome of the rear wall 6" trap I've built?

It's working, for sure! Your decay time went from over 530 ms to around 440 ms, and the additional treatment brought it down to around 340 ms. It can still improve more, but it's much better, and a lot smoother.

If, after bass trapping the left and right wall in the next stage, you will still find that I need to do more bass trapping,

Yep! It still needs more. Lots more. As much as you can fit in to the corners.

Do you think that that might give us enough bass trapping for the whole room?

It would help, for sure, but I think you'll likely still need more. It's a small room...

I guess that now I should move on to treat the front part of the side walls right?

Right! That should take care of the flutter echo and early reflections, and hopefully also help with some of the modal issues.

1) a simple broadband absorption angled trap with fabric and no slats, maybe starting at 6" deep (or more?) from the middle of the 2 front superchunks and gradually going towards 0 at the end of the listening position and finally getting flush with the untreated side wall (left) and the window (right),

You could do that, but a simple panel of 703 on each of the side walls at the first reflection point would also be a good option, and much easier to build.

2) the same but as a tuned device with slats?

I wouldn't do that at the first reflection points. Further back in the room would be good, but not in the area between your head and the speakers.

Q4 - REAR SIDES - And for the portion of the side walls that are behind the listening position, should those be left untreated (and reflective) just with fabric directly on top of the gypsum board? Or maybe just 1" of mineral wool underneath?

I would do some REW tests after all the above treatment is in, to see how you are doing, then decide what to do at the back. So for now, just leave them untreated. You might need slot walls back there, or something else...

Q5 - FRONT WALL - I wish I could get away with just 2" absorption on the front wall to save space, do you think I could?

Try it and see! Do a REW test with 2" just placed there temporarily, then another with 4", and a third one with 6", then decide which one is most effective.

Q6 - CEILING - The ceiling above the listening position is angled 15° so you suggested to treat that later on only if necessary. But in that case will it need to be all absorptive all the way to the rear, or just in the section that's behind the listening position, considering that is not angled at all there?

Probably just a section behind the listening position. Once again, you could do something temporary (held up on step ladders, or some such) while you do REW tests to see where / how thick / how much area.

- Stuart -

[callaghan]

Dear Stuart,

I've finally built the treatment layer on the front and front sides of the walls!

Callaghan-front-and-sides-treatment.jpg

I must admit that after this step I finally realized that I will have to lower my aim a little , 17.2 x 12.8 x 8.5 is not bad a size for a home studio, but as you said it is smallish for a full blown pro studio mixing room, I finally came to terms with that and have to accept it. I know I will probably disappoint you, since your philosophy is: I want this studio to amaze people. "That'll do" doesn't amaze people.

…but I finally realized I will have to agree to a compromise between ideal control over BASS freqs and the contrasting necessity of having enough space for comfort during work, and will have to resort to double-check bass freqs on my Beyerdynamics while mixing. In fact, apart from the 6" bass treatment in the rear wall I had already put up, I really think I cannot afford to go over 2" in the front and sides as you see in the pics, I know it's not much... I finally accepted that I will have to stick with the best result I can obtain with bass freqs given these space limitations...
Of course I will be able later on to build some more super-chunk bass trapping on the horizontal corners of the room as you said, but I really would like to build them in the END since they would sit over the final wall and ceiling treatment, if that is ok with you.
What do you think about this new "compromising" approach? I hope you'll understand and will still be willing to help me out...

Here are the new MDAT files measured after installing the front and front-sides 2" treament, for an easier analysis I've consolidated them accordingly to your advice (random disturbances in the very low freqs are due to some construction works going on outside...)

http://web.tiscali.it/webspace7/Callagh ... -files.zip

In the light of this, I would like to ask 2 questions if I may Stuart:



Q1 - CEILING - Luckily you said that I might not need a cloud due to the 10° slope in the front, however the wooden tiles floor is not very reflective at the moment on account of it still being covered by cardboard and plastic protection layer, but once uncovered I guess it'll be much more reflective right? do you think I should go for an absorptive ceiling or not? would any of these options be a good idea for the ceiling?

A - buy (or DIY) some classic rectangular fabric covered mid-highs rockwool panel absorbers, or some kind of pre made mid-highs absorptive foam tiles, attached directly over the gypsum ceiling?

B - or go for a full absorptive surface, leaving no gypsum surface free, building a wooden frame and filling it with rockwool panels.
My only concern over this solution is the extra cost of getting the ceiling professionally upholstered too (I'll already be paying a pro to upholster the walls), the possible sagging of the rockwool, and the added dust problems that more fabric on the ceiling would create later on… But of course if you think this is the best solution I'll definitely go for it!



Q2 - MID-REAR SIDE WALLS - Considering that I will be putting a plastic layer over all the rockwool treatments just underneath the upholstering as you frequently suggest, should I go for a 2" absorptive layer for these areas too, just like in the front early reflection sides, or maybe some wooden surface or slats to keep the room lively?
At the top of the page is a Sketchup screenshot that can give you an idea of the room's layout.. The RIGHT wall's surface is mostly occupied by the internal leaf window (see pic above in this post), while the LEFT side is free, apart from the door and the encased radiator on the back as you can see in the pic below:

Callaghan-left-side-wall.jpg

As always, thanks for your patience and crucial support Stuart
Pat

[Soundman2020]

My reply is probably coming a little late, but yeah, I would always go with an absorptive ceiling, especially when it is not too high.

should I go for a 2" absorptive layer for these areas too, just like in the front early reflection sides, or maybe some wooden surface or slats to keep the room lively?

Here too I would go with the absorption first, then see how it goes after you measure it with REW and listen to it. If needed, you can always add some reflective surfaces to live it up a bit.

- Stuart -