John Sayers' Recording Studio Design Forum Archive

Thanks Stuart for the feedback. I think I have worked out what I was doing wrong in the calibration stage so that is sorted. (For some reason that uncalibrated test showed my low end problems had vanished but all the tests I did before and since show them to be there, so I think it was a dud). More tests below.

I stuck with the neoprene top and bottom (time/money constraints) & strapped it down. I understand it won't be floated properly.
Rear bass traps are in I couldn't see how I could fit hangers in the front corners, so just stuffed with pink stuff. Baffles are now up - still waiting for the fronts There may be some diffusion from all the tools! 3 new REW tests:

Test B is with the speakers soffit mounted behind the baffles but no other changes - so basically all your notes still apply to over absorbtion of mids and lack of absorbtion of highs and bass. I will get to this soon

B: https://www.dropbox.com/s/88wu6jk13a3ms ... .mdat?dl=0

Tests C & D are with the four corners stuffed with pink fluffy - the only difference being C is with the rear traps covered in that foil and D is not. So again mids and highs not dealt with yet, just trying to see how much the bass traps have done to the bass and whether to use the foil or not

C: https://www.dropbox.com/s/o7umniw798dw2 ... .mdat?dl=0
D: https://www.dropbox.com/s/5zz7h59oxogxd ... .mdat?dl=0

Notes:

- I was a bit disappointed after seeing these tests - the bass traps have not miraculously taken away all the bass problems on the waterfall graph like I hoped! And there is now a null at 100Hz. It is improved by 10dB by the bass traps, but still nasty looking. Any ideas on what causes this and how to solve it given I've already stuffed all four corners with fluffy stuff? I'm guessing the slats don't help here given that we talked about aiming for the 250-450 Hz region for them.

- The foil (test C) does seem better in the low end e.g. nasty waterfall peak at 65Hz on non-foil pink graph. Non-foil is better in highs, but I will easily get to those highs soon just by taking off some of the thin plastic off the wall and ceiling.

- This sounds like I'm only unhappy! In fact, doing a listening to music test, I was absolutely blown away by the sound stage, by how clearly I could hear the instruments and the panning decisions. And the phantom centre was spooky - I thought the singer was in the window in front of me! I don't know if this would have happened anyway by putting the speakers on stands in the corners to give that big equilateral listening triangle or whether I can safely praise the soffits for it? I know my graphs aren't looking great yet, but I am already hearing things in songs that I never heard before - and I worked in that room for a few weeks with the speakers on the desk and it sounds completely different now.

And there is now a null at 100Hz.

Welcome to SBIR, Nick! It's your new worst enemy. The wave-eating SBIR monster has moved into your room, and it will never leave, now.

OK, that sounds a bit drastic, but here's the deal: A large dip at around 100 Hz is very common in studios, even high end ones. It is due entirely to the location of the speakers in the room, with respect to the floor, walls, and ceiling. It is called "SBIR", which stands for Speaker Boundary Interference Response (some also call it "Speaker Boundary Interface Response"... same thing).

Basically, the sound at low frequencies moves outwards from your speakers like an expanding bubble or balloon. (In your case, with soffit mounts, it expands as a "half-bubble") At some point, that wave will hit a hard, solid, rigid room boundary surface (wall, floor, ceiling, window, door, etc), and bounce back, time delayed. As it moves on from there, it will interact with new waves that have since left the speaker, after it did, cancelling some of them out, and reinforcing others. The frequency where the cancellation occurs is related only to the "extra" distance that the wave traveled before it meets up with a later version if itself. The larger the distance, the lower the frequency of the first "dip" in response. By placing the speaker in a soffit, you already eliminated the largest problem: SBIR from the front wall. But you can still get it from other surfaces in the room. In this case, it is more correctly called "LBIR" ("Listener Boundary Interference Response"), since it is related to the location of the listener in the room, not the speaker (which isn't in the room, if it is soffit mounted), but the effect is exactly the same: phase cancellation of certain frequencies at the listening position.

There's a procedure that you can perform using REW to determine if this really is the problem, and even to identify which surface(s) is(are) causing the problem. The procedure is very boring, slow, and time consuming, but it will tell you a lot about the room, in addition to identifying where the monster is hiding! If you are interested in getting really bored over several hours as you run multiple tests in carefully chosen locations, then PM me, and I'll tell you about the procedure.

It is improved by 10dB by the bass traps,

Great! So the monster is whimpering a bit, as you are containing it! Good news! Now you just need to know where it is, so you can kill it completely.... or at least beat it into substantial submission....

Any ideas on what causes this and how to solve it given I've already stuffed all four corners with fluffy stuff?

Only FOUR? A room has twelve corners...

I'm guessing the slats don't help here given that we talked about aiming for the 250-450 Hz region for them.

Right! You guess very right. In fact, you can't treat an SBIR null with a resonant device, because SBIR does not ring. It is purely frequency related, not time related. You can only treat it with absorption or diffusion.

I'll download your REW files and take a look at the other stuff, but your biggest issue is that SBIR behemoth.

I was absolutely blown away by the sound stage, by how clearly I could hear the instruments and the panning decisions. And the phantom centre was spooky - I thought the singer was in the window in front of me!

Yep! Welcome to the world of soffit mounted speakers done correctly!

I don't know if this would have happened anyway by putting the speakers on stands in the corners

Nope! Definitely not...

to give that big equilateral listening triangle

Unlike SBIR, which is a very real monster, the "big equilateral listening triangle" is a mythical monster that doesn't really exist. Lots of people talk about it, and hunt for it, and even draw diagrams showing where it is, but nobody ever seems to have found a real live one...

or whether I can safely praise the soffits for it?

Yep! You can!

I worked in that room for a few weeks with the speakers on the desk and it sounds completely different now.

So you went from the worst possible speaker geometry, to the best possible speaker geometry. Not surprising you found it vastly superior!

If you'd let me, I'd love to take your words above, and re-use them all over, maybe even as a "sticky", so people can see that setting up speakers properly really does make a huge and worthwhile difference, especially when compared to having them set up badly. May I use your words, and quote you?

the only difference being C is with the rear traps covered in that foil and D is not.

Clearly visible effect of foil! Great!

It looks like you chose the right thickness of foil, but have a bit to much area. You will probably need to cut holes or slots in it, one by one, to "tune" it until you get the best response.

Nice going, Nick! It's working out well.

- Stuart -

checking your REW data...

One thing I did notice, is that you need to turn down your right speaker by about 1 dB, and carefully check your calibration. The right channel is consistently louder than the left, but the difference changes: First it was about 0.5 dB, then 1 dB, and now about 1.5 dB- Not sure why the level is changing, but you should search for that and fix it.

The other thing I'd like to highlight in flashing neon signs, is the huge difference you got from putting the soffit baffles in place! The red line is before you put the soffit faces on, the green line is after. Pretty stunning: you extended the low end response downwards by an entire octave, and a bit more. Your speakers are now giving you very decent response down to 38 Hz, whereas you only had 81 Hz before. And that, coming from a fairly modest speaker that, according to the manufacturers specs, only goes down to 64 Hz... You are getting down to nearly an octave below what the manufacturer says...

There's pie in the face for people who don't believe that soffit mounting does anything useful....

Aren't you glad you decided to take our advice, and soffit-mount yours?

...

Next item: on double-checking, I think you are going to need thicker plastic on your traps, and then cut holes/slots in it to not kill the highs. The traps are sucking too much out of the mid range still, so the plastic is not thick enough.

I'm also seeing a lot of what seems to be very low frequency background noise in some of the later graphs: Was there something noisy going on right outside the room?

Overall, I have to say I'm impressed by what you have accomplished so far, and very hopeful that your room is going to turn our great!


- Stuart -

Welcome to SBIR, Nick!

Argh – I had read pages like this (http://arqen.com/acoustics-101/speaker- ... erference/) that said I'd do away with it with my soffits, but they were only talking about the front wall!

On the topic of the front wall – am I right in thinking it's optional whether you have a baffle extending down below the speaker – e.g. John's design has a board (covered with some insulation) down to near the ground (to let the vent air in), but I've seen others with no board and just using that space as a bass trap there. What I'm wondering is - is there any chance I'm getting SBIR from the back of the speaker hitting the front wall behind it then reflecting downwards and coming out the lower front. Or in other words, do I need to put a board down there?

There's a procedure that you can perform using REW ...

I don't care how boring it is (plus I'll probably enjoy it!) I'll definitely PM you for those instructions.

Only FOUR? A room has twelve corners...

I kind of had your voice in my head saying this even when I wrote that! I had thought about going horizontally across the top of the front and back wall. Maybe that test you mention will help me see if this is worth it. One thing I'll say is I hadn't quite appreciated how much smaller the room seems with all four vertical corners cut off. Though equally it feels more like a studio.

Welcome to the world of soffit mounted speakers done correctly!

Well, I'm glad I can thank the soffits (and you and this forum of course or I never would have done them!) They were a lot of work, but somehow quite do-able now they're done.

If you'd let me, I'd love to take your words above, and re-use them all over, maybe even as a "sticky", so people can see that setting up speakers properly really does make a huge and worthwhile difference, especially when compared to having them set up badly. May I use your words, and quote you?

Of course. Apart from a donation to the site, the other thing I can give back here is to fully communicate a modest budget beginner's journey for others to hopefully learn from, even if it's mainly from my mistakes!

One thing I did notice, is that you need to turn down your right speaker by about 1 dB, and carefully check your calibration.

Yes, I'd noticed that too – have done it now.

the huge difference you got from putting the soffit baffles in place

I'm worried that my bad calibration in REW might have thrown you onto a bad conclusion here. The reason I say that is when I had the calibration wrong another time, it also gave that bass roll off at 80Hz and when I re-calibrated correctly, it went back to rolling off at 40Hz - this was without changing anything in the room. I don't know why running a test at 100dB instead of 86 would make any difference though, apart from moving the curve up on the page, but it did seem to. I'm sorry I've been so useless with REW seeing as it's the only “eyes” you get into the studio.

To help, I have done another test just now with the baffles off – i.e. like the photo above

Test E: https://www.dropbox.com/s/acpyber4ifhod ... .mdat?dl=0

The speakers are still placed right up against the front wall and so maybe do get some of the benefit of bass boost? Comparing here the two tests (both have bass traps in place, the only difference is whether the baffle board is there or not) I do see a massive improvement in the SBIR with the baffle on. The 30-60 Hz is pretty similar, though the 20-30 is better if that means anything.

But I'm still a bit confused because my speakers are listed as rolling at 64Hz in their manual, so actually I don't understand why the low end SPL is so good in the 40-60Hz band with the baffle off.

I think you are going to need thicker plastic on your traps, and then cut holes/slots in it to not kill the highs. The traps are sucking too much out of the mid range still, so the plastic is not thick enough.

I thought maybe the slats would help bring back a lot of the mids? Or at least, perhaps I could put up the slats and take off plastic on my first reflection points, and then tune the remaining absorbers and traps at that stage.

I'm also seeing a lot of what seems to be very low frequency background noise in some of the later graphs: Was there something noisy going on right outside the room?

Well, I live right on a dual carriageway And as you have mentioned the lowest frequencies are the hardest (slash most expensive!) to keep out. I can occasionally hear the deepest of the car noises coming in.

Overall, I have to say I'm impressed by what you have accomplished so far, and very hopeful that your room is going to turn our great!

Thanks!

I'm worried that my bad calibration in REW might have thrown you onto a bad conclusion here.

Ahh! That might be it.

I'm sorry I've been so useless with REW seeing as it's the only “eyes” you get into the studio.

There's a learning curve, and it takes a while to get it all under control. No problem.

The speakers are still placed right up against the front wall and so maybe do get some of the benefit of bass boost?

... and also from the side wall. The speakers are nto too far from the side walls, so they'll be getting a boost there too... as well as artifacts!

though the 20-30 is better if that means anything.

Yep! But that might also just be noise. As we discussed in the PM, do use the "sweeps" setting on at least 2 to help minimize non-constant noise.

thought maybe the slats would help bring back a lot of the mids?

Right, but the superchunks are still sucking up a bit too much in the mids. Slats will help, but they don't have as big an effect as you might imagine.

Or at least, perhaps I could put up the slats and take off plastic on my first reflection points, and then tune the remaining absorbers and traps at that stage.

You are getting the right idea! Room tuning is all about balancing and trade-offs, and doing something here that undoes something there . . .

- Stuart -

Some new things today

- added another bass trap along the top of the back wall (will leave fully foiled & not yet put cloth over it for now so I can cut holes if needed)
- got the fronts on the baffles
- took the thin plastic off all the walls (it is still covering the ceiling for now)
- put loads of the calico cloth up
- put some thick plastic on some of the walls at the back that are neither first reflections nor going to be slatted
- listening to music test is even better (tighter bass) and I listened to a few references and then listened to a test recording I did with a local singer - my test sounded terrible! So good to be able to see it under a microscope to see (hear!) what's really going on.
Ideally I'd do a test for each change but I have other work to do while my carpenter beavers away so I can only test at the end of the day

Here is the new REW test F with all these changes https://www.dropbox.com/s/kgzy7z50b7mi9 ... .mdat?dl=0

What I'm seeing in the graphs:

- Comparing test C (previous best test) to F, the SBIR is 3db better at 100Hz & there's none of that funny stuff happening sub 30Hz. I did do 2 sweeps this time so presumably it was car noise and the average of 2 sweeps takes it away from the test results. It's often written that 6dB is a doubling of volume so I must admit I'm a bit worried about the SBIR dip - which is currently a 20dB difference between 40 and 100Hz. I do hope to get a parametric EQ at some point to adjust for the extra bass that flush mounting brings. Perhaps if the region of 40-80Hz is brought down it might help the 100Hz dip?

- The waterfall shows a good improvement <1kHz

- The RT60 is a tiny bit better

- The spectrogram I have no idea what it means! But I think the disappearance of those blue blotches is good - though perhaps that's just the car noise already mentioned.

Today I got some thick plastic from the garden centre. It weighs in at 0.268 kg/m2, meaning F = 336Hz, even lower than the foil.

I tried putting it up on the ceiling, over the thin plastic for now, covering the rear 2/3's of the ceiling - the area that isn't first reflections (and I actually bothered to do the mirror test rather than eyeballing it and realised the first reflection point was much further back in the room than I thought.)

Test H (https://www.dropbox.com/s/kkp0wiu7n4kdd ... .mdat?dl=0)

Here is the before and after SPL for both speakers. The main thing I see is the weird dip at 12k. This isn't present on either the Left or Right speaker tests. For the T30 graph (not sure which of the RT60 graphs is best to show), there is quite a bit of improvement in the 500-5000Hz range. But still my mids are well below what I'm aiming for. I noticed on the Bob Gold site that 100mm of 703 is actually more absorbent in the mids than the highs. And given that I started by covering my entire room pretty much in exactly that, that could be part of the problem. I can still add some more of the thick plastic to the bass traps and a couple of other areas, but I also wonder if I just need to take some of the fibreglass batts out of the room? I also checked again the Bob Gold calc (http://www.bobgolds.com/Mode/RoomModes.htm) and for my dimensions (4.02m L x 2.74m W x 2.53m H) it recommends around 300 sabins = 28 sq m of absorbtion. My rear wall plus left and right walls alone are 27. At the moment I have about 40 sq m of absorption. I had thought that people who did inside out walls did fill every section with 4" of fibreglass and then just use thick plastic and slats to deal with the too-many-sabins problem, but I'm not seeing the results (yet) on REW tests, plus it would have been cheaper to buy a few less fibreglass batts so I may have made a mistake here. Hopefully in the next few days I'll have all the slats I ordered up and see how that affects things.

The calc also gives this to aim for "Recommended RT60 161ms, ±50ms from 200Hz to 4kHz = 111 to 211ms" which made me feel a bit better in that I'm not miles from that at the moment & am working to get everything above 111ms.

Did a test with more covering on the rear bass traps. https://www.dropbox.com/s/y4cy9vns5bvxo ... .mdat?dl=0

Test H is the "before" with just the foil

Test I is around 30% cover with the 3mm ply veneer covering the traps (saw a thread where this was suggested) Test J is full cover over the foil with the thick plastic (therefore not a very fair test as I don't have enough 3mm ply lying around) Q - is it fine to do foil and plastic or best to take the foil off if I decide to keep it (fully or partially)

I'm looking at the area below 111ms on the RT60 graph for improvement. There is quite a bit of improvement in both tests and the most with the full plastic cover. But the waterfall is a bit worse and the dip at 390Hz on the SPL is worse. What I don't quite understand is how with most of the ceiling and all of the back wall covered in thick plastic, my RT60 is still far too low. How do I ever get liveliness back into this room?! (Lots of slats are going up soon so we'll see what they do, but from what I've read it's more the highs than the mids that they'll help with in terms of reflection).

Had to wait a long week while my carpenter was unavailable – he's the only guy round here I've found who understands what I'm talking about so I had to wait for him. I know I'm going quickly compared to some, but I find I'm so impatient to see it done. Gave me time to get the slats varnished and then he & his brother put them up. Slats! Slots! I feel I've joined some kind of club! (photos in next post as I've hit max attachments)

REW test K with the slats up (https://www.dropbox.com/s/nwbza2awb093r ... .mdat?dl=0)

Comparing back to test F as the tests after that were non-permanent (now using variable smoothing for the pics – hope that's ok, it seems to be similar to what you suggested Stuart though not quite as strict. Compared to 1/48 smoothing, it's not hiding any problems in lows or mids, but is tidying up the graph in the highs)

- SPL is similar apart from >4k there's a dip and the SBIR at 100Hz is a tiny bit worse.
- RT60 is a whole lot better
- Waterfall is a bit smoother in the low end (also shows the same improvements in mids/highs that the RT60 graph shows.)
- Listening is much more enjoyable, less fatiguing

I then did a whole bunch of tests to hunt for more of the elusive liveliness that's still somewhat lacking.

For test L (https://www.dropbox.com/s/qm2wfd59ioi7f ... .mdat?dl=0) I took out all of the ceiling fibreglass that wasn't in the first reflection point area (and tried a few small things that you can see in the file, but the main test I included in the next file so this can be safely skipped)

For test M (https://www.dropbox.com/s/54brkn4lf726k ... .mdat?dl=0) I tried putting back some of the ceiling fibreglass, but 50mm (2”) this time to try to hit highs and not mids and I also tested putting some board directly under the speakers to extend the baffles to nearly the floor to see if that affected my SBIR. M.1 is with extended baffle, M.2 is without. There is also some testing with the listening position moved forward slightly to help with the SBIR dip.

- SPL shows test L being much worse for the 100Hz SBIR dip, but putting back some of the ceiling 703 solved that in test M. Test M also solved the >4kHz problem, not quite sure why. The extended baffle didn't seem to help the 100Hz dip by more than a few dB.
- RT60 greatly affected in test L by removal of ceiling absorption, but the highs have gone too high. Test M seems to be a good middle ground, it's the best I've seen the RT60 so far. (Incidentally, I've seen some debate as to whether RT60 means anything in a small room, but I've found it very useful in getting the deadness/liveliness balance right where for example you can make good decisions on reverb in a mix.)
- I'm not really seeing much for or against the extended baffle - the low end on the waterfall is perhaps a little smoother, but it doesn't seem to be hear or there unless I'm missing something.

Some pics

Testing the covering of the bass traps to try to reflect mids Slats!

WOW! that looks great with the slats up, suddenly like a studio!

These graphs and explanations of your trial and error are really interesting so thanks for taking the time to share. I guess you're almost finished with this now? I wonder if there's anything that can be done to sort out that 100Hz, but I'm guessing you've got a 3.4m dimension somewhere in the room that's the mode for??

Nice work
Gareth

Thanks Gareth. What's also great, it the slats really seem to make a difference to how people perceive the studio - like they actually believe in it.

I must admit I've run out of ideas to deal with the 100Hz dip. The only thing left up my sleeve in the room is to use a parametric EQ to deal with the 6dB excess bass caused by the soffits. On my SPL graph at 50Hz, 200 Hz and 500Hz, it's pretty much spot on 6dB too high at 92dB (test ran at 86dB for both speakers). So my hope is by lowering that to 86dB, the relative drop between 50 and 100 Hz will be less. But that assumes the 100Hz won't just drop down 6dB (but then maybe I can boost it at 100Hz). My theory is that if it is destructive interference, then having less energy at that frequency in the room will mean less interference and less dip/smoother curve. But I did try running a REW test at 70dB to see how it affected the dip and it didn't reduce it relatively - the whole graph just moved down. So that's not a great sign, but I'm hoping anyway. I'm sure having done all I can with treatment I'll be able to get something out of EQ'ing.

Anyway I hope Stuart will correct me if I'm talking nonsense.

Hi
Random question, what dimensions did your silencer boxes end up? I'm still trying to sort mine out and we have similar sized rooms. Do you need to run that fan on high speed at all or is it OK on low speed?

Thanks!
Gareth

garethmetcalf wrote:Hi
Random question, what dimensions did your silencer boxes end up? I'm still trying to sort mine out and we have similar sized rooms. Do you need to run that fan on high speed at all or is it OK on low speed?

Thanks!
Gareth

Mine are 100cm x 59cm x 26cm and made from 18mm chipboard with 25mm liner. My external one is letting in a bit of sound, but I can see daylight through the join from the room so they didn't seal it right - it should be better once I've finished that. But I can see why Stuart likes two rather than one for the in and out. I know a lot of people would maybe double up the board and maybe add green glue but I was keeping my costs down.

The fan I run on low and I feel like even that is much more than I need, so I wish I had got a variable one, but it wasn't easy finding anything round here. When I had it blowing into the room from outside I could hear a lot of the fan noise in the room and when I have it blowing out I still hear a little (Stuart also posted about this earlier in thread). I think part of my problem is I have no duct at all - just fan plugged into silencer which is on the external wall with a hole through to the room. I think more ducting would give a chance for the fan noise and turbulence to get absorbed.

We are now off the to the UK for 6 weeks and I've basically finished the room bar a few cover strips that will cover the staples, and the registers/vent covers (Gareth did you find any in UK? any links? I'd love some with built in air filters that are washable). Could also use a small sofa if I can get one. REW tests

Test N: https://www.dropbox.com/s/0b3xnftd4xy2h ... .mdat?dl=0

Test N.1 is putting the lower baffles permanently under the speakers
Test N.2 is testing covering some of the bass traps with 3mm ply to help boost the lower-mids in the RT60

Test P (I skipped O): https://www.dropbox.com/s/f0mu0i80qu5y5 ... .mdat?dl=0 (80MB includes test N.1)

Test P.1 is with the door now covered with 50mm insulation to match the wall opposite it & the back bass traps now permanently covered in 3mm ply in two patches
Test P.2 is bringing in the dreaded desk - which made the SBIR dip worse at 100Hz but better at 400Hz
Test P.3. (only done at +20cm) is trying the desk at a 6 degree angle (spoiler - it didn't bring the magic I'd hoped for)
Test P.4. (only done at +20cm) is trying some cushions on the desk first reflection points to see what they were doing
Also included K,L,M,N for comparison

(note for REW newbies, in REW you can use alt-up/down arrow to quickly switch between measurements to see the change)

What I'm seeing in these tests is that there's pro's and con's for moving the listening position between 0cm and 20cm (having done Stuart's walking mic test I zoomed in on this area) - moving forward helps the SBIR dip but worsens the 40Hz mode. The highs also dip at some positions. Also my waterfall was better a few tests ago than it is now! Maybe I still need to try some things when we come back, like taking off the rear 3mm ply or taking out the lower baffles.

I'm very happy with the improvements in my RT60 from the start though. It is now within +- 50ms albeit a bit low overall.

And overall, what the graphs don't show is that I'm hearing my reference tracks like I am in the studio with them. It feels like cheating! So happy with how it sounds and looks.