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Post by devnull on Apr 19, 2016 1:33:27 GMT
Hi Ethan, I've had one exchange with you in the past (not on this forum I think). I'm starting on a new mix room in my home that I would like to also do a little recording in, and would like some thoughts on possible treatments. I've included a pdf showing the room (tried to send REW mdat but too big- waterfall charts in pdf). Room is nominally 15'x12'x8', tile floor, drywall on all walls and ceiling except right wall is masonry block. I've read through most of Cox and D'antonio's 2nd Edition, have done a fair amount of online reading on acoustics web sites and forums, and am a EE by background. So, I was planning on putting in a fair amount of porous absorption - super chunks in corners (16x16x22), 6" panels on rear wall and behind monitors on front wall, 4" panel on right wall, 4" panels at left/right primary reflection points, 4" panels on ceiling. super chucks around most of wall/ceiling (16x16x22) - maybe MLS slats on some of wall panels. However, the room measures a lot 36-44 Hz that I don't think the porous is going to cover. I was considering building some 2'x2' membrane resonators (say 6 - 8) tuned to about 40 Hz and putting them in the primary corners. I was also thinking about skyline diffusers on the left and rear walls (I'm using QRDude and probably will limit to minimum distance of 40" (design frequency is 1126 Hz) to allow for possible mic'ing in the rear of the room). Some things that may be a problem: - I was thinking I'd put the membrane resonators behind the super chunks - I'm assuming the LF will just pass through the porous absorbers. - I was thinking of using cellulose blown insulation (GreenFiber) (at 3.5 lbs/cuft density) in the panels and super chucks - after doing some validation that it works - there's a far amount of indications that it does - but I thought I'd try to get some measurements to be sure. Any thoughts? Attachments:Studio_Ethan_160418.pdf (230.49 KB)
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Post by Ethan Winer on Apr 19, 2016 18:53:18 GMT
That all sounds excellent. You are perhaps the most prepared person to ever post here.  I'm sure porous absorbers in front of "membrane" traps will be fine. Whatever the porous traps don't absorb will simply pass through. I'll add that if this was my room, I'd probably load it up with as many porous traps as will fit, each as thick as possible, and call it a day. But there's nothing wrong with making tuned traps if you're up for the additional effort and expense! --Ethan |
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Post by devnull on Apr 20, 2016 20:38:10 GMT
Thanks for the response Ethan. Sounds like I'm not totally confused at least. I think I'm just to much of an engineer to not play with making some membrane traps.
I'm kind of new using REW - do you have a post that might explain what to look for in REW responses? What I should be the objective in my REW measurements for a room my size when it is well treated?
I kind of understand I'm trying to get rid of the peaks and nulls in the waterfall and limit the decay to some maximum time. For example, in my first waterfall chart, there's a peak about 38 Hz that hits about 67 dB and the decay goes well beyond 300 ms at about 40 dB. If I shift limits to a lower dB at bottom, the decay goes further out. Should I expect a "generally" flat response across more of the spectrum with peaks at say X dB and have all the decays below say Y ms at X-Z dB? What would Y and Z be? Are there other things in REW that I should be looking at?
I was a bit confused by the response I'm seeing at about 15-20 Hz. My calibration files on the RTA mic (-3 dB @ 12.5Hz) and my audio interface (UR824 -3 dB at 5.8Hz) are basically flat. My sub is spec'ed to 35 Hz (I assume -3 dB). Do you think this might be some 'artifact'? At 15'5", my calculated lowest resonance should be at 36.6 Hz.
I'll let you know how the cellulose works out once I do some testing.
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Post by Ethan Winer on Apr 20, 2016 21:00:29 GMT
This tutorial shows the way I like to see the various REW graph types scaled and presented: Room Measuring PrimerAs you'll see there, I set the vertical range to 40 dB with the highest peak within 10 dB of the top. That puts the most audible parts on the graph, since once stuff is 30-40 dB down it's not much of a problem. If you can get a small room like this to within a 10 dB window you're doing very well. In most rooms the peak / null distance is 30-40 dB or even worse below 300 Hz. If your sub goes down to 34 Hz, I wouldn't worry about data below 25-30 Hz. --Ethan |
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Post by devnull on Apr 20, 2016 22:10:03 GMT
Thanks again Ethan. You advise and feed back are most appreciated.
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Post by Hexspa on Apr 22, 2016 4:18:42 GMT
Correct me if I'm wrong, please, but 6" thick 705 or 8lb mineral wool will have an impact below 44Hz. I'm sure it did in my room at least. I understand you're using less dense material but, since you're superchunking, I'm guessing you'll have a similar result.
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Post by devnull on Aug 1, 2017 0:58:58 GMT
Hexspa. My plan would be to deploy most of the absorption before I go after any membrane resonators. I'll leave room for them, and if I don't need them, I'll probably just put some more absorption in that location.
As far as low frequency absorption goes - from what I've read - porous works where the sound wave has velocity - maximum velocity is at 1/4 wavelength - it's zero at the wall. So if you're trying to absorb 44 Hz, which has a wave length of 25.6 ft, ideally, you'd want an absorber at about 6.4 ft from the wall that is associated with this mode (actually no one wants a 6' absorber or one setting 6' into a room). One text said it needs to be at least 1/10 a wave length to have some affect - so perhaps on the order of 2.5 feet which is about what the ray path length is through a typical superchuck (32" wide). That's not to say that something smaller wouldn't have some affect.
703 is 3.0 lb/cuf. 705 is 6.0 lb/cuf. Folks seem to lean toward the lower density as far as I've read. I've talked to one vendor (not Ethan) that said they use 1.6 lb/cuf and don't notice a difference. Cox's book has graphs of the absorption of the specular reflection attenuation for four densities 30/70/120/175 kg/m3 (i.e. 1.87 lb/cuf to 10.9 lb/cuf) - alpha vs freq vs incindent angle) - there is variation, but perhaps not as much as you might guess- they all seem to work - though the 70 seems to be a bit better to my eyes. 703 seems to be the product of choice.
(But see post below for some measurements with green fiber).
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Post by devnull on Aug 1, 2017 1:08:19 GMT
Hi Ethan, I know it's been a while, but I though you might find these preliminary measurements that I made on Green Fiber interesting. The green fiber is still in the factory packaging with about a 9 lb/cuf density - not ideal, but the measurements I took seem to be showing that it is working rather well. I'm proceeding toward implementation and should have some more realistic density/packaging and measurement in the future. The absorption looks good down to say 125 Hz and it is having an impact below that - that being said - I did have 20 packages spread around my 12x15 foot room. Studio_Ethan_REW_170731.pdf (360.87 KB) |
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Post by Ethan Winer on Aug 4, 2017 14:38:08 GMT
Something strange is going on at the very lowest waterfall peak frequency, but overall this is obviously a very big improvement!
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Post by devnull on Aug 14, 2017 15:25:31 GMT
I assume you are referring to the uptick on the 20 Hz line. Attached is another shot of the untreated room focused on the low Hz. The room is surrounded by house/garage on all sides. There seems to be a lot of LF stuff running around - some of it not decaying. My sub is 34Hz -80Hz. Could that be rumble from a nearby roadway? I'm one lot over from a 4 lane road (2 per side) and about 100 feet up on a hill (e.g. there's a cut in the hill and the road is below line of sight where it is close). Can hear some traffic noise when I'm outside usually, but not in the house. Mostly cars, but sometimes trucks. More importantly... is there something to be done about it other than putting 20Hz HP filters on any recordings? Studio_Ethan_REW_170814.pptx (136.75 KB) |
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Post by Ethan Winer on Aug 14, 2017 19:49:58 GMT
You can tell if "activity" on a graph is noise by measuring three times in a row. Anything that changes from one graph to the others is noise.
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Post by devnull on Aug 15, 2017 17:30:32 GMT
Thanks Ethan.
I'm going to make another pass at measurements soon. I've got the 'green fiber' blown out into some 55 gal trash bags and the density should be around 2.0 lb/cuf - so should get a more realistic picture of how this stuff performs.
I think REW has an SPL meter. Will play with that to see if I can locate some noise and resonance problems. I'm pretty sure my metal file cabinet is buzzing at 110 Hz (Why have a metal file cabinet? It had to go somewhere and I lost the contest). Will try turning off AC and/or TVs in other rooms in house, maybe isolate mike stand from floor, see if can associate noise with window, A/C ports, door, etc.
I have another question on room design. If one follows the typical advise on small room treatment (absorption for first reflections, corner super-chuck traps, triangular ceiling/wall traps, cloud) you probably end up with more absorption surface than say the Eyring-Norris equation would recommend for mode decay times to be on the order of the desirable 500 ms (RT60).
Rather than cut back on the bass traps for low end, I was toying with doing something to change the absorption on the corner and ceiling traps behind the listening position to say something like 50% with perhaps ternary or quadriphase sequences that have diffusion as well as absorption. I was assuming the front end of the room will be left dead. Looks like I could get up to around 400 ms (at 500Hz+, assuming 4"+ absorption is a=1 (probably accurate) and everything else is a=0 (maybe optimistically low)). Since I'm optimizing the number sequences, I could pick any absorption % I want. Is 50% a good number or should I go higher like say to 75% or higher?
I know how I can fabricate the sequences, but is this likely to actually help in a room this small or am I going down a rabbit hole?
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Post by Ethan Winer on Aug 22, 2017 14:52:28 GMT
Much simpler that trying to stick a diffuser on top of bass traps that won't benefit from diffusion, just glue thin card stock to the front of the insulation. Or use Contact Paper which already has sticky glue and comes in many colors. That keeps all the bass absorption, but reduces absorption at mid and high frequencies.
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Post by devnull on Sept 19, 2017 18:56:30 GMT
So I did a quick noise survey using the NIOSH app on my iPhone (https://www.cdc.gov/niosh/topics/noise/app.html).
The following readings were takeng over about 90 seconds on a Tuesday around 11AM and reflect:
LAeq: Equivalent (averaged every second) continuous sound level in A-weighted decibels [dB(A)].
Max Level: Maximum instantaneous dBA sampled over period.
Outside my house in front of garage facing roadway: Max 56.4 dB, LAeq 50.1 dB
Inside Garage (which studio one wall and window with): Max 35.9 dB, LAeq 29.6 dB
Inside Studio with computer on (at listening position): Max 28.5 dB, LAeq 27.0 dB
Inside Studio with computer off: Max 26.3 dB LAeq 21.1 dB
No treatment in room. house AC is on, but otherwise house was quiet. Haven't done anything to noise isolate the window or computer yet. With computer off, I think I could hear some LF traffic rumble at one point.
I've read through Master Handbook of Acoustics (MHA) (6th edition) now and it filled in a lot of holes in my understanding. I'm guessing you're familiar with this book. Just off the top of my head, to make things better what I would probably do is:
1) Try to isolate the computer away from mic'ing positions and perhaps make some sort of an acoustic box for it to be used while recording or mixing.
2) Perhaps do something with the window like they recommend in (MHA) in the section on home studios (4 laminated MLV sheets over window with air tight seal).
After (1) I'll probably take a look at mic'ing the quiet room and looking at frequency content with say protools and spectrum analyzer, but my guess is that the LF content in the waterfalls is from the roadway traffic.
Question to you is what would you suggest and how bad off do you think this situation is (know there is a spectrum of answers for quality of silence, but perhaps you could give some views on what would be an acceptable level of noise for tracking).
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Post by devnull on Sept 20, 2017 0:22:27 GMT
Oh - one more thing I forgot - the A/C duct feeding the studio also feeds the room across the hall. This room is on the roadway side of the hose - so some noise could also be bleeding from that room through the duct.
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