CR Treatment and Tuning---The home stretch!!

of course. My expectations are well adjusted... 8)

I can also imagine how complex it would be to figure out what to build in order to do it effectively without over killing it or lacking.

:thu: There's also the issue of being careful to use the right equation! There's actually a wrong version of the Helmholtz equation circulating, and it has even been printed in several text books: It's a simple "typo" error, where there's a "plus" sign instead of a "multiplication" sign.... so it's sort of important to check that you even have the right equation, when designing slot based devices! Wrong equation: fo = 2160*sqrt(r/((d*1.2*D)+(r+w))) Right equation: fo = 2160*sqrt(r/((d*1.2*D)*(r+w))) :) - Stuart -

Here's the Bass Trap... A lot of work... Fortunately Stuart is pleased with the results... :mrgreen: I'm sure he'll be commenting soon. Very encouraged!

Update on what all of Frank's hard work actually accomplished. It's rather nice, actually. So here's some pairs of "before" and "after" graphs from acoustic measurements he did. The "Before" measurements were taken immediately prior to him starting to put in treatment, in the otherwise-empty room. The "after" measurements were taken yesterday afternoon, as soon as he had that rear-corner monster in place. First, the waterfall plots, which show how the sound pressure levels decay over time, for every frequency. In this graph, the frequency axis runs across the page from left (lows) to right (highs), the intensity is shown in the vertical axis, up the page, and the time axis comes out of the page towards you. In this case, we are looking at just the bottom end of the spectrum, from 18Hz up to 500 Hz, because that's the most critical part. In fact, what's REALLY important here is the part under 200 Hz: BEFORE: AFTER: You can clearly see how the response has smoothed out very significantly, with all those huge mountain peaks very much flattened and rounded into hills. In a previous post, prior to Frank building this thing, I did mention that one of the tuning points was 131 Hz, so I marked that with the cursor in this graph (a white line overlaid on top of the "mountains". You can see that it is working very nicely, with both the intensity and the ringing greatly attenuated. But the biggest issue here is what happened down lower, towards the bottom end, which is always the hardest to treat: There were some pretty big modal issues at 50Hz, 59 Hz, 69 Hz and 94Hz (HUGE!) that have almost completely disappeared now, and the big one at 114 Hz is greatly reduced, as are several others. Another very interesting one, is the RT60 graph. It shows the reverberation time for many small frequency ranges (just one third of an octave wide) across most of the spectrum. It's not really technically accurate to call the low end response "RT60", since there's no statistical reverberant field in small rooms for low frequencies, but most people still call it that. More accurate would be something like "energy decay times", but RT60 is fine. BEFORE: AFTER: Those cover almost the entire spectrum, from about 35 Hz to 11 kHz, and you can clearly see how the rear corner devices are having a major effect on smoothing things out. Before the treatment, the overall RT60 time for the room was around 900 - 1100 milliseconds: Now it is neatly and smoothly under control, at around 400ms. Also note that the high end is still there, not killed, and it's looking good. We are aiming for something around 250ms for this room. Maybe a little more. Next up; the spectrograms. These show the same data as the waterfall plots, but in a different representation. 2D, not 3D. Here the frequency axis is still across the page from left to right (lows on the left, increasing to the right), the intensity is shown in different colors, and the time axis runs up the page: higher peaks mean longer ringing, slower decay. BEFORE: AFTER: The improvement is pretty spectacular, actually! Each of the long thin spikes in the "before" image is a room mode. You can see how narrow and sharp they are: very thin frequency bands, very intense. In other words "high Q". And you can see how all of that has flattened out and smoothed over very nicely, with the monster at the back of the room. The SBIR hole that was there before at around 110 Hz is gone, and overall it is a lot smoother. Once again, I highlighted the 131 Hz issue, so you can see that the tuning is working out fine. The monster is eating the modes! And finally, the graph that everybody loves to look at first but actually doesn't tell you a lot: frequency response curve. All it shows is the sound intensity for each frequency. Not very useful, but here it is anyway: BEFORE: AFTER: The smoothing is pretty clear here to. Once again, look at the 131 Hz point, to note that the device is doing it's job, and you can clearly see how the peaks at those other problematic modal frequencies I mentioned before (50Hz, 59 Hz, 69 Hz and 94Hz) have all come down. The results at 59 and 69 are very satisfying, as that low down is very hard to treat. That's the deep bass region, always a tough one, but it's coming under control rather nicely. The change at 94 Hz is also rather substantial. OK, one more: this is a comparison of "before" and "after" on one single graph, so you can see the difference more easily: That one is zoomed in to show just the most important part: 18Hz to 200 Hz. Purple is "before", green is "after". The overall result is better than I had hoped for, so I'm pleased with it. I think the graphs speak for themselves! :) Consider that this is a corner control room, and also consider that the ONLY treatment in the room at this point is the rear corner. We still have the ceilings and walls to play with, plus the cloud, and the soffits. So there's plenty of room for playing still. The rear corner bass trapping usually gives the most spectacular improvements, as you can see here, and subsequent treatment accomplishes less and less with each round, but I'm confident that there's enough room to get things rather nice for Frank. The ceiling is up next: Frank is starting to work on that right now. That should smooth out some of the other spikes that didn't change much. They didn't change because the are associated with the vertical axis of the room (ceiling and floor), not the walls. So treating the ceiling should take a bite out of those. After that we'll be doing the cloud, and the soffits. Not sure which one will come first... STAY TUNED! - Stuart -

That's once again great work guys! A lot of work for sure Frank but you did great man looks awesome! So Stuart you design these bass traps/treatment once you've made a proper analysis of the test results correct? In other words you can't fully predict what's required when it comes to treatment until you know how sound gets thrown around the room? I'm amazed at the level of details that go into this sort of thing..I love it!

So Stuart you design these bass traps/treatment once you've made a proper analysis of the test results correct?

Right.

In other words you can't fully predict what's required when it comes to treatment until you know how sound gets thrown around the room?

Right. Prediction will get you close, especially for a rectangular room, but this room is not rectangular: it is closer to square (it has to be: it's a "corner control room"), but with one corner chopped off, angled side walls, and soffits. It's more of a 7-sided trapezoidal polygon, so it's hard to predict the outcome. And it's always best to check the actual response, even with a perfectly rectangular room. Construction materials are not perfect, dimensions can vary, there might have been changes during the construction for unexpected reasons, etc. - Stuart -

In case Stuart doesn't reply shortly, the short answer is yes.. he can predict the behavior of a room when he designs it, but not completely.. What you're seeing is how the process works when you hire Stuart. LOL... looks like we did it again Stuart

Excellent! Thanks for the explanations! I love learning more about this stuff! Keep up the great work!

Frank has put the insulation on the ceiling, and sent me his latest REW measurements, so here's some more "before" and "after" pairs, where "before" is with just the rear corner treatment in, and "after" is with the ceiling insulation in too: WATERFALLS: You can see the very nice effect that this is having on the modes at 113 Hz, 177 Hz, 205 Hz, and 232 Hz. Those have pretty much gone now, as well as general low end smoothing. This is a real "slap in the face" for those folks who insist that a couple of inches of insulation has no effect below about 400 Hz or so! Much to their chagrin, the result here is glaringly beautiful: That's some pretty major changes, especially at 113 Hz... and all Frank did here was to put insulation in between the ceiling joists. Just a few inches thick, nothing more, and nothing special: just ordinary insulation. Got any photos of that Frank, to permanently silence the noisy "insulation doesn't work down low" mob? Showing what you did, and how thick it is? (For those who don't know what I'm referring to here: there's a vocal bunch of detractors of the benefits of thin porous absorption, who loudly voice their ignorance all over the internet, insisting that insulation can only be effective if it is 1/4 wavelength thick. According to them Frank would need insulation that is two and a half feet thick to have any effect on 113 Hz (the wavelength is 10 feet, so the quarter wave is 2.5 feet). Even worse, what Frank did here also had a nice effect on the problem at 86 Hz, where the Wavelength is 13.1 feet, so you'd need 3 and a quarter feet, according to the silly folks. But you can clearly see that just 8" of insulation is having a very, very nice effect, even down at these low frequencies. 8" is just 6% of ten feet, and 5% of 13 feet, which proves Andre's theory that good porous absorbers is still effective down to 3.5% of the wavelength for random incident sound, and 7% for normally incident sound. And if you look closely, you'll see that this treatment even had some effect on 74Hz. Not a lot, but it's there. The wavelength for 74 Hz is 15.25 feet, so according to the nuts, he would have needed nearly four feet thick insulation to do that... :) 8" is 4.3% of that, still within the 3.5% theoretical range... --- Sorry for the rant, Frank, and for hijacking your thread to do it, but this is one of my pet peeves! People spouting off about stuff they know absolutely nothing about. I just wanted to mention that, since your carefully done tests highlight their ignorance so beautifully! The ONLY thing you did was to put the insulation in, and the results clearly show that it works very decently.) SPECTROGRAMS: You can see the same thing here, and also something more important: The SBIR null at 144 Hz is filling in nicely too. I highlighted that with the cursor. (I'm pretty sure that is SBIR, not modal, since there's also a phase shift going on at that point.) (You have to be careful when looking at this pair, since REW has automatically adjusted the scale between the two! Because the overall level went down, REW modified the color scale and intensity, so it looks like the low end got louder, but it didn't. REW just shifted the scale so that the highest level is now at 89 dB, instead of 91 dB before. So refer to the key color-strip on the right side of the graph for clarification. FREQUENCY RESPONSE: Nice changes, smoothing out neatly.... The 144 Hz issue is highlighted there, with good improvement. RT-60: The decay times are coming down, especially in the high.mids and highs. But I'm a little concerned that we might be going too far in the high end: the very top end is already down to about 230 ms, with plenty of treatment still remaining. So well need to do something about that. So, Frank, you'll need to get some very thin plastic, around 1 mil, and cover SOME of that insulation in the ceiling, but not all of it. I'd suggest maybe 50% coverage. The type of plastic you need is the very thin stuff, that painters use as drop cloth to protect furniture while they paint. You can buy it in hardware stores (Home Depot, etc.). Cut off reasonably broad strips, maybe 8" or so wide and as long as you need to span between joists, then leave 8" open spaces between them. Don't make them too wide, or they'll be reflecting too much. Also, don't make them too narrow, or they won't do their job. It's not too critical: anywhere around 7" to 9" wide is fine. So put those up on the ceiling, and try another REW test. There won't actually be very much difference in the graphs with this test at this point, since there's still way to much other stuff going in, that is going to be masking the effect, but it needs to be done now, before moving on to the rest. - Stuart -

Stadank0 wrote:

In case Stuart doesn't reply shortly, the short answer is yes.. he can predict the behavior of a room when he designs it, but not completely.. What you're seeing is how the process works when you hire Stuart.

Thanks for the promo and commercial break, Frank! :thu: :) - Stuart -

I'm guessing ill only need to do the half of the ceiling that won't be behind the cloud. I do need to put in a correction in there....The insulation in the ceiling consists of standard rockwool but also an inch of duct liner in the covers... :mrgreen: I forgot to mention it, Sorry Stuart... So we have 3.5" of rockwool and 1" of fiberglass duct liner. I'll take some pics when I'm adding the plastic food wrap...back in a bit!

So we have 3.5" of rockwool and 1" of fiberglass duct liner.

Even more impressive! Four and a half inches of typical insulation is giving you some pretty substantial improvements! 4.5" is 3.9% of the 113Hz wavelength, 2.8% of the 86 Hz wavelength, and 2.5% of the 74 Hz wavelength. So Andre's theory was actually pessimistic! Reality turns out to be even better. The effect goes down even lower than he predicted: porous absorption is slightly effective at thicknesses as small as about 2.5% of the wavelength, and very effective for thicknesses greater than about 3% of the wavelength. When done correctly, of course!! :) - Stuart - (PS. I would do 50% coverage of all of the ceiling, just to be safe: you never know what reflections might make it up above the. Theoretically, it should only be lows and mids getting in there, but in practice, I would not be surprised to find some highs too. The whole concept of RFZ is to get a decent "diffuse field" after the ITDG of around 20ms, that then dies away slowly. So I'll take that field from wherever I can get it! If there's some above the cloud, then that's good for me! :)

Here are the pics you wanted Stuart. I'm ready to move onto the back walls. Let me know if you want anything else done first....

I'm assuming you want me to do the same 1 mil plastic on the back walls?

Not yet! First the insulation, then we'll see how it's going with REW. I might want to do slats on some of that, in which case you don't need the plastic. - Stuart -

Actually, I would prefer that because the walls will look boring with just fabric... 8) oooo...I just had a thought. What about hanging guitars on the wall as part of the reflective scheme? That would look pretty!

oooo...I just had a thought. What about hanging guitars on the wall as part of the reflective scheme? That would look pretty!

I'm not a fan at all of hanging resonant things in control rooms! :) Acoustic guitars are designed to resonate.... Perhaps a couple of electric guitars would be OK. - Stuart -

of course, I was thinking solid objects for sure...that actually occurs to me. Not a critical need for me either way. Will have a test up for the back walls today or tomorrow morning!

OK, So here are some pics of the back walls insulated with panels on them. Nothing exciting to see, but they are now crushing the decay so I'm sure changes will be made. This is not unexpected, so no big deal. Some problem areas got worse and some better. Again, I expect this kind of behavior as the process goes on. Stuart will probably elaborate more on this.

To be on the safe side I went back and did a measurement without the covers on. very little change but wanted to eliminate any suspicions all the same. I'll let Stuart post pics of the graphs if he wants cause He'll explain it better... :shot: There is little difference in a dip at around 175 hz so I'll put em back on!! I absolutely hate insulation! Ready for the next round! Which will be next? the cloud or the soffits?

So after looking at the results of these measurements and nothing better to do until I hear back from Stuart, I have a theory... There is now a significant dip at around 177hz that was not there before. Of course there are improvements as well so this is not something that I'm worried about, but more interested in understanding. I understand that as a broadband absorber, insulation absorbs some amplitude and also affects phase and decay as well(on a broad spectrum). If the null was not there before the insulation was installed, then I theorize that phase shift at that frequency with regards to SBIR is causing the null. I noticed no appreciable difference between the covers on and off save the null being slightly worse with the covers on. This of course adds the 1" of duct liner. I further theorize the the duct liner would further exacerbate the phase shift and increase the null with regards to SBIR. Can't wait to see what techniques Stuart has up his sleeve in this game of whack a mole! Before: After:

If the null was not there before the insulation was installed, then I theorize that phase shift at that frequency with regards to SBIR is causing the null.

It might have been there before, just masked by NOT having all that absorption around: In other words, the absorption trimmed the long grass way down, so now you can see a few rocks and stones laying down on the ground that were there all along, but not visible before. Or you cut down some tall trees, and now you can see the shorter ones that were hidden behind them. I think we might be looking at an issue that was there before, but moved up the spectrum slightly and is now much more visible because you "trimmed the jungle". If you take a close look at the spectrograms, you'll see that there was a vague hole around 165 Hz in the untreated room, which moved up to about 167 with the rear trap, then 171 with the ceiling absorption, and now 174 with the side wall absorption.. It's definitely phase-related, though. You can't see it on the straight phase plot, but you sure can on the minimum phase and excess phase plots!: No time to go into what those two graphs actually mean (minimum phase and excess phase are a bit complex to explain), but in general they are a lot more useful than the plain phase data, and here you can clearly see some major stuff going on at the same point where that dip is happening. So it's pretty clear that a reflection of some type is responsible for this. So . . . I'm wondering if that is a floor bounce, perhaps, or maybe from the front walls... Quick check on that: Put a thick pad of insulation on the floor, between the speakers and the mic, like this: Lay down a full panel of 4" thick 703 in front of each speaker, as best you can fit it in, then throw some pink fluffy on top of that, and do a REW test. It will be interesting to see which way that null moves (if it moves at all), and how the amplitude and reflections change. Next point: Your graphs: when you look at your REW graphs, apply some smoothing so you can see what's really important. If you are looking at the low end, then apply 1/48 octave smoothing, or maaaybe 1/24: Anything higher will hide modal issues. If you are looking at the mid range, then apply something like 1/24 or 1/12, and if you are looking at the high end, then 1/6 or maybe even 1/3 would be more appropriate, depending on what you are looking for. As rule of thumb, when I'm hunting for "needles in a haystack", I use 1/48 for lows (or maybe no smoothing at all), 1/24 for mid and 1/12 for highs. If I'm looking more for "How will the human ear perceive this?", then I'd go more like 1/24 for lows, 1/12 for mids, and 1/6 for highs. Of course, the specs for control room acoustics usually define things in very low res, such as 1/3 across the entire spectrum, but that does not show the details. And as the saying goes, the devil is in the details! If you follow the above "rules of thumb", you'll see a much cleaner graph that is far more useful. In addition, you'll probably want to zoom in a lot more, so you can better see those details where "the devil is"! :) For looking at the low end, I normally set the horizontal scale to cover 18Hz - 500 Hz, and the vertical to cover 40 dB - 110 dB. That's where nearly all of your major issues will be. For some issues, I go even tighter, and use 18Hz - 200 Hz with 50db - 100 dB. You get to see all the ugly stuff more clearly like that, and everything below about 200 Hz is critical for bass response. On the other hand, if you take a quick look at the second graph you posted above, in the gold color, you'll notice that your unsmoothed frequency response is already within +/- 10 dB of flat, across the full spectrum (except for that biggie at 170 Hz)! That already puts the room well within acceptable response for home studios: +/-10 dB is pretty good. +/-6 is really good, +/-3 is high-end pro-level, and better than that is magic! So you already have "pretty good" frequency response, unsmoothed, but we are only just getting started! OK, enough rant, on to practicals: As expected, all those hundreds of square feet of pure absorption sucked out all the life from the room: it's dead now. That's good, actually, since it took it out fairly evenly across the spectrum: So we are starting from a good position, where the decay is nicely even across all frequencies, just too low over all. And all we have to do now, is to add some life back in to the room with carefully crafted reflections. OK, that sounds way too easy... the concept is easy, but the implementation... not so much! We need some more data to see which way to go here. So first test (after you do the "insulation on the floor" test above, and remove that insulation from the room again!); if you have a couple of full sheets of plywood, OSB, MDF or something similar on hand, lean those up against the side walls, symmetrically (same on left and right). No nee to attach them: just lean up temporarily. Thickness not too important: it's surface area we are after. So try putting one full panel on each side of the room, do a REW test, then two full panels on each side, and another REW test. That will give us some data points to work with, in figuring out how much total reflective area you need to get things looking nice again. Don't expect the response to be even, though! It's gonna mess up that nice smooth decay curve, but that's fine. If you don't have full panels, then use whatever large scraps and off-cuts you have (hopefully nice long ones that go almost all the way up the wall), trying to keep the surface area balanced between the walls. You could even use a couple of old doors, or any other large, flat, rigid surface that you happen to have on hand: counter top, table top, deck planks, whatever. So there's some things you can play with today, to help figure out what is going on, and which way to go to fix it. I wont be available for the rest of today and tomorrow, as there's a yearly planning meeting I have to attend (and it's also the weekend!), but I'll check the data as soon as you sent it to me. If you wanted more things to do to keep you busy, after the above tests (remove the wood panels from the room first...) you could do 40-50% coverage of the insulation with that 1mil plastic, but do it in horizontal strips above and below ear height, and not going all the wall to the floor and ceiling. In other words, leave a strip of bare insulation at the top and bottom of the walls, and another bare strip about 18" - 20" wide centered at ear height (48" above the floor). Do yet another REW test once that is in. That's not going to make a big change here, but it will help with some of the lost life, at the high end. - Stuart -

I'm on it! will report back soon with results... Things are definitely progressing in a hopeful way. I started this project with home studio ambitions but those have changed after 3 years and lots of money invested. My hope is to get to a professional level result so that I can monetize the space. I know that was not the scope of the original design, but I'm hopeful that we can get within range based on how things are shaping up.... :mrgreen: Thanks for the graph lesson by the way. Useful for sure :thu:. Was posting those quickly to show the 177 null but will definitely use the guidelines going forward...

My hope is to get to a professional level result so that I can monetize the space. I know that was not the scope of the original design, but I'm hopeful that we can get within range based on how things are shaping up....

I kind of figured it would go that way! :) It often does... However, it might have been easier if I would have known in advance! But we can still get it good, I reckon... - Stuart -

i knooooooowww...if only!! I never would have guessed back then, I can tell you that. :lol: In all reality, I still believe that the quality of work done in this space will speak for itself. I have no doubt that outstanding engineering work gets done is worse spaces that is totally passable and respected in the real world. There are several local studios (excluding Universal Audio of course) that have produced hundreds of projects on a professional level that were not done to this level of detail. I'm certainly not trying to be a full blown LA studio or the like..That would be unrealistic.