Getting the most out of a small tracking room

We have been using this room with a few baffles for a year or two but have enough money now to really try and get the room sounding good. I have uploaded a few sketch-up drawings, REW measurements, and pictures of the room empty. Two of the walls are cinder block, and the other two walls are drywall with no insulation inside them. The floor is tiled with a concrete slab underneath. The ceiling is made from thin pieces wood similar to beadboard, and the roof is about two feet higher than the ceiling (I doubt there is any insulation between the ceiling and the roof and I will also note the roof is metal). This room was basically a quick addition to the existing building in the 1970’s and due to some shotty carpentry, the walls are all slightly different lengths. The ceiling is 8’1” on one side and 7’ 10” on the other. The long concrete wall is 12’6” and the opposite wall with the window is a length of 13 feet. The short wall with the window is 9' 3" and the short concrete wall is 9'5". We originally installed 8 baffles with 8 lb. 4” roxul insulation along with 8 2x2 skyline diffusors. This made a dramatic improvement over the empty room, but even with treatment, the room still had a small unpleasant ambience and modal ring to it around 560Hz. We have made a new design shown below as we are giving the room a makeover. I would love to get some feedback. We are basically filling the walls with baffles, superchunks, qrds, and skylines. Any input would be really appreciated. We have built all of the baffles, qrd's, and skyline's, and have been pleased with the results so far. The QRD's we are building scatter down to 358Hz, diffuse down to 717Hz, with a cutoff around 3100Hz, according to QRDude. The skyline diffusors have a good diffusion coefficient down to 860Hz according to the BBC papers they were modeled after. And the suggested additions...

Hi "boogle1234", and welcome! :)

We have made a new design shown below as we are giving the room a makeover. I would love to get some feedback. We are basically filling the walls with baffles, superchunks, qrds, and skylines.

First, get rid of those diffusers! The room is way too small to be able to benefit from most types of diffuser. You need a large room for that. You say that your diffusers work down to 358 Hz, implying a wavelength of about 3.2 feet. Depending on who you listen to, you need at least 3 full wavelengths, or seven full wavelengths, or ten feet, for the lobing issues caused by numeric diffusers to even out. That implies that you need to have both your instruments and mics at least 9.6 feet, or 22.4 feet, or ten feet away from the diffuser. In a room only 13 feet wide, that is really hard to do! So number-based diffusers are not an option for your room. Despite what some manufacturers and some web sites tell you, QRD based diffusers aren't suitable for small rooms. Putting instruments and mics too close to such a diffuser produces unpredictable response in both the frequency and time domain. It will be hit and miss every time you set up, so trying to repeat the sounds of an instrument tracking session by setting things up roughly the same is not going to work: you'd have to take very careful measurements of exactly where everything is for each session, and reproduce the precise locations of all items in order to get the same sound again. Maybe....

We are basically filling the walls with baffles, superchunks, qrds, and skylines.

The superchunks are fine, but forget the QRDs and skylines. They will do more harm than good. And I'm not sure what you mean by "baffles", as that can have several meanings in acoustics.

The skyline diffusors have a good diffusion coefficient down to 860Hz

Once again, you'd need to be at least 4 feet away from those, or 9 feet, or ten feet (depending on who yo listen to), in order to get consistent, repeatable results. Number based diffusers act in the frequency domain, 3D space domain, and the time domain. Because of the way they work, they produce lobing patterns around them, and that requires both space and time to even out. They do good things for dispersing sound, provided that you are far enough away from them, but get too close and you are going to be affected by those lobes. In your room, number based diffusers should be tuned higher than about 2 kHz in order to be usable. But of course if they are tuned that high, then they are not dealing with the low frequency problems evident in your graphs.

I have uploaded a few sketch-up drawings, REW measurements,

The room clearly has some serious modal issues which are not being treated. The frequency response is all over the place, and the RT-60 seems to show that the room must be sounding rather "honky". In fact, the RT curve looks like it is pretty much the inverse of what it should be, according to ITU and EBU recommendations! It's hard to understand how the decay can be 1.2 seconds at 1 kHz while also being 500 ms at 200 Hz! What type of music do you plan to record in there? It's a tracking room, not a control room, so the response does not need to be flat, but what the graphs show doesn't seem to be what you'd expect for a good tracking room. My $0.02, for what it's worth. - Stuart -

Thanks a lot! First, the "baffles" are 2'x4' frames with 4" thick, 8 pound roxul insulation wrapped in burlap. The main reason we put the diffusors into the plan is the strong mid presence in the room. Both the RT60 and frequency response show a lot of problems in the mid frequencies. The diffusors cover the range of problem frequencies and will hopefully help get rid of some of those long sustaining mid frequencies in the room without killing the high-end like the corner traps and baffles would. There are some great videos Ethan Winer has put out where he is using QRD's in rooms about the size of ours and the results sounded really good. We also had the skylines in the room before and a/b the room with/without them and they def. seemed to help. At the same time from what I understand about the science behind the diffusors I completely understand what you are saying. My question is why does the videos Ethan has put out, and our experience with diffusors in smaller rooms seem to contradict the science behind the qrd's, or at least suggested implications. Even Massenburgs studio chttp://www2.digidesign.com/digizine/dz_main.cfm?edition_id=101&navid=907 has diffusors in a tight room. Thanks!

Even Massenburgs studio chttp://www2.digidesign.com/digizine/dz ... &navid=907 has diffusors in a tight room.

There's a few huge differences between that room and yours: First, it's a control room, not a tracking room. Second it was designed as a mastering studio originally, (even though it is also used for mixing). Third, it is MUCH larger than yours. Fourth, it was designed, in part, by D'Antonio himself, who happens to be one of the "fathers" of QRD design theory, and has written paper on it. He's also the guy who recommends at least ten feet between the diffusers and your head, or 3 full waves, and if you look at those photos, he does seem to be taking his own advice! Also, the depth of those diffusers is orders of magnitude greater than yours. And finally, that studio cost hundred of thousands of dollars to design and build.

There are some great videos Ethan Winer has put out where he is using QRD's in rooms about the size of ours and the results sounded really good. We also had the skylines in the room before and a/b the room with/without them and they def. seemed to help.

I think you missed the point: Yes, diffusers will change the sound of the room, and yes I have no doubt that Ethan found a good place in his room to record that video. But that's not the issue. The issue is that he'll have a hugely hard time finding the SAME place again, which he would need to do if he had to add a few extra takes to an song! The lobes on diffusers cause large variations in sound close up, which evens out with distance. Even moving the mic or instrument an inch can change the sound. So while you might be able to get great sounds by playing close to a diffuser, you also might never be able to get that same sound again when you go for the second take next day, or want to do another song that sounds the same a week later. As I said in my first post: the issue is repeatability, consistency. So if all your artists always play perfectly every time, and never, ever need to come back for a second take, then your room can work fine. But I don't know too many musicians like that! :)

Both the RT60 and frequency response show a lot of problems in the mid frequencies.

Exactly! That's my entire point. And those graphs are showing the result AFTER putting in your initial round of diffusers. The issue is simply this: those diffusers are not removing the mid-range energy from the room! They re-direct the energy in both time and space, sure, but do very little to REMOVE it from the room, which is what you need. It is still in there, bouncing around, and taking a different path through space, phase and time on each occasion where it hits a diffuser, but still there. So the sound field is diffuse, for sure, but still overpowering in the mids. That's the reason for your very high RT-60 times in the mids. No amount of diffusion is going to change that. Yes, on each "bounce" you do lose a little energy, and you also lose some due to the air itself, but it's a tiny fraction, and clearly from the graphs you are not losing anywhere near enough. And that, indeed, is the entire purpose of measuring RT-60: it tells you about how long the diffuse REVERBERANT field is hanging around in the room, after the sound sources cuts off. The only way to get rid of some of that overwhelming mid-range energy, is to absorb it. And that needs to the next issue: in order to absorb that much energy, you need a large surface area. Theoretically, for a room with your dimensions, you need 285 sabins of absorption, (implying 285 square feet of open window). But since you have all your walls mostly covered in diffusers, you don't have enough wall space available to achieve that. The entire surface area of your room is about 582 square feet, so you need roughly half of the entire room surfaces covered with perfect absorbers. You obviously cannot do that at all with the floor, which removes 117 possible square feet, leaving you with about 465 ft2 of wall/ceiling space for absorption. So you'd need to cover more than 61% of the entire available wall and ceiling surface with perfect absorbers to get the mids under control. And since no absorber is actually perfect in real life, you actually need more area than that. Then you have the issue of the space taken up by the windows and doors, so the actual amount of wall area that needs to have absorbers on it, is going to be very large. That's the other reason why diffusers are not recommended for small rooms: The smaller the room is, the more absorption it needs, but smaller rooms have less surface area available to do so! Catch-22.

The diffusors cover the range of problem frequencies and will hopefully help get rid of some of those long sustaining mid frequencies in the room without killing the high-end like the corner traps and baffles would.

I already said this, but it bears repeating: diffusers remove very little energy from the room, so they don't change "sustain times" much. And the traps and baffles won't suck out highs if built correctly, with a suitable layer of plastic, thin wood, or perforated panel in front of them.

First, the "baffles" are 2'x4' frames with 4" thick, 8 pound roxul insulation wrapped in burlap.

:shock: :!: 8 pounds? as in "8 pounds per cubic foot"? As in "8 PC"F? That's way too dense, and explains why you have such severe bass problems and no highs in the the room! Contrary to popular belief, higher density insulation does not work better for low frequencies: it works better for HIGH frequencies, and WORSE for lows. So what you have is doing the opposite of what you need. You might want to consider replacing that with something more useful, such as maybe 2 to 3 PCF. That stuff you have in there right now has such high gas flow resistivity that it is just sucking up the highs and doing very little to improve the lows. I just noticed something you said in your original post:

We originally installed 8 baffles with 8 lb. 4” roxul insulation along with 8 2x2 skyline diffusors. This made a dramatic improvement over the empty room, but even with treatment, the room still had a small unpleasant ambience and modal ring to it around 560Hz.

If it is at 560 Hz, then it isn't a modal ring! Your modes at that frequency are so closely spaced as to be irrelevant. Your room has serious modal issues below about 300 Hz. Although the Schroedinger frequency for that room is theoretically 147 Hz, I'd still expect problems one octave higher, and that seems to be borne out by the REW graphs. Finally, you also seem to be lacking energy in the highs, and I suspect that that might also be related to the diffusors: since highs act more like rays than fields, they tend to be more easily dissipated by obstructions and distance, which is what the diffusers are. I'd still suggest replacing the diffusers (at least some of them) with absorbers covered in thin plastic. Make some large 2" and 4" panels (703, fiberglass, or mineral wool), and scatter them around the room, more or less randomly (walls AND ceiling), some up tight against the surface, others spaced ab inch or two away, until you are covering maybe 200 square feet with them, and see how that goes. But make sure that each one has a layer of plastic over the front surface of the absorber (behind the cloth cover), as you do NOT want to lose more highs! You are already suffering from a lack of highs, so you don't want to absorb any more of those. I say "200 ft2", rather than the theoretical ITU 285, since you also don't want to kill the room entirely! It's a tracking room, so I imagine you want to keep some of the ambiance and warmth in there. So I'd go for maybe 200 ft2 to strart with, and see how that looks, then decide if you still need more (or maybe even less, in which case you could "double up" one panel on top of another at on location, in order to improve the lows and not waste a panel). So if you were to make 4' x 8' panels, you'd need seven of those around the room, and if that were too much you could put #7 on top of #6, thus making it twice as deep but reducing the surface area by half. You also seem to need even more bass trapping than you already show in your SketchUp model, to deal with those severe modal issues in the low end, but there doesn't seem to be enough room to do that. Besides, the absorber panels outline above would help with that, to a certain extent. I'd bet that doing the would get you a good improvement in overall room acoustics, both in the frequency domain and the time domain. Anyway, that would be my plan for a room like that: get rid of some or all the diffusers, which are doing more harm than good and are also taking up the valuable wall space you need for absorption. You might even consider making one wall into a slot wall, which you could then tune to problematic frequencies, or ranges of frequencies. - Stuart -