Hi. Please read the
forum rules for posting (click here). You seem to be missing a couple of things!
In my Vocal Booth (2.2 x 2 x 2.1m) I have to use some Diffusers.
Why do you "have" to have diffusers in your vocal booth? Who told you that? Whoever it was, you can ignore their advice from now on, because they don't have a clue about acoustics.
Vocal booths do not need diffusion like that. And even if they did, it would not be possible to use a numeric-series diffuser in such a small room. You should never use a diffuser within 10 feet (3m) of your ears or a mic, because of this:
QRD-Diffusion-lobing--pattern-graph-SML-ENH-2.PNG
That shows the lobing patterns formed in the air close to a diffuser. All of those artifacts do not get fully smoothed out into a really diffuse field, until a long distance away from the diffsuer: at least 3m (10 feet), and perhaps more, depending on what frequency you tune your diffuser for.
Which leads to another question: What frequency range WOULD you tune your diffuser for, in a vocal booth? And why would you choose that frequency range?
And another question: Where would you put such a diffuser in a vocal booth? The wall you face? The wall behind you? One of your side walls? The ceiling? The floor?
That's a 2D Diffuser, a lot of work to build.
Yes it is a lot of work to build, and yes it is 2D, but it is not a diffuser (despite the name on the website). It claims to be a QRD diffuser, but it very definitely is not. Numeric diffusers (including QRD diffusers) have to be based on prime numbers, which I'm sure you know are all odd numbers, never even. Yet that "diffuser" has 6 wells in each direction! "6" is not a prime number, so that is not a QRD diffuser. Prime numbers are necessary for a very simple reason: it's the only way to get the diffuser to send out the same acoustic power in all directions, equally. If you don't use a prime number when you design it, then you do not get flat power response: there will be more power going out in some directions, and less in others... thus, the sound field around the diffuser will be even more contorted and mangled than in the image above, and it will never smooth out completely... (well, it won't ever smooth out before the diffuse field blends into the background field of the room, and is lost...) That device might scatter a bit, but not evenly, not smoothly, and it is not a true diffuser. So you don't need to build it anyway!
The other one is a Tim Perry design, and truly is a proper diffuser: he has some good designs, and I have used some of his designs in studios that I have done for clients. You'll notice that the elements in his diffuser do have an odd number of wells, and are based on prime numbers, so they do work correctly. Here's a link to an example of a control room I designed for a client, that is currently under construction, and uses Tim's LeanFuser:
thread about Steve's high-end control room in New Orleans
However, I would still not use such a device in a vocal booth. I doubt that Tim would recommend that either. To start with, it is rather large to fit in a vocal booth! But even if it could fit (using just one module, for example), I still would not do that, for the reasons I mentioned at the start.
There are a lot simulations of Diffusers in the internet with different mathematical models, but I've found almost no measurements. Sometimes I think they are afraid of the measurements
They are not afraid of measurements: the problem is that it's very hard to measure "diffusion" in a real room, and even harder to interpret the results. The untrained eye would likely not be able to understand the resulting graphs. For example, here's one of the many tests we did in that very same room you see in the link, just before and just after the diffuser went in, and at the same time we took out some old absorption panels:
STN--REW--FR--20-20k--With-[GREEN]-and-without-[BLUE]-leanfusers.png
Can you look at that frequency response graph, and explain what the diffuser did there? Can you see where it made a difference, and where the absorption panels made a difference, just from looking at that?
Here's the actual impulse response graphs for that (in the form of an ETC):
STN--REW--IR--100ms--With-[GREEN]-and-without-[BLUE]-leanfusers.png
Can you look at that and determine if the diffuser is doing it's job? Which part of the changes in IR, are due to the diffuser, and which parts are from the absorption changes?
Here's a third graph, that also shows the Impulse response, but this time smoothed to 0.2ms to make it easier to see, and including a third curve, which is after additional treatment was added to the room:
STN--REW--IR--100ms--With-[GREEN]-and-without-[BLUE]-leanfusers-and-other-[RED].png
Can you see what the diffuser is doing in there? The ETC graph is the most useful of all, for an acoustician, because it reveals everything about the room: exactly how it responds to a sudden sharp transient impulse. So everything you need to know about the diffuser is in there... but it's pretty hard to see!
In fact, if you look at the blue curve and the red curve, most people would say that they are pretty much the same, with practically no differences, so they would assume that the treatment is not working.... but this is what the studio owner said after he listened to some music in there for the first time after installing the diffuser (excuse the graphic language!):
I have no way of listening to reference CDs on my JBL mains right now -- so we actually are listening on a Panasonic "boom box" with the speakers up on music stands, in front of the JBLs at the same height, aimed the same way and a mere 2" from the soffits. We listened to a bunch of commercial reference CDs. The room has to be truly amazing [because] it transforms a shitty boom box into high fidelity, larger than life listening experience with really well mixed material. All I can say is that we never heard those mixes sound that good and that clear, and I had to keep reminding myself this is just a boom box ! And the mixes still sounded big and powerful walking all the way back to the diffuser, euphonic and euphoric and never feeling sucked out or dull.
So basically, Hell yeah!
That diffuser is making a huge difference to the room. And that comment comes from a mix engineer with many years of experience in all types of studios. He's blown away by what it does for his room. It even makes lousy speakers sound good.
But you'd never know that from looking at acoustic response graphs, without an in-depth understanding of what you are seeing!
So that's why diffuser designers and manufacturers don't usually bother posting test results: It's hard to measure, harder still to present, and REALLY hard to interpret.
Now, before you get all excited about the studio owner's comments, and think that you need such a device in your vocal booth, take into account that this is a heavily treated control room, carefully designed to be as good as possible, and the room is TWELVE TIMES larger than your booth. The air volume of your booth is about 9 cubic meters. The air volume of that control room is about 112 cubic meters....
So that's the sad truth: you don't need a diffuser in a vocal booth. Even worse, your booth is practically a cube! 2.2 x 2 x 2.1m. The only real option you have is very heavy damping, then some reflective surfaces on the wall you are facing and the side walls, such as varying wood slats, angled, and maybe a shallow-curve poly hung from the ceiling to provide a little smooth diffusion in the vertical plane. That's what I would do in such a room. It's still going to sound somewhat "boxy" though, because of the size. You can't make a small room sound large.
- Stuart -