Really! I didn't know diffusers needed so much space.
Yup! It's a function of the lowest frequency and therefore the longest wavelength. The issue is that QRD (and other numeric sequence based diffuser designs) work by changing both the angle and phase of the reflected wave in each well, so you get a difference in both direction and in timing. The result is "lobes" of reflection, where the intensity of the scattered sound field is slightly higher/lower in some directions than in others, due the differing reflections from different wells. That takes some distance to even out again, and become more smooth. Depending on which expert you listen to, the minimum distance to have your head away from the diffuser is either 3 times the longest wavelength, or 7 times the longest wavelength, or ten feet. If you are closer than that, then you are probably in an area where the lobes have not yet smoothed out. And since there is ample evidence that diffusers still scatter sound down to one octave lower than the "lowest" tuned frequency, that's the wavelength you should consider.
That's why you'll often see the recommendation here on the forum to not use diffusers in small rooms. Of course, the manufacturers of diffusers don't tell you this stuff. (Perhaps it has something to do with the fact that a large chunk of their market is home studios and small rooms!
) But if you read the literature and research put out by the people who originally came up with the mathematics behind how these things work, you'll find the reality: So absolute minimum ten feet, a bit better is 3 full wavelengths of the lowest frequency, or best of all 7 full wavelengths of the lowest frequency. Which shows that you need a fair sized room to be able to use diffusers properly.
So how do I eliminate standing waves in a small room like that
Well, in strict truth you cannot actually "eliminate" standing waves, since they are a function of the actual dimensions of the room. They are "modes", and will always be there, at specific frequencies, simply because the room has the dimensions that it does. But what you CAN do, is to damp the modes, so that their "ring" or RT is brought down to suitable times, depending on the frequency band. In any event, diffusers are not much use for dealing with the biggest modal issues, since the modal issues will always be below about 200 or 300 Hz in a a small room, and a diffuser capable of dealing with such low frequencies would have to be very large. (Yet another reason why diffusers are not suitable for small rooms.)
So the only real option open for damping modes in a small room, is absorption. Lots of it. Very thick. And in the corners. Since all modes terminate in corners, that's where trapping is most effective.
But like you say:
I'm trying to reduce my reliance on broadband absorption cause the room is sounding a little dead.
Exactly. Put too much absorption in a room, and it sounds "dead". Murphy strikes again!
OK, so the reason it sounds dead is because in order to have enough absorption to damp the low frequency modes, you now have way too much absorption for the highs! So the solution is to put something in front of your bass traps that reflects the highs back into the room without allowing them to "see" the absorption, while allowing the lows to get through. Fortunately, that isn't hard to do: just put some plastic across the front of the bass traps. Thinner plastic works for higher frequencies, thicker plastic reflects down to lower frequencies. For example, 6 mil plastic works down to about 600 Hz, which is probably about right for most small rooms. So frequencies lower than that will get through and be damped by the thick absorption inside the bass trap, and frequencies higher than that will be reflected back into the room. Of course, that isn't a sharp "cut-off" point! It's more of a slow roll-off above and below. That number is just sort of the place in the spectrum where the plastic is reflecting roughly the same amount as it allows through. Above that it progressively reflects more and more, while below that it progressively allows through more and more.
But you can experiment a bit with other things too. If you think you need to go down even lower, then try very thin plywood or something like that instead.
And of course, the key to tweaking the room is to actually measure what it is doing, using REW. Your ears can tell you that "something is wrong", but REW can tell you exactly what is wrong, why it is wrong, how much it is wrong, where it is wrong, and suggest what to do to make it right again.
So I'd suggest that you download REW and analyze your room with it, then post the results here for us to look at.
- Stuart -