In a previous post we've been looking at how a "good sounding" room for recording means, essentially, a room with controlled and low reflections.
But when it comes to mixing, is it the same story? That is, if we have a good sounding room for recording, is it good for mixing as well?
Broadly, the answer is yes (and it's certainly better than a bad sounding room!), but with a few important qualifications.
First of all, let's get the cat out of the bag: when mixing, it is perfectly possible to ignore the room completely, simply by mixing on headphones.
Actually many engineers (me included) prefer to use headphones when mixing under pressure in an unfamiliar environment, since it saves time (you don't have to "learn" the room) and ensures consistent results (you "know" your headphones so you have an idea on how things should sound).
For example I know how bass sounds on my Beyerdynamic 990 Pro, so I can take any bass line and balance it up quickly so that it matches my references, without even listening to them.
So that's said.
However, let's assume that we're talking of mixing on monitors.
In room sound terms, mixing is different than recording in a couple aspects.
The mixing sphere
First, unlike a microphone, the mixing position is relatively fixed in the room (unless you want to move the desk for every session!). The "microphone" is really your ears... so what matters is how the room sounds at the mixing seat, more than the room in general.
This means that design optimizations and room treatment will be directed towards that area where your head lies.
Since we may move a little, the relevant area is around, say a 1 or 1.5 meters diameter sphere, centered on the mid-line between the monitors. So that you have a little wiggle room.
That said, in order to avoid reflections at the mixing desk, you will still have to prevent sound from bouncing around too much. That means that a mixing room will need to be well behaved even outside the mixing sphere, aka have low-ish reflections all around, and sound generally good.
As little reflections as possible
Second: with recording, a little leftover reflections (especially high frequency) can be beneficial, as they impart a sense of space, while leaving the shape of the waveform (that is, the essence of the sound) largely unchanged.
A typical "room mic" for drums, for example, can do wonders to glue a multi-miked drum kit (so long the room is good sounding, of course). Not so, however, at a mixing seat. There, we really want to have as little reflections as possible. Down to none if it's achievable (which generally isn't).
Well, what is the point of having these uber-detailed (and uber-expensive) low-crossover three way monitors, if the tiny high frequency details you want to hear get smashed by slightly-less-powerful, slightly-delayed high frequency reflections?
Yes - the right answer is "none".
To get value for money from your monitors, you want to have the space around your head totally reflection free, so that the only sound waves the reach your air are the ones produced by your monitors, and not reflected ones.
This is harder than it sounds. Why?
Because the desk is not your friend.
Neither are your computer screens, or anything sticking in front of the speakers.
Thing is, every flat surface in your room reflects sound, not only the walls.
And guess what: a very reflective surface - bouncing slightly-delayed waves right back at your ears - is the mixing desk right in front of you! Or, of course, any computer screen in front of the speakers.
So, when placing near-field monitors, don't put them directly on the desk. Use column stands or raise them up (that's why many desks have a baffle) and when placing them on the baffle try to make so that the speaker boxes come "a little out" of it. Also make sure your computer screens are further away than the speaker grilles.
You can also orient the monitors slightly upwards (to create a larger angle between the low side of the emission cone and the desk) - there's a few desktop stands which are shaped for you to do just that.
If you can, cover your desk in some absorptive material or at least scattering shapes, which help dissipating at least higher frequency reflections (yeah! finally a solid excuse for all these chip bags and coke cans lying on your desk!).
It is also important to make sure that your monitors are placed firmly and do not vibrate or - even worse - induce vibration in your desk. Vibrations will, of course, move air in sympathy with your direct sound and generate a new energy wave, which will have a shape a least partly similar to the direct sound... This wave will reach your ears with a slight delay, causing filtering. That's why monitor stands need to be heavy, and have some dampening properties (like thick rubber pads under the speakers).
Placing monitors on the walls may help (even if it depends on the monitor design, and their EQ controls), and even more placing them in the wall - which is exactly what actual studios end up often doing, using larger mid-field monitors which can be kept further from the mixing desk.
Keep a low profile
Another good way to keep a mixing room sounding good, is to keep mixing volume low (especially for bass emitted by monitors on the desk).
Low like in "I can have a conversation with someone over it without making an effort". That low.
That thundering bass you hear when you crank up means that lots of bass energy is unleashed in the room and bounces back on the desk or the floor, comb filtering the sound that reaches your ears and giving you a very skewed notion of what's going on.
This is why it's always good to check bass on headphones when you need to hear how the mix works at higher playback levels.
Incidentally: if you use a subwoofer with a high enough crossover, you have less of this problem, since low freq waves will hit the desk less if they are generated away from it. Unfortunately, a subwoofer usually comes with much larger problems on its own. Higher volume for low frequency waves equals higher energy levels, which in turn makes the bass much harder to absorb. So a subwoofer and low levels may be good, but most people buying a sub want to hear it loud, which usually doesn't end well. Unless your room is very good at avoid low frequency reflections, better avoiding a subwoofer altogether.
So there you have it: a good sounding mixing room is slightly different than a good sounding room for recording. It is dryer at the mixing position, but can be more lively elsewhere (so long reflections don't disturb the mixing sphere); however, when using near-field monitors, there will almost always be a degree of reflections at the mixing position, due to the close proximity of the monitors to the desk.
A little positioning and keeping levels low will help the room - and your mix - to sound good!