The room is the most expensive thing you own

The order most people buy in is interface, then microphone, then monitors, and eventually — reluctantly, after everything else — some treatment. For anyone making mix decisions, that order is exactly backwards.

Why it is backwards

A monitor you cannot trust turns every subsequent decision into a guess, and the room contributes more to what arrives at your ears than the speaker does in almost every domestic space, by a wide margin.

Bass is where it is worst. Room modes in a small room produce peaks and nulls of ten to twenty decibels at low frequencies. No monitor upgrade addresses that, because it is not the monitor.

You cannot mix what you cannot hear, and the room decides what you hear.

What a room does to a speaker

Three things, in descending order of damage.

Modal behaviour at low frequencies. Every dimension of the room supports standing waves at frequencies whose wavelengths fit it. At those frequencies you get large peaks in some positions and near-total cancellation in others. In a typical bedroom the worst of these sit between forty and two hundred hertz, which is where bass and kick live.

Early reflections in the mid range. Sound reaching your ears from the walls, ceiling and desk a few milliseconds after the direct sound, combining with it and producing comb filtering — a series of peaks and notches across the mid band that moves as you move your head.

Reverberation at the top. A bright, untreated room adds decay that makes everything sound more spacious than it is, which leads directly to mixes that arrive dry and thin everywhere else.

The cheap fixes, in order

Position first, because it is free and it is the largest single improvement available to most people.

• Sit facing the short wall, symmetrically, with equal distance to the side walls. Asymmetry produces a stereo image that is not real.
• Do not sit at exactly half the room's length. That is where the worst modal null usually sits, and it is where most desks end up.
• Pull the speakers away from the rear wall if you can. Close to a wall adds low-frequency reinforcement that is not in the mix.
• Speakers and your head form an equilateral triangle, tweeters at ear height, pointed at you.

Then absorption at the first reflection points. Get someone to slide a mirror along the side walls while you sit in the listening position; wherever you can see a speaker in it, that is a reflection point. Thick broadband absorbers at those points, and on the ceiling above the desk, remove the comb filtering that makes the mid range unreliable.

Then bass trapping in the corners, which is where low-frequency energy accumulates. This is the least visually appealing and the most effective per unit of money once position and first reflections are handled. Thick, porous, floor to ceiling, in as many corners as you can live with.

What treatment cannot do

It cannot make a small room behave like a large one. The modes are a function of the dimensions and they do not go away — trapping reduces their severity and shortens their decay, which is a large improvement, not a cure.

This is why the other half of the answer is references and cross-checking. Know what three or four records you trust sound like in your room, and use them as the calibration. Check the mix on headphones, on a phone, in a car. You are not trying to build a perfect room. You are trying to build one whose errors you know.

A known bad room beats an unknown good one. What you need is not accuracy, it is predictability.

The spending order that works

Treatment, then monitors, then interface, then microphones. Reverse of the usual, and the reason is simple: every item further down that list is judged through the items above it. A better microphone heard through an untrustworthy room does not help you make a better record, because you cannot evaluate what it did.