Since the question is so short, we thought it would be an interesting challenge to keep the answer as simple as possible.
The first thing you need is quiet. Yes, quiet is a noun. Audio Masterclass thus spake it so.
Quiet is relative. In our simple studio what we need is quieter. So presuming you live in a building rather than a tent, there will be a certain amount of outside noise coming in. You could hire a $400-an-hour acoustic consultant, then order in the concrete. Or you could find the quietest place in your apartment or house in which to record. Finding the quietest place costs nothing, and you will get better recordings. (The tent doesn't have a quieter place. Moral: Don't record in a tent unless you have to.)
The second thing you need is dry. Oddly enough, dry is a noun too.
By 'dry', I mean acoustically dry. The $400-an-hour acoustic consultant would correctly say that in an ideal world you would have a combination of absorption and diffusion, fine-tuned to provide a reverberation time appropriate for the size of room, and gently falling in proportion with rising frequency. Standing waves and excessive low-frequency reverberation would of course be attended to.
But we're thinking simple. So what's the simplest thing you can do to make an improvement to an ordinary room? Yes, make it dry. Bring in all the soft materials you can find until your recording room is as dry as a dinosaur fossil.
Mr $400-an-hour is now saying that this isn't right. No, it isn't a fully correct solution. But it's better than recording between hard, bare plaster walls and it can cost nothing if you're a bit clever about sourcing your soft materials. Yes, the room will sound dull due to too much remaining low-frequency ambience. But it's better than no treatment, and it can cost nothing.
In summary, you can take a typical domestic living space and move it closer towards perfect recording conditions, for little cost and little effort. You can either record in an untreated living space and make poor-quality recordings, or not record at all because you can't work with anything less than perfection. Or you can make simple improvements and get better recordings than you did before.
You have made a studio!
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