It's the sound absorption, stupid!
Updated: Jun 2, 2020
Ok, my attempt to a Clitonian presidential slogan is perhaps not the best, but after reading the n-th question about sound proofing, I feel like ranting a bit.
And what I feel like ranting about, is a simple but apparently largely unknown truth of the universe that you can read in the picture below:
Repeat with me: SOUND TREATMENT IS NOT THE SAME AS SOUND PROOFING.
"Treating a room" means reducing the amount of reflections along all the audible frequency range.
In a regular room, you do that by hanging absorption panels and setting up bass traps in the appropriate places on walls and ceiling, and perhaps putting a carpet on the floor.
In a purpose built room (a studio) you have specific shapes to deflect directional frequencies, different materials in different areas, resonators in the wall, and other paraphernalia to get you the amount of reflections you want (not much) where you want them (where you record acoustic guitars but not where mixing goes on).
The amount of sound that comes in or out of the room, however, will be largely unchanged.
Sure, some low frequencies will be removed, but a way to do it may well be to make them escape outside of the room (what better way of getting rid of unwanted low frequency energy waves than getting them outs?).
Same for noise coming in... if you have a regular room and bass traps, you soon will be aware of the First Law Of Low End Absorption:
You Cannot Have Enough Bass Trapping
So you will be already struggling to deal with the bass that is generated within the room. There's simply no way you are gonna be able to do something to the low frequency vibrations due to that subway nearby.
Move the studio, or time your recordings between trains!
However (nearby subway tunnels excluded), it's relatively easy and cheap to sound-treat your room so that you significantly reduce reflections and therefore make it sound way better.
Sound proofing is a completely different ball game.
It's about preventing sound to get out of the room, and sound getting in.
Now, "sound" is a shortcut for "moving air". Better, is a shortcut for "a wave of kinetic energy making air molecules vibrate". If they have enough energy, vibrating air molecules will induce vibrations in anything they come in contact with, so long it can move a little. They're pushy things, energy waves.
You know already that bass frequencies have much more energy than highs (longer wavelength, move a lot more air, etc etc). And you know what moving air in a room is in contact with?
And most walls can - and will - vibrate, at least if sufficient energy hits them.
So "sound proofing" essentially means:
preventing air to come in and out of the room
for low frequencies, preventing walls from vibrating (which would carry low frequencies in or out). And that includes floor and ceiling.
That means two things:
Sound proofing is a bitch.
There's no way to do it on a budget.
Even if you make room airtight (and you die of asphyxiation within 10 minutes of closing the door... but what don't you do for the love of music), you will still have to deal with the wall vibrations.
That usually means building a room into a room, and making sure the bearing points of the internal room do not carry vibrations in and out, and the space between the walls does not conduct too much energy either, and all pipes and plugs and holes are made to dissipate energy and you have a Star Trek like airlock instead of a door, and so on and so on.
(And no, you can't make a vacuum between walls. The strength of material needed for surfaces as large as room walls would make the budget for the Space Shuttle look like peanuts. I jest, but not so much. If you ever have tried to create a little vacuum in anything you would know how hard it is. Literally all the air from the height of the thing to up to the stratosphere it's pushing to get in.. and that's a lot of air. And the pressure is not linear. So a light bulb.. fine. A ball-like volume.. hard. A room, built with regular materials ..no way.)
So, is it not possible to make a room soundproof?
Yes it is, but it is not for the faint of heart or for anyone on a budget. Most of the times, you are really better off finding a place in the quiet of the countryside (and making sure there's no heavy industry in a few km/miles radius.. low frequency likes to travel. A lot.).
But absorbing sound will reduce it, right?
Yes it will, a little.
You can reduce the sound coming in or out (mostly out), a little bit. But the kind of sound proofing that allows you to play drums at 2 AM without disturbing the neighbors.. it's very tough and very expensive.
And the same goes, unfortunately, if you are the neighbor and the drummer is on the other side.
Where does this leave us?
Nowhere, really. This is a rant.
Perhaps, it will leave you with a better idea of what you're talking about when you're talking of soundproofing.
One can always hope.. :)