Let me try to help here by explaining a few things.
Sound is just energy, energy that vibrates air molecules that happens to be in a range that your eardrums can vibrate to, and in turn transmit into your brain.
Sound saves, light waves, radio waves, it's all the same stuff - energy, just at different frequences - which is why you hear sound waves but can't see them - and can see light but can't hear it.
Sound, like water, will gladly fill the space it has available to it.
There are two aspects of sound control useful in studios. The first is sound isolation and the second is sound absorption.
Simply put, sound isolation is basically keeping sound in one room and not allowing it to pass into another.
So to understand how sound gets from one room to another will then allow you to understand how to stop it.
Sound is just vibrations, so if the air is shared between two rooms, sound can pass through.
If the rooms do not share air, but share structure (walls, glass windows, etc), those items can vibrate passing the sound from one air chamber to another.
If you close all your windows and you can hear your neighbor's dog barking, either you have an "air leak" or the dog's barking is vibrating material in your home (drywall, glass, whatever), and those materials are passing the sound. In most homes it's probably both actually.
So to achieve true sound isolation, you have to eliminate both paths of sound transmission - air leaks and forms energy conversion (sound --> wall --> sound).
There are only two methods to achieve sound isolation that I know of, and they're often necessary to be used together:
1. Increase mass
2. Suspension.
Increasing mass just means making the room to be isolation out of materials that are physically heavier, denser, etc. An ordinary home window transmits outside sounds very well - even closed you can hear your dogs barking. Yet, if you stand in the lobby of a "corporate" building and look outside, you can't hear the people outside who are talking while smoking, nor the cars zooming by, and in some buildings you can't even hear emergency vehicles! That's sound isolation through mass - the glass in corporate buildings is far thicker, heavier, and actually constructed differently. This applies to walls, floors, ceilings as well as glass windows.
The thicker and heavier the materials are, the less sound energy can vibrate them.
Suspension is often a hard concept to understand because most of us do not construct suspended rooms. But, all this means is that the room to be soundproofed is fully suspended by some means that doesn't transmit sound - generally rubber blocks are used because rubber is a terrible for sound transmission - this is why most luxury cars have rubber bushings (springs!) between the body of the car and the chassis - and in turn why they are very quiet inside as compared to a car 1/5th it's price - the extra rubber.
So lets put these concepts together:
for true sound isolation, you need to build the room out of heavy, dense materials, keep it air-tight, and suspend it in some way that does not transmit sound.
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Sound absorption, which is what most of us focus on, and that's treating the sound waves inside a given space so that it doesn't reflect on the walls, ceilings, and objects in the room if there are any. This is different than sound proofing, which is keeping the sound out, or in, depending on the purpose of the room.
Sound absorption, regardless of material, shape, brand, or what have you, works the very same way conception - the sound waves (energy) comes in, and the sound waves (energy) does not leave.
These devices do this by creating friction for the sound wave. Since energy is neither created nor destroyed, and only converted from one form to another, the sound waves are being converted, by friction, to heat.
That's it, that's all there is too it on a high level.
Now, different frequencies have different wavelengths and different amounts of energy for a given "volume". Bass frequencies at 120db have much more energy than high frequencies at 120db. Bass frequencies also have longer waves than high frequencies, measured in feet. Rooms, coincidentally, are also measured in feet. Generate a frequency that has the same wavelenth as your monitor-to-back wall dimension, and wow, that's going to hurt your ears real fast.
These two things together is why high frequencies are very easy to slow down, stop and convert to heat - less energy to start with and the waveforms are far smaller than absorbers structure whether it be bubbles, pockets, or strands.
Bass frequencies are the tougher ones for the very same reasons - and that's why foam-based bass aborbers tend to be far larger/thicker than foam-based wall treatments. Bass frequencies also tend to "collect" in a sense in corners, nooks, and behind equipment (your monitors!).
Treating bass frequencies requires a lot more than an egg crate stapled to the wall - by itself this does nothing.
This is why it's often suggested to construct recording rooms that aren't rectangular (or square) - because as the bass frequencies are reflected, the non-square dimensions will cause the sound energy to change direction - this helps prevents "dead spots" and "hot spots" where the waves cancel each other out or overlap - making it appear as if there is no, or twice the sound energy in that spot. It also weakens the sound wave by causing it to change direction a bit...
So, hopefully this "theory lesson" helped someone. How far you take it, is entirely up to you.
But at least we have access to knowledge, math and physics, whereas 25+ years ago it was all guesswork and copy-cat behavior.
Like I said, way back when I lined an entire studio with egg crates. I bought into the the BS too.