Two issues need to be distinguished:
1) increasing the insulation
2) Improving the sound of music in the room
These are 2 different goals that are achieved by different means. The terms "soundproofing" "damping" so often used,
they do not give clear information for which purpose.
For example, our company produces panels that are designed to improve the sound of music in a room, but we do not deal with the issue of acoustic insulation.
When it comes to insulation, some information:
It is not easy and cheap fun. Building the room indoors ensures good results. If we have a recording room and a control room, we are building 2 rooms.
When building walls and floors, we have the following measures at our disposal:
1) add mass: plasterboard, e.g. double. It is best not to repeat the thickness - plasterboard has its own resonance frequency and by using boards of different thicknesses we will get 2 less severe resonances.
2) separate the walls from each other so that they do not transmit vibrations. When building partitions, we can build 2 that do not touch each other.
A good intermediate solution is a wall of the so-called staggered stud wall.
3) add damping material - most often mineral wool
4) plug any gaps
In the case of amateur solutions, first try to plug all gaps: windows, doors. Double seals on the door. The crack under the door.
Place the studio monitors on a sponge so that they do not transfer dragging to the floor.
In a situation where there is a lot of bass at the ceiling and little at the listening position, the solution is to use bass traps. They will increase the amount of bass at the listening position, and will blunt any audible resonances which, in turn, are too loud at the listening position.
I recommend reading the following article (free):
http://akupro.pl/pdf/Produkcja%20Muzyki%20A%20Akustyka.pdf " target="_blank" rel="nofollow noopener ugc" class="postlink inline" title="" >
http://akupro.pl/pdf/Produkcja%20Muzyki%20A%20Akustyka.pdf