In order to soundproof, fill the joist cavities in the wall with blown cellulose, and use a fiber mesh membrane to hold it in place.
Due to the complicated nature of installing blown cellulose for soundproofing, it is always best to contact a professional installer if possible.
Cellulose soundproofing sections off sound from different parts of a house so that noise does not bleed from room to room. It is also ideal for musicians because it limits the sound leaving the room and gives the sound in the room a richer tone.
Cellulose soundproofing allows for optimal noise reduction over other options, such as fiberglass, because of the material's density.
Blown cellulose is often more expensive than other solutions, and the labor to install it is sometimes costly.