I have lived in apartments and condos with walls so thin I could hear my neighbors daily conversations and other things. Things I didn’t want to hear. And I am sure they could even hear all of the noise we made on our side of the paper thin walls. I have never been so self conscious about my shower singing.
Now imagine if you were practicing singing, guitar, flute, or playing video games. Especially at odd hours. Yep, you would not make the neighbors happy. Unless of course you had your own mini soundproof studio.
The Danbocchi Soundproof Cardboard Studio ($1,800) is just 66.6-lbs and measures 3.6 x 3.6 x 5.4′ when assembled. Not cheap, but certainly better than trying to make your own system.