My ”mancave” is 16m2 in area. It has walls made of plasterboard. Ceiling is MDF panel and floor is concrete. Room has lots of furniture which already diffuses sound reflections.
Still some additional acoustics experiments is under work to make room acoustically better.
First try is with https://www.thomann.de/fi/the_t_akustik_diffusor_wave_eps_set.htm which is cheap for testing purposes.
