Room acoustics describes how sound waves interact with the physical surfaces of a recording or listening space. Hard, parallel surfaces create reflections; small rooms create standing waves (room modes) where certain bass frequencies pile up at specific positions; large rooms with little treatment produce noticeable reverberation.
In home recording, untreated rooms are the most common cause of recordings that sound amateur — not the microphone or interface. A $5,000 microphone in an untreated bedroom still picks up flutter echo, room modes, and reverb that is difficult to remove in post-production.
Basic acoustic treatment is surprisingly affordable and dramatically effective. Heavy curtains, bookshelves filled with books, a sofa, and thick rugs all absorb mid and high frequencies. Bass traps in corners reduce room modes in the low frequencies.
Dedicated acoustic panels (rockwool or rigid fiberglass in a frame) are more effective than soft furnishings and are commonly used in home studios. A treated room sounds noticeably drier and more controlled — recordings made in it translate better to other playback systems.