Acoustic design of the space is needed in order to make it "sound-neutral" and pleasant to the ear, depending on the application, as well as provide the solution to soundproof the given space. Acoustic design covers two aspects of sound improvement of the building, such as acoustic treatment and soundproofing (sound insulation). Let's clarify these two terms.
Acoustic treatment aims to improve the "inner" acoustic of the given space by eliminating unwanted reflections and resonances, reducing the reverberation time, flattering the frequency response, etc.
Soundproofing takes care of not spreading the sound from the given space and isolate the given area from external noises such as structural and airborne noises. Acoustic design is a process of designing the wall, floor and ceiling structures, as well as acoustic elements to be installed to achieve the desired soundproof rating and the desired acoustics.