A soundproof enclosure comprising: a plurality of acoustical wall panels interconnectable each to another at opposing vertical side edges for enclosing an interior space, each of the acoustical wall panels having a bottom edge provided with a bottom flange adapted to be supported on a floor base unit or on a floor and a top edge provided with a top flange, wherein at least a portion of a front wall panel is movably connected at one vertical side edge between an open position and a closed position for providing and covering an access opening in which the portion forms at least one access door panel of rigid material; and a multilayer decoupling ceiling unit disposed on the top edges of the acoustical wall panels for forming the enclosure, the multilayer decoupling ceiling unit comprising: a plurality of ceiling layers having an intermediate core layer supported by the top flanges at the top edges of the acoustical wall panels, the ceiling layers are vertically spaced apart from one another; and a vertical wall member upwardly extending from an outer periphery end of the intermediate core layer, wherein the intermediate core layer of the multilayer decoupling ceiling unit which is disposed on the top flanges at the top edges of the acoustical wall panels, includes a decoupling element interposed between contacting surfaces of the intermediate core layer and the top flanges for isolating and decoupling the ceiling unit from the acoustical wall panels to provide the soundproof enclosure with a desired sound impedance, and to enhance the sound absorption and insulation characteristics of the soundproof enclosure 10); the soundproof enclosure being characterized in that the intermediate core layer includes a decoupling element comprising a layer of low acoustic impedance material or sound insulating material. a plurality of acoustical wall panels interconnectable each to another at opposing vertical side edges for enclosing an interior space, each of the acoustical wall panels having a bottom edge provided with a bottom flange adapted to be supported on a floor base unit or on a floor and a top edge provided with a top flange, wherein at least a portion of a front wall panel is movably connected at one vertical side edge between an open position and a closed position for providing and covering an access opening in which the portion forms at least one access door panel of rigid material; and a multilayer decoupling ceiling unit disposed on the top edges of the acoustical wall panels for forming the enclosure, the multilayer decoupling ceiling unit comprising: a plurality of ceiling layers having an intermediate core layer supported by the top flanges at the top edges of the acoustical wall panels, the ceiling layers are vertically spaced apart from one another; and a vertical wall member upwardly extending from an outer periphery end of the intermediate core layer, a plurality of ceiling layers having an intermediate core layer supported by the top flanges at the top edges of the acoustical wall panels, the ceiling layers are vertically spaced apart from one another; and a vertical wall member upwardly extending from an outer periphery end of the intermediate core layer, wherein the intermediate core layer of the multilayer decoupling ceiling unit which is disposed on the top flanges at the top edges of the acoustical wall panels, includes a decoupling element interposed between contacting surfaces of the intermediate core layer and the top flanges for isolating and decoupling the ceiling unit from the acoustical wall panels to provide the soundproof enclosure with a desired sound impedance, and to enhance the sound absorption and insulation characteristics of the soundproof enclosure 10); the soundproof enclosure being characterized in that the intermediate core layer includes a decoupling element comprising a layer of low acoustic impedance material or sound insulating material.