A module with variable acoustic properties configured for covering boundaries, such as walls or ceilings of a room, for instance a multi-purpose room, in which it must be possible to change the acoustical properties of the room, such as the reverberation time, according to each specific use, where the module has a front face (4, 55, 61, 62, 70, 75, 87, 103) provided with one or more through-openings (6, 104) through which sound energy can enter an inner region of the module, and where the module in the inner region comprises a sound absorbing device (12; 38, 39, 95) in acoustic communication with the through-openings (6, 104) in the front face (4, 55, 61, 62, 70, 75, 87, 103) such that sound energy can pass from an exterior region or space outside the module to said sound absorbing device (12; 38, 39, 95), where the through-openings (6, 104) can be closed, such that sound energy cannot enter the sound absorbing device (12; 38, 39, 95) via the through-openings (6, 104) and opened, such that sound energy can enter the sound absorbing device (12; 38, 39, 95) via the through-openings (6, 104), where the front face (4, 55, 61, 62, 70, 75, 87, 103) is provided with an opening and closing mechanism comprising a closing member and a retaining mechanism, which retaining mechanism is configured such that it maintains the closing member in contact with substantively the entire circumference of the through-openings, when the module is in a closed state, wherein - said opening and closing mechanism is configured such that it never extends beyond the front surface of the front face (4, 55, 61, 62, 70, 75, 87, 103), that faces said exterior region or space, in any state of the opening and closing mechanism, whereby it becomes possible to attach a covering sheet on the outer surface of the front face (4, 55, 61, 62, 70, 75, 87, 103) or in the vicinity to the outer surface of the front face (4, 55, 61, 62, 70, 75, 87, 103) without the opening and closing mechanism interfering with the covering sheet in any state of the opening/closing mechanism; characterised in that - said opening and closing mechanism is configured to obtain a substantially acoustically tight closing of said through-openings (6, 104), where the mechanism comprises a first and a second closing region (106, 110), where the first closing region is provided on the front face (4, 55, 61, 62, 70, 75, 87, 103) and the second closing region is provided on the closing member, where the mechanism is configured such that it closes the through openings (6, 104), when the first and second closing regions (106, 110) are in contact with each other and opens the through openings (6, 104), when the first and second contact regions (106, 110) are not in contact with each other, where said substantially acoustically tight closing of said through-openings (6, 104) is obtained by the retaining means preventing the closing member from undergoing movement in a direction substantially perpendicular to the front face (4, 55, 61, 62, 70, 75, 87, 103). - said opening and closing mechanism is configured such that it never extends beyond the front surface of the front face (4, 55, 61, 62, 70, 75, 87, 103), that faces said exterior region or space, in any state of the opening and closing mechanism, whereby it becomes possible to attach a covering sheet on the outer surface of the front face (4, 55, 61, 62, 70, 75, 87, 103) or in the vicinity to the outer surface of the front face (4, 55, 61, 62, 70, 75, 87, 103) without the opening and closing mechanism interfering with the covering sheet in any state of the opening/closing mechanism; - said opening and closing mechanism is configured to obtain a substantially acoustically tight closing of said through-openings (6, 104), where the mechanism comprises a first and a second closing region (106, 110), where the first closing region is provided on the front face (4, 55, 61, 62, 70, 75, 87, 103) and the second closing region is provided on the closing member, where the mechanism is configured such that it closes the through openings (6, 104), when the first and second closing regions (106, 110) are in contact with each other and opens the through openings (6, 104), when the first and second contact regions (106, 110) are not in contact with each other, where said substantially acoustically tight closing of said through-openings (6, 104) is obtained by the retaining means preventing the closing member from undergoing movement in a direction substantially perpendicular to the front face (4, 55, 61, 62, 70, 75, 87, 103).