The device for improving the sound of accordions and bandonions is characterized in that it is comprised of materials having a minimum hardness of 70 shores D, particularly of carbon fibre or other fibre compounds, or of composites (which may be coated for decorative purposes with materials having a lower or higher hardness), comprising platelets (1) arranged in the facing of the chest (4) by means of fixing elements (2). The chest (4) is fixed to an edge inside the case of the accordion (6) by screw elements (8). The sealing between the valves (12) and the plate on which they are bearing is improved by one or two lips (14), (15).