Abstract: In order that holes may be produced in a simple and safe manner in a plaster plate (1) to be used as sound adjuster in a room, holes (15) are punched out of the plate, e.g. in the shape of slits, by means of a cutting tool. Hereby the considerable dust and noise inconveniences caused by the hitherto known machining are avoided. By slits (15) that must lie close to each other, it is possible to punch in two stages, i.e. first with double distance and then in the space between. This prevents fracture of the plaster and tearing of any paper that may be on the surface. The cutting tool is provided with punches (7, 8), in a shape corresponding to the holes. Each punch is provided with an inclined blade (10) which forms an angle of the horizontal of 13.degree.. The cross sectional shape of the blade is V-shaped, with two blades (11) running along the sides of the punch.