A process for producing a panel for acoustic treatment including an acoustically resistive porous layer, at least one alveolar structure, a reflective layer with openings, and a drainage plate having at least one furrow for delimiting at least one evacuation pipe when the drainage plate is flattened against the free surface of the reflective layer, includes: shaping, if necessary, the reflective layer according to its final geometry, affixing a demolding film on at least one portion of the free surface of the reflective layer, positioning on the demolding film at least one core whose cross-section is adapted to that of the evacuation pipe so that the at least one core covers the openings of the reflective layer, molding the drainage plate on the demolding film and the at least one core, demolding the drainage plate and removing the at least one core and the demolding film, and making the drainage plate integral with the free surface of the reflective layer.