The invention relates to a method of manufacturing a composite acoustic panel employed in an inlet passage of a gas turbine engine (1). The acoustic panel comprises a permeable face-layer (8), an impermeable backing sheet (9) and a sound absorbing layer (10) disposed therebetween. The method comprises a double polymerisation process for the face- layer and the remainder of the acoustic panel and finally a perforation step to perforate the face -layer according to a pre - determined perforation distribution (11.1, 11.2, 11.3, 11.4, 11.5).