Abstract: A noise attenuation panel for an aircraft engine nacelle structure comprises a cellular core having a multiplicity of cells and a facing sheet having a multiplicity of holes which provide for gaseous fluid communication between the cellular core and the front face of the facing sheet. Each hole in the facing sheet has a bore diameter which increases along the bore in the direction of the rear face of the facing sheet throughout the bore or a part of the bore and which does not at any point of the bore decrease along the bore in the direction of the rear face of the facing sheet. Prior to assembly of the facing sheet and the core the holes are formed by electron beam drilling from an entrance face of the facing sheet through to an exit face and the facing sheet and the cellular core are assembled to form the panel in such disposition that the exit face forms the front face of the facing sheet and the entrance face forms the rear face of the facing sheet.