engines is disclosed various construction details which enhance the acoustic performance of the structure and prevent the formation of ice in the structure are developed a sound absorbing panel in the structure is spaced from an engine case to form one or more cavities which extend circumferentially about the engine application_number US 10500979 A description a gas turbine engine embodiment of the invention is illustrated in fig1 the principal sections of the engine include a compression section 10 a combustion section 12 and a turbine section 14 an annular flowpath 16 for working medium gases extends axially through the engine a stator structure including an engine outer case 18 bounds the working medium flowpath a rotor structure 20 is disposed inwardly of the outer case and has an axis of rotation a in the compression section the rotor structure includes a rotor disk not shown and a plurality of rotor blades 24 the rotor blades extend outwardly from the disk across the working medium flowpath into proximity with the outer case the outer case has a rub strip 26 facing the blades and contains a sound absorbing liner 28 the sound absorbing liner extends circumferentially about the interior of the outer case and faces the working medium flowpath the liner in combination with the case forms a sound absorbing structure 30 fig2 is a cross sectional view of a portion of the outer case 18 and associated structure such as the rotor blade 24 and the sound absorbing liner 28 the case is adapted by a circumferentially extending groove 32 to receive the liner the liner engages the case and includes one or more sound absorbing panels 34 each panel has a first wall such as outer wall 36 and a second wall such as inner wall 38 an.