Abstract: An acoustical muffler for a turbofan engine is disclosed. The muffler is utilized on the tail cone plug and comprises a plurality of circumferentially spaced slots oriented in parallel relationship to the grazing flow (the exhaust stream of core engine and fan discharge air). The slots have depending walls extending into the hollow central portion of the plug. Barriers at the upstream and downstream ends serve to define the volume of the plug. The relationship between the slots and volume is selected to tune the muffler to the desired frequency of peak attenuation. Radial extending barriers discretely mounted within the plug help to prevent airflow into and out of the slots.