An air duct silencer can be used at the inlet of an air supply fan unit for a building or other large structure. The silencer has an exterior housing with exterior walls forming outer surfaces of the housing and interior walls arranged in the housing, connected to the exterior walls and defining an annular airflow passageway extending between an air inlet located at one end and a circular air outlet adapted for connection to the fan unit. The outlet defines a primary central axis extending through the center of the outlet and perpendicular to a plane in which the outlet lies. A central airflow defining member has a second central axis extending from an outer end thereof to an inner end and this axis is substantially coaxial with the primary central axis. This airflow defining member is substantially circular in transverse cross-section along its length and has a relatively wide bulb like end section at its outer end and a narrow end section at the inner end. There is sound absorbing material in the housing which is covered by the interior walls and also in the airflow defining member. The transverse cross-section of the airflow defining member causes the air velocity to be substantially higher at the air outlet than at the inlet.