Methods and apparatus to reduce instabilities and/or turbulence of an airflow of a HVAC system are described. The method may include positioning an airflow moderating zone in an airflow passage to reduce instabilities and/or turbulence in the airflow. Apparatuses to create the airflow moderating zone are also provided. The apparatus may be configured to have a plurality of openings and an airflow resistance structure so as to provide airflow resistance to instabilities and/or turbulence in the airflow. The airflow moderating zone may be, for example, a sheet-like material including a plurality of openings.