A method and an apparatus for monitoring the production of blow molded plastic containers (20) includes a columnator (26) and sound detector (24) responsive to a defect sound of pressure fluid escaping from a container being formed in a mold cavity (14). The columnator is effective to filter the defect sound from ambient noise. An output signal generated by the sound detector is processed to generate a control signal used to reject a defective container. Preferably, the open end of the columnator is positioned between 20° and 150° from the preform loading station in a rotary platform (12) blow molding machine.