Distinct noises are emitted when the PET flakes impinge on the walls of the zigzag air classifier during air classification. On-line monitoring of air classification was investigated by measuring the noises. The relative energy of the noises that are emitted at the feed or top part of the air classifier was found to be increased with feed rate or top recovery throughput rate, respectively. The label grade of bottom product was correlated with the bottom yield and the air velocity inside the air classifier. From these results, the technical feasibility of the on-line monitoring of the PET flakes yield and grade was demonstrated by measuring the relative energy. This monitoring system for air classification was successfully tested at a PET recycling factory.