A chemical mechanical polishing apparatus includes a platen to support a polishing pad, a carrier head to hold a surface of a substrate against the polishing pad, a motor to generate relative motion between the platen and the carrier head so as to polish an overlying layer on the substrate, an in-situ acoustic monitoring system comprising an acoustic window having a top surface to contact the substrate, and a controller configured to detect a polishing endpoint based on received acoustic signals from the in-situ acoustic monitoring system.