A chemical mechanical polishing apparatus includes a platen to support a polishing pad, a carrier head to 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, and a controller. The controller is configured to detect exposure of an underlying layer due to the polishing of the substrate based on measurements from the in-situ acoustic monitoring system. The in-situ acoustic monitoring system may detect exposure of an underlying layer based on comparison of the signal to prior measurements of acoustic signals generated by stress energy of test substrates.