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 in-situ acoustic monitoring system includes an acoustic signal generator to emit acoustic signals and an acoustic signal sensor that receives acoustic signals reflected from the surface of the substrate. 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.