Endpoint of a chemical mechanical polishing process is detected by monitoring acoustical emissions produced by contact between a polishing pad and a wafer. The acoustic information is resolved into a frequency spectrum utilizing techniques such as fast Fourier transformation. Characteristic changes in frequency spectra of the acoustic emissions reveal transition in polishing between different material layers. CMP endpoint indicated by a change in the acoustic frequency spectrum is validated by correlation with other sensed properties, including but not limited to time-based changes in amplitude of acoustic emissions, frictional coefficient, capacitance, and/or resistance. CMP endpoint revealed by a change in acoustic frequency spectrum can also be validated by comparison with characteristic frequency spectra obtained at endpoints or polishing transitions of prior operational runs.