Abstract: A method measures the rotational speed of an engine turbocharger. A microphone picks up sound generated by the rotation of the turbocharger and a microphone signal containing signal components is supplied. Low-frequency signal components are filtered out from the microphone signal generated by engine operation including noise from valves, fans and gas flows. An electronic bandpass filter is tuned to the remaining signal component having the highest amplitude in the filtered microphone signal and the frequency of the remaining signal component having the highest amplitude is determined. With a knowledge of the turbocharger geometry, the value of the frequency of the remaining signal component having the highest amplitude is converted to the rotational speed of the turbocharger.