Method for determining the rotation speed of a supercharger in an internal combustion engine

A method for determining the rotation speed of at least one supercharger which compresses the sucked air in an internal combustion engine (1); the control method, during the normal operation of the internal combustion engine (1), comprises the steps of detecting, by means of a microphone (25), the intensity of a sound signal generated by the rotation of the supercharger (14); determining a frequency content of the sound signal generated by the rotation of the supercharger (14); identifying the frequencies with maximum amplitude within the frequency content of the sound signal generated by the rotation of the supercharger (14); recognising the frequencies with maximum amplitude which are each other's multiples or submultiples; and determining the rotation speed of the supercharger (14) using exclusively the frequencies with maximum amplitude which are each other's multiples or submultiples.