The acoustic RPM counter is based on the high resolution spectrum analysis on the signal sampled by the sound card installed in a personal computer.
To get a RPM count is sufficient a personal computer running Windows with a cheap microphone connected to a sound card (Sound Blaster or compatible) correctly installed and working.
The programs is based on a very simple concept: each running motor generates a sound proportional to its RPM count. The system calculates the spectrum for the signal and interpolates the higher spectrum peak in order to get a precise frequency reading. The system always "tracks" the highest peak and calculates the RPM from this reading.
The signal can be sampled at differents rates, useful to accomodate the differents RPM ranges in a wide variety of applications. For most of the applications the sampling at 8KHz or lower is sufficient. The user must set the scaling factor for the signal: in fact at each motor revolution the motor itself generates one or more "sounds" that represents the "fundamental" frequency that can be observed and examined. The scaling factor can be evaluated with differents methods: by direct measurement, using the nominal motor data or in other ways. This scaling factor remains constant at all the speeds and characterize the motor functionning: for example the Graupner 700BB Neodym motor has a scaling factor of 10 events/revolution. The user can also set the trigger level (if the sound level is under this threshold no measurement is performed), the number of averages (to get a more stable reading) and a frequency limits of a bandpass filter, useful for spurious harmonics erasing.