RPM from Sound
If you are looking for a Windows or Pocket PC version of this program, we no longer support those platforms for RPM from Sound. Instead we recommend "Engine RPM", an iPhone app that does a much better job.
The sound that an engine makes is complex. The sound is generally described as a collection of harmonics of a single fundamental frequency, or pitch. If you know that pitch then you can figure out the RPM.
Engine RPM calculates the autocorrelation function of the sound produced by the engine. Based on the location of the peaks in the autocorrelation function, the pitch is determined and the RPM is displayed. For more information see Engine RPM in the App Store.