Engine RPM is an Android app that listens to the sound of an engine and calculates and displays the RPM of that engine from the sound made by that engine.
Here is what the main screen of Engine RPM looks like. The large digits at the top of the screen are the RPM reading. The numbers in green show the current search range used by the analysis. You can set this range as hint to the software by swiping through the two bluish bands below the minimum and maximum values.
Below that is the engine configuration. You can configure Engine RPM for 2-cycle or 4-cycle engines with up to 12 cylinders. The analysis software uses an advanced signal processing algorithm called the autocorrelation function, which is shown in graphical form at the bottom of the screen. This graph helps during setup and to confirm the reliability of the readings.
There are three other modes of operation in addition to this one, such as Tracking Mode where the search range is automatically adjusted around any good readings so that if the RPM changes, the search range will change accordingly.
There is also an Auto Ranging Mode that does not require any hints or restrictive search range for use in cases where the engine sound is very clear.
You can download a detailed Operations manual and the app has a built-in context-sensitive help system.
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