Engine RPM for iPhone, version 2.4

Engine RPM is an iPhone app that listens to the sound of an engine and calculates and displays the RPM of that engine from that sound.  The RPM range is 400-90,000 RPM. Engine configurations include 2-cycle and 4-cycle  up to 12 cylinders.

You can find the app in the App Store by clicking here:

Here is what display looks like.  At the top in large digits is the calculated RPM.  The RPM value is calculated from the peaks in the autocorrelation graph (in green at the bottom).

Below that are the hint controls, which specify the RPM range.  By restricting the search to a specific range of values, it is more likely that the true engine sound can be found from amidst the background noise.  The hint is specified by a center value and a tolerance value in percent.  By swiping left or right in the blue bands below the hint, you can adjust both the center RPM value and the tolerance, which determines the minimum and maximum RPM values shown.

Instead of a fixed hint band, you can also use tracking mode, which can track RPM changes over the entire 400-90,000 RPM range.  In that mode the hint controls are replaced by a control that lets you force the tracked RPM to be close the actual RPM so that true tracking can begin.

Below the hint controls is a graph of the autocorrelation function.  This allows you to verify the reliability of the displayed RPM and it a guide to adjusting the RPM range.  The vertical yellow lines in the graph correspond to periods in the sound produced by the engine.  If those yellow lines line up well with the peaks in the graph, you can rely on the displayed RPM value.

The conversion from audio pitch to RPM depends on the engine configuration.  You can choose an engine configuration by specifying the number of cylinders (up to 12) and whether the engine is a 2-cycle or 4-cycle engine.  You can also specify a generic factor that can compensate for any gear ratio between the engine and the shaft whose RPM you wish to display.

Besides this view, there are setup/configuration pages.  Each page has it own context-senstive help, which gives more information on how to use the app.  There is also a detailed Operations manual that can be downloaded from here.

If you have any questions about this app, you can e-mail us at: info@tunelab-world.com

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