If you want to do it the other way,
RPM = 2 X the lowest frequency peak * 60 for a 2 cylinder bike
RPM = 1X the lowest frequency peak * 60 for a 4 cylinder bike.
There are some settings on the program that will make it have better resolution at the low frequencies:
Leave the audio sampling rate set at the default: 48000 Hz
set the FFT size to 8192 bins (5.9Hz/bin)
set the decimation to 5 (0.18 Hz/[email protected]
set the window function to Hamming
Desired transform interval to 20mS (50Hz)
exponential smoothing factor to 0.3 (default)
Frequency axis scale = logarithmic
Waterfall off (dont go chasing waterfalls! )
Max-hold trace = OFF
when you run the program two finger zoom in so you only see from about 1 Hz to 50Hz or so.
It will show you the frequency on the bottom scale. If you touch the screen a cursor will show up you can put on the peak to read the frequency.
Its also useful for other audio applications, if you zoom it back out to 1 to 10,000 hz, you can see the frequency response of music that is playing, you could tune a guitar, hold it by a fan and see how fast the blades are spinning...