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5,380 Posts
Discussion Starter #1 (Edited)
Last year I was playing with the idle on my bike, and used my cell phone to record the audio of the exhaust sound, played it back on an Oscilloscope, and measured the time interval to see what my idle RPM speed was.

Tedious and not very useful.

There is an ap you can get for your smart phone (android) called Spectroid that makes it very easy to see your engine rpms live in real time in living color!

Its a spectrum analyzer program, it captures sound and shows you the frequency components that are present, from 1 Hz to 10,000 Hz.

Without getting too technical, you run the program, hold it near one of your exhaust pipes, and look at the graph for the lowest frequency peak. Its showing you the frequency of the exhaust puff, which is the revs per second of one cylinder.

For example: my 650 idle speed should be 1200 RPM. Divide by 60 and you get 20 RPS (Hz). Since each cylinder fires on every other rev, you get one fire per rev coming out the exhaust pipes: put put put put put...

If you have two mufflers, each one will be half that frequency: 10Hz

So if you hold the phone mic right next to one exhaust, and your engine is running at 1200 rpm, you will see a peak on the spectrum graph at 10 Hz.

If you have a 4 cylinder road star at 1200 rpm you will see 20Hz, because cylinders are firing twice on each revolution. Road star idle might be a lower RPM - check your user manual.

Spectroid is a free ap, its easy to use, and you can hold your phone by your bike and adjust the idle speed while you watch it. Very cool!

5,380 Posts
Discussion Starter #2 (Edited)
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:

Under audio:
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)

under display
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...
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