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Gas engine tachometer phone app? Answered

Does anyone know of a tachometer app for a phone or PC that can estimate gas engine rpm by sound? One would enter options like 2-stroke or 4-stroke and number of cylinders, then hit the analyze sound button. I found one for RC helicopter rotors but couldn't find anything else.


Found it.


That's going to be an interesting little challenge. There are a lot of parts in a gas engine, spinning at many different speeds, so there's a lot of noise to sift through to find the frequencies you're interested in. Might be easiest if you took your recording at the exhaust; there will still be a lot of noise but the firing frequency should be the strongest.

My first-guess approach would probably be to convolution-filter down to the frequency range you're actually interested in, then (if that isn't clean enough to just count peaks per second) run that through FFT to get a frequency graph and report the strongest frequency or frequencies.

As far as whether such a thing already exists: I'd have to websearch, and you can do that as well as I can.

I was thinking of recording at the exhaust as well, should be the most consistent rhythm to record.

.  If the muffler isn't "too good" you should be able to count the pulses at the exhaust without all the fancy filtering.
.  It might be possible to replace the photodetector on an optical tach with a microphone, but, as orksecurity points out, you can probably find what you want (schematic and/or finished product) on the Web. I'd start by searching for "acoustic tachometer", "audio tachometer", or similar.