Mobile Science Kit RPM Frequency Counter

I just came across an interesting use of a mobile phone for sciency things… a audio frequency analysis to count RPM.

Another great video by Matthias Wandel, or what I new him from for many years before I found his youtube channel,

The premise is you find the audible frequency spike, combine that with the knowledge of what is actually compressing the air to make the noise, and do some simple math.

Simple, really. All great things are simple.

I thought I would give it a try with my small battery Dremel tool. A bit of tape to compress the air and get it to make noise as it spun, and I get some numbers. They are less than the rated speed, but that is to be expected as the tape flap is pretty big and slows things down a lot.

Testing an app on my cell phone

Notice in the following images in their top right corner, there is a red and green labeled numbers. Those are the peak frequencies. Frequency of the loudest noise it hears, plus the ‘local’ or whatever is on the screen. So you can zoom in on something and get the number of just what you are looking at. Red is the ‘Peak’ and green is current.

The low speed setting on my Dremel with tape reads at 97.85Hz times the 60 seconds per minute gives me 5871 which is a bit less than the 6500 listed on the label for low speed. Battery drain, load, etc. I think this sounds about right.

Trying the high speed, I see 170.89 or 10,253 RPM. Much lower than the 14000 listed.

I remove the tape and tried again now that I have an idea of what to expect.

Low Speed, the first spike is at 103.36Hz which works out to be 6201RPM, pretty close to the 6500 claimed RPM.

Looking beyond that first spike, we can see the harmonic frequencies. The first big harmonic is a multiple of 3. If I rotate the shaft by hand, I feel 6 ‘clicks’ so this makes some sense, but I don’t yet fully understand this. Something to look into.

Here we see the the 2nd spike at 311.46hz.

Zooming way out, we see all sorts of spikes in frequencies, fairly evenly spaced out.

Lastly, lets look at that high speed setting without the tape.

The high speed is coming in at 208.1Hz or 12486 RPM. Much closer to the 14000 listed on the label.

A different App

Trying a different App I get slightly different numbers. They are all less, so it could be that I drained the battery a bit and it’s running slower. Or I could be seeing a difference in the maths under the hood. Not sure, more investigation will be required.

I see 102Hz

I tried kicking up the FFT ‘bucket’ size as recomended by Matthias in his video and don’t see much difference here.

Looking at the higher speed I see 204Hz.

With the higher FFT size I see 207Hz instead. I think the higher FFT sizes gives smaller ‘slices’ of the data so it can better find the peaks.

Things to try

I am wondering if basically covering the microphone on the phone with something like a stick and touching the stick to the frame of whatever you are trying to count RPM might be a way to ‘hear’ the frequency. Kinda like a stethoscope.


I think this is a really great technique. Thanks to Matthias for creating an exceptional video explaining what is going on.

This is certainly a great way to use a cell phone in creative ways.  Multiple apps are available to accomplish this cool trick.

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