> You could just use a microphone and record it’s sound. You will see different frequency modes depending on the RPM. Yes, I believe this is what Phyphox does

> You could just use a microphone and record it’s sound. You will see different frequency modes depending on the RPM. Yes, I believe this is what Phyphox does for you. Most people have a microphone in their smartphone. But of course you could use a dedicated microphone and do a frequency analysis yourself instead.