There's blade-water friction as well as water-water friction. The link you provided does not quantify their relative contributions. [edit: Actually, I'm not even sure how much blade-water friction there is, due to the boundary layer effect.]

There is also heating from cavitation bubbles collapsing in the water. If you look up cavitation heaters you will find a wacky rabbit hole with unsubstantiated claims about achieving heating efficiencies over unity. However, apparently there are some industrial processes that benefit from cavitation heating by getting even liquid heating and avoiding heating elements that can develop scaling. More info: http://hydrodynamics.com/cavitation-technology/scale-free-he...

That's a great idea! Did you find a good app? If so I'd like to hear. I just looked at a few free ones, and they didn't have enough control to be able to read off frequencies in the relevant range very well.

Reading your parent, I decided to (on Android) dl and try "Spectrum View" (there's an SpectrumView for remote control of a spectrum analyzer) and "Spectrogram", both from the same publisher. I would say both are useful and fun, particularly Spectrogram. Nice, barebone apps that are free and no ads, while apparently very well made. (no, I have no affiliation with the publisher).

Actually, I got so mesmerized during my bus trip that I missed my stop :) Fun facts: the brakes have a very sharp 5kHz resonance sound, while the beeper ("I'm closing the doors") were spread out over a few fairly sharp bands. Very fun! When I get home I'm gonna check what the freq contents of my infant daughters laugh and yelling looks like. :)

I use SpectrumView but I have not done an exhaustive search of the space.

(Blog author here) If you check out the spectrograms you'll see that there are a lot of different frequencies in the audio. I wasn't sure if the most powerful frequency would be the true rotation rate, so that's why I wanted to have a separate method to check. However, as TheLoneWolfing mentioned, it turns out that the most powerful frequency is indeed once per revolution.

I’m not sure if you saw it, but the last figure in the post is a spectrogram of the motor under load. I'd say the load results were the most interesting because they revealed differences in the speed control of the different models.

I'm the blog author. I'm glad you guys are getting a kick out of my write-up! I'll answer a few of the questions/comments here, and I'm happy to answer any other questions.

This is why we read Hacker News :)