Yes, and your images were taken
from here. Pretty good write up on fan speed control too.
Most of the fans I have come across have been open collector types. A few years ago I played around with fan speed control to some extent. MOSFETS were my new best friends for a few weeks driving piles of fans. Some of what I did
can be found here. Since that was a few years ago the page is left open with a classic "I'll be back".
When it came to home computers I actually took the time and patience to instrument my workstation and measure accurately case temperatures. The images from that science experiment
can be found here. The images were used with text files explaining everything but the object was to observe case inlet and outlet temperatures as well as PSU inlet and outlet temps with pair of CPUs at idle and 100% loads. I actually looked at case volume, ambient temps, fan blade pitch, fan volumes of moving air as well as fan size. You name it and I looked at it.
This was when fan manufacturers were hyping home computer fans. I literally beat it to death. I think it is cool that you are getting into it. Like I mentioned I never had a problem using lower frequencies in the audio range but then too my hearing isn't what it once was.
The original experiment was more aimed at seeing just how much heat a large system emits into its environment.
Ron