Throughout my research, I was working under the false assumption that 20 Hz sound waves are not perceptible to humans. This is why I targeted fans which operate at 20 Hz. It is so often repeated, that I did not look at the literature. I have since learned that sound can be perceived down as low as 4 Hz, however, the volume required to detect these frequencies increases significantly.
https://assets.publishing.service.gov.u ... curity.pdf
If I was to continue the research into noise reduction of the cantilever fans, a simple approach would be to reduce their operating frequency to 15 Hz or below.