How to add additional speeds to a box fan
I used the variac and got the noise level down from 74-84db to 45.  Pleasant white noise, vs Is that Industrial Music you're playing?

I researched various approaches, that fall into the basic camps of a 
variable resistor (like the sun court-variable speed, speed bully) and variable transformer.

The resistor ($20-$30) absorbs part of the power, thus generates heat, and the reviews of which almost always were super low, with burnout of the device and or fan, heat noise etc.

After trying to find the power requirements of the fan (which was not well documented anywhere), i figured it was about 1.5 AMP Max.  So I got a 3AMP Variac Variable Transformer ($65) which there are several that all look like rebrands of the same thing from China.   It is big, beautiful in an industrial manner, smooth dial, and works perfectly silently (no buzz hum), so I can choose whatever noise/airflow I need.   That said it's not calibrated correctly, so 100% on it's dial does not correspond to 100% when no variac is used, but it does go above 100% to make up for it.