Hifi Fans Recommendations

So I just saws Zeo’s video on how to mod a regular room fan to make it quieter. I’m extremely interested in trying this out in my own room, but i was concerned whether or not anyone on here would know if undervolting/underpowering box fans causes them to break faster or if there is any concerns with AC fans versus DC powered fans. Zeos recommended that you use a Variac variable transformer would this be safe for fans that run AC power, DC power, or both?

For reference I’ll link the video that he posted because it is still fairly new: https://www.youtube.com/watch?v=BYlOGF64IfU

I don’t think you would have any problems with fans that run on AC, like the one zeos used, but DC motors operate differently. Here’s something i found that pretty much summarizes it:
Hope this helps.