How to reduce the speed of a 3 speed fan

Hi, I bought a standard 3 speed 20" box fan. I am building a frame to hold a permanent filter in the back of the fan to make a DIY air purifier. The problem is the fan is quite noisy even at the slower first speed. It is not a vibration noise but just the air that flow through the fan box. I would like to lower the speed of the fan so it does less noise at night. I read a bit on the subject and it seems more complicated than I thought. I even searched here but I did find not anything but controllers for a 3 speed fan. I wanted to use a variac but it does not seem good about what I read so far. I have some knowledge in electronic and AC and I can solder or install electrical parts if I needed to. Is there a solution?

 

Thread moved to electrical.

 

A variac should work fine in that application. I use one to vary the speed of the fan in my shop.

I've also used a couple of these exact controls.
amazon-Motor-Control

 

Ok I tried a variac and it works fine. The variac is quite big so I might tried the control you suggested. Thank you.

 

It'll certainly run cool. Most variacs are rated for 500+ watts and your fan is probably like 45w or so.

 

It is very big and it is rated 20A - 3KVA

 

Well.... that will never heat up.

 

I was going to suggest a ceiling fan speed control . Is that what the link leads to ? They also used to make a ceiling fan speed control called the " De-Hummer " . This is one example I found online .

God bless
Wyr

 

I do not need to vary the speed anymore. With the filter installed, it is less noisy and the noise will not bother me at night. In fact, it may help me to sleep. I might dream I am camping hearing the sound of a cascade. Thank to all for the help. If I ever need to reduce the speed, I will have 2 options.