This is a radial silencer for 40x40mm fans. I use one on my MKS Gen L case and was able to reduce the volume of the fan blowing into the case. In fact it does not work well on the hotend fan.
The silencer works by reflecting the sound waves coming from the fan. It acts as a kind of a sound maze. At the same time no airstream is beeing blocked by the design of the silencer.
Try it out and please post your makes and comment about your opinion whether it works for you or not! Thank you :)