I gave in and tried the felt mat under a typewriter. Apparently they reduce sound, but I did not think it would work for me since my only typewriters are ultra-portables.

Also, where I use a typewriter does not have a large space underneath, like an open desk or table, where the sound would echo and the 5mm of dense felt would help.

Still, it does create a sense of space, a cordoned off area when it is there, that I enjoy.