You can read about it in different manufacturing sites, eg. Yaxi, Dekoni etc.

Just one example: https://dekoniaudio.com/articles/so-you-want-some-focal-measurements-with-the-dekoni-audio-ear-pads/

When interested in experiencing even subtile changes: Cut out rounds from coffee filters and put in your cans … you’ll be astonished how noticeable the differences are.