After a year of listening to people talk through face masks, it appeared to me that the sound is not merely muffled, but it has a very specific and recognizable tinge and texture to it.
It reminds me less of sound through a muffling pillow, and more of old speech recordings from the 1950s.
This is apparent whether you listen, today, to recorded speech on TV or radio or through the phone. Sometimes you hear it in person too.
I suspect that the sound is not just muffled or low-pass filtered, but that there is more happening. Perhaps it's a band-pass? A notch? Are there non-linear or time-variant effects from turbulence due to the interaction between plosives and the fabric?
Has anyone looked into this?