[boomskats]

Just the fact that your brain 'sums' those signals somewhere, to let you hear that interference frequency, has always fascinated me.

Do you have a link to the pmeumatic headphones study you mention?

[RobotToaster]

I did some digging, I couldn't find the paper but I found a reference to it on a wiki page, https://en.wikipedia.org/wiki/Robert_Monroe#Hemi-Sync