Excellent advice, Dave. The less you think about it, the better.
White noise is most useful. I like to be around water and hear the waves "roaring".
You know, it may not be possible to "cancel" out the ringing, as if it were a sine wave. Maybe nothing is in motion in the ear, which could be canceled with another wave 180 degrees out of phase.
Maybe it is the brain pretending to hear ringing. Who knows.
For now, it's foam ear plugs and ear muffs. That's a good idea about wearing something while mowing.