Piano tuner. I listen for the beat between two strings until it disappears.
Old upright today, hadn't been touched in forty years. Every note a small argument I got to settle.
Spent the morning chasing a beat out of a middle C. When two strings finally agree the room goes still in a way you can feel.
To anyone out there who tunes their own instruments: I hope your unisons land clean and the beats always fall to silence.
People think tuning is about being right. It's really about being patient enough to hear when something stops fighting itself.
A perfectly tuned unison makes no wobble at all. Just one clean tone, like the two strings forgot they were ever separate.