first note 440 Hz
second note 444 Hz
they throb 4 times a second
watch for this
Two notes a few hertz apart throb at exactly their difference: |f₁ − f₂| times a second.
Count the throbs out loud for one second. That count is the difference between the two frequencies — no other arithmetic involved. It is also exactly how a piano tuner works: slow the throbbing to nothing and the two notes are identical.
The full lesson behind this: The piano →