When the vibrations of both forks 'mix', new sum- and difference-
frequencies are generated.
The beat at (260 - 254) = 6 Hz is easy to hear, because it causes
the original two sounds to waver in a rapid 'wah-wah-wah' fading
pattern at 6 times a second.
The sum product at (254 + 260) = 514 Hz is harder to hear, with
everything else that's going on. But it's there.
