Coupled oscillator modes smoothly trade character as one natural frequency passes the other

Evidence

The frequencies are shown by the curved solid lines, with the amount of red oscillator motion contributing more red to the lines, and likewise for the blue. The dashed lines show the individual oscillator test frequencies. The higher frequency pure mode (the top line) is seen to go smoothly from mostly red motion for a short red oscillator to mostly blue motion as the blue becomes shorter than the red. In the middle, when they have equal length, the color of both lines is purple (an equal admixture of red and blue). — Eric J. Heller, Why You Hear What You Hear- An…, 1036

Categorization

• XREF: Connects to the classic avoided-crossing / level-repulsion phenomenon shared across quantum mechanics and acoustics.