A 180-degree phase shift occurs crossing any isolated resonance

Evidence

The phase change of 180 degrees of the drive relative to the system as the frequency passes from below resonance to above resonance for a damped oscillator suggests that we revisit wave resonance to look for relative phase effects there. First, we consider a resonance of a driven string. On the one hand, we know that a string supports waves, but on the other, we know that any motion of the string is decomposable into a succession of individual oscillators of ever higher frequencies—the modes or standing waves. The corresponding mode is really a harmonic oscillator in disguise, so we already know the end of the story, that the 180-degree phase shift must occur in going from below to above any isolated resonance. — Eric J. Heller, Why You Hear What You Hear- An…, 1013

Categorization

• XREF: connects to: Changing a component’s phase by 180 degrees is equivalent to subtracting that partial — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 454