Motion shifts a wave's pitch — up as the source nears you, down as it leaves. The engine computes the classical acoustic shift live: f′ = f (v + vobs)/(v − vsrc), for source and observer moving along the line of sight through a medium of wave speed v. Rendered, not quoted.
source Doppler, Über das farbige Licht der Doppelsterne (1842), Abh. Königl. Böhm. Ges. Wiss., ser. 5, vol. 2 — Wikisource transcription
One medium, one line of sight. Sign convention: vobs > 0 when the observer moves toward the source; vsrc > 0 when the source moves toward the observer.
Source in the denominator, observer in the numerator — the medium is the frame both are measured against. That single split is the whole physics.
Motion shifts pitch — Doppler 1842. The formula is asymmetric in who moves, because the medium sets the frame: the moving-source and moving-observer cases are genuinely different physics, not the same shift relabelled.
This is the moving-frame face of the-wave-equation — the same f(x − ct) travelling solution, now watched from a seat that is itself in motion.
Live re-check of the load-bearing invariant — an approaching source must raise the pitch. Flips red the instant the TAMPER inverts the sign.
"The shift is symmetric — only relative motion matters." → False for sound. The medium picks a frame, so moving-source ≠ moving-observer. Symmetry is recovered only in vacuum (relativity).
"Approaching raises pitch because it gets louder." → Pitch, not loudness. Frequency shifts from wavefront spacing; amplitude is a separate effect.
Doppler (1842): double-star colours are the Doppler shift of their light. → Mostly wrong — stellar colour is thermal and radial velocities are tiny. But the principle held: Buys Ballot verified it for sound (1845); Huggins measured stellar line shifts (1868).
Planted void: swap the sign convention so the source term is +vsrc in the denominator — now an approaching source lowers the pitch. The Witness (7) catches it live.