Doppler Effect

The Doppler effect, described by Christian Doppler in 1842, occurs whenever there is relative motion between a sound source and an observer. When a source approaches, each successive wave crest is emitted from a position closer to the observer, compressing the wavelength and raising the perceived frequency. When the source recedes, wavelengths stretch and the perceived frequency drops.

In professional audio, the Doppler effect manifests in several practical scenarios. Rotating loudspeakers (Leslie cabinets) deliberately exploit Doppler shift to create vibrato and chorus effects — the approaching and receding sides of the rotating horn produce continuous pitch modulation. In measurement contexts, a moving loudspeaker during a frequency sweep introduces pitch errors that can corrupt the measured frequency response.

For large-displacement subwoofers, the Doppler effect creates intermodulation distortion. When a subwoofer cone reproduces both a low bass note (causing large excursion) and a higher frequency simultaneously, the cone's physical movement toward and away from the listener frequency-modulates the higher note. This Doppler distortion is audible as a "wavering" or "smearing" of mid-frequency content and is one reason why dedicated subwoofers separated from mid/high drivers produce cleaner sound.

In SonaVyx measurements, the Doppler effect is generally negligible because both source and microphone are stationary. However, the problem detector's modulation analysis can identify Doppler-like artifacts from rotating equipment (fans, HVAC) near measurement positions. The environmental noise monitor detects vehicle pass-by events where Doppler shift is prominent in the spectral signature, useful for transportation noise assessment.