This paper discusses the importance of the spectral distribution of the energy in the sound to the dB(A) level of the usual passby test. A described technique obtains reproducible “real-time” spectra from the nonstationary sound field. Doppler effect and source frequency shift are quantified in relating these spectra to the tread pattern repetitions. Inverse square law fitting is viewed through the two-source-microphone relation which changes during the spectral window period. Spectra are shown for 6, 12, and 50 ft (1.83, 3.66, and 15.2 m) passby microphones and for a microphone carried on the truck.