Technique for measuring the sound of a moving source

Abstract

A measurement and analysis technique has been developed to determine the narrow-band spectra and the radiation patterns of sound emitted by a moving source. The sound is measured by a semicircular array of stationary microphones as the source passes by the array, and is recorded on a multichannel tape recorder. The sound pressure signals are digitized and the analysis of the data is completed on a digital computer. The analysis procedures make corrections for the effects associated with a moving sound source, such as the nonstationarity of the signal due to the time-dependent transmission path, the Doppler frequency shifts associated with each microphone, and the determination of accurate narrow-band spectra from a short segment of a rapidly varying signal. A relationship between the finite Fourier transform and the Doppler effect has been derived and serves as a basis for the Doppler correction procedure. The output from the analysis consists of the spectra of the sound emitted in the direction of each microphone and the radiation patterns for given frequency ranges or for weighted overall sound pressure levels. The technique was developed as part of a study of the mechanisms of tire noise generation using a single-wheel trailer. Results from the tire noise analyses are used as examples.