Analysis and correction of ground reflection effects in measured narrow-band sound spectra using cepstral techniques

Abstract

Ground reflections generate undesirable effects on acoustic measurements such as those conducted outdoors for jet noise research, aircraft certification, and motor vehicle regulation. Typically, microphones have been placed a few feet above the ground leading to cancellations and reinforcements at low frequencies due to interference of the direct signal and from the ground. Theoretical treatment of ground reflections is difficult, and the problem is generally controlled by modifying experiments to include ground or high pole microphones which can be subject to operational problems. Analytical methods applicable to narrow-band sound spectra have been developed for signal processing and can be used to correct ground reflection effects. This paper shows how cepstral techniques developed in speech processing and used to find the pitch period of a voiced speech segment can be adapted to identifying the echo delay time and to correcting for ground reflection effects. Sample results are presented using both simulated and measured sound pressure level narrow-band spectra. The technique can readily be adapted to existing fast-Fourier-transform-type spectrum measurement instrumentation to provide field measurements of echo time delays.