The cepstrum: a viable method for the removal of ground reflections

Noise data from open air test facilities is contaminated by the effect of ground reflections causing cancellations and augmentations of the sound at certain frequencies. This problem is generally dealt with by placing microphones near the ground and subtracting 6 dB from the SPL spectrum. However the high frequency part of the spectrum obtained in this way suffers from problems due to temperature gradients near the ground on sunny days. The present empirical method used at Rolls-Royce for the estimation of one third octave band free field spectra involves the use of two microphones, one at 0·051 m above the ground (ground level) for frequencies up to 1 kHz and the other at 1·524 m above the ground for higher frequencies. Corrections are applied to the one third octave spectra from these low and high microphones to take account of intensity increases. These spectra are then "married" at 1 kHz to produce a single free field spectrum. At present there is no similar method to obtain narrow band free field spectra. In this paper cepstrum analysis is proposed as a satisfactory method to produce both narrow band and one third octave band free field spectra from high level microphones only. A series of tests has been carried out in an anechoic chamber facility in which ground reflections were simulated. The cepstrum technique was applied to this data to deduce the free field spectra. These compared very well with free field spectra obtained under anechoic conditions. Data is also included from open air tests on the Viper 11 engine and the spinning rig jet noise facility to show that the cepstrum technique is a viable way of removing ground reflections from high level microphone data.