APPLICATION OF THE CEPSTRUM TO REMOVE ECHOES FROM ROTOR ACOUSTIC SPECTRA

The power cepstrum technique is shown to be an easily applied and useful tool in removing undesired acoustic reflections from measured rotor spectra. The technique is found to be most effective when the reflections are exact duplicates of the signal, that is when the echo transfer function is linear. In addition, the technique is best applied to a measured spectrum which is due entirely to the signal and its echoes, when no non-source related signals are present. For an exactly echoed signal, the cepstral echo spike at the echo delay time is linearly related to the echo’s relative amplitude in the time domain. A previously applied correction to the reconstructed spectrum, which required prior knowledge of the echo amplitude or path length is found to be incorrect. The technique can be applied without prior knowledge of the echo characteristics, although Information of the echo amplitude and delay time are helpful. Aliasing of the echo delay time occurs if the data record length is less than twice the echo delay time. An additional source of aliasing of the echo delay time in the cepstral domain is explained. This aliasing occurs when the source spectrum is not continuous, which often is the case for highly periodic signals.