Time variances with MLS technique: Theoretical considerations concerning rotating microphones in building acoustics

For some time now, the technique of maximum length sequence (MLS) measurement has been successfully applied in building acoustics. Rotating microphones which are otherwise useful, especially for the determination of sound reduction, cannot be used here because the time invariance of the system would be violated. The measurement errors arising when rotating microphones are used in experiments were already investigated previously. However, in view of the future process of standardization of measuring techniques in building acoustics, theoretical understanding is necessary. Theoretical analyses are presented which exhibit the deviations of the measurements. Emphasis is placed on the differences arising when a synchronous or asynchronous relation between microphone round trip time and MLS period is chosen.