Repeatability of spatial measures using figure-of-eight microphones

The purpose of this study was to evaluate the measurement deviations of the spatial measures lateral energy fraction (LF) and late lateral energy level (GLL), obtained using figure-of-eight microphones. The following factors that may contribute to measurement deviations were evaluated: (1) microphone orientation, (2) spacing between figure-of-eight and omni-directional microphones, which varied from 64 to 152 mm, and (3) differences between individual microphones. Impulse response measurements were taken in a 100-seat lecture hall at four receiver positions using a total of eight figure-of-eight microphones. The average differences due to microphone orientation and spacing, were found to be relatively small: 0.20 ± 0.18 dB (4.3 ± 3.8%) for GLL and 0.02 ± 0.02 (2.0 ± 2.4 JNDs) for LF. (The just noticeable difference (JND) of LF has been established at 5%, however the GLL JND has not yet been established.) The largest differences were found when comparing the results between the individual microphones with differences on the order of 0.92 ± 0.73 dB (16 ± 13%) for GLL and 0.022 ± 0.018 (2.6 ± 1.9 JNDs) for LF. These larger differences are likely due to the variations in microphone sensitivity. Based on the measurement conditions used in this case study, it is possible to compare GLL and LF measurements with a given figure-8 microphone in order to evaluate relative differences, since the measurement deviations for a given microphone were found to be relatively low. However, it is difficult to compare the results of data obtained using different figure-8 microphones due to the measurement deviations of the microphones.