UNCERTAINTIES OF MEASUREMENTS IN ROOM ACOUSTICS

The influence of several sources of error on room acoustical measurements have been investigated in a collaboration between the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany and the Norges Tekniske Hogskole (NTH) in Trondheim, Norway. The measurement method is the technique using maximum-length sequences (MLS). At first, basic impulse response measurements of a one-third octave band filter and of a loudspeaker using quite different MLS-systems are compared and the differences in the results proved to be negligible. Then algorithms for the determination of room acoustical parameters are introduced and the possible influences on the results are discussed. Finally, intercomparison measurements of room acoustical parameters are described, which were performed in the PTB auditorium by participants from seven institutes all using their own equipment. The algorithms for the determination of room acoustical parameters used by different analyzers introduce systematic differences caused by differences in time-windowing and filtering, in reverse-time integration and in noise compensation. The overall standard deviations of the room acoustical parameters measured in the PTB auditorium by the different teams are about 5 to 10% for T-30, EDT (Early Decay Time), D (Definition) and TS (Centre time), and approximately 0.5 dB for C (Clarity) and G (Strength), all measured in the 1 and 4 kHz octave bands. Larger uncertainties were found in the 125 Hz octave band, and for the parameter LF (Lateral Fraction) in all frequency bands. The results of this investigation show that if measurements of room acoustical parameters are performed according to ISO/DIS 3382, the overall uncertainty is of the same magnitude or a little higher than subjectively perceivable changes in these parameters. However, the draft standard allows various procedures to be applied in the processing of impulse responses. To reduce the overall uncertainty the standard should, in future revisions, be more specific in the respect. This also applies to the specifications of the omnidirectional source.