Inst. fur Tech. Akust. RWTH Aachen, Templergraben 55, 5100 Aachen, Germany
In halls with horizontally arranged seats the direct sound propagates nearly parallel to the audience, which causes a frequency-dependent attenuation. Although the additional attenuation is remarkable not much experimental data are available in the literature so far; most measurements were carried out in empty halls. The present study examines the effects on sound passing over audience and unoccupied seats at grazing incidence. Measurements were carried out in two halls and for scale model seat rows, using maximum-length sequences in combination with the Hadamard transform. To improve the dynamic range the spectrum of the maximum-length sequences was digitally pre-emphasized in some cases. The investigation showed that the audience causes a considerable, almost constant attenuation in the range between 500 Hz and 1.5 kHz with an exponential relationship between distance and sound preasure decrease. For higher frequencies the attenuation was dependent on the horizontal angle of incidence with regard to the seat rows. In some cases a significant decrease of sound preasure due to interference effects was observed. In the low-frequency range the well-known ``seat-dip'' effect occurred.