Analysis and perception of the seat-dip effect in concert halls

One of the acoustic phenomena in concert halls is the so-called seat-dip effect, which is the selective low-frequency attenuation of the direct sound and reflections arriving at grazing angles to the audience. It has been considered detrimental to the concert hall's acoustics, and efforts to understand and model the effect have led to several design suggestions for its remedy. Perhaps somewhat surprisingly, the perceptual significance of the seat-dip effect has been very little studied. Most conclusions have been drawn from objective measurements with a few source-receiver positions in the concert halls, scale models, and simplified simulation models. This work extends the analysis and study of the perception of the seat-dip effect with a relatively large database of measured room acoustic responses in concert halls and with a purpose-built scale model. The concert halls are measured with a loudspeaker orchestra that approximates an orchestra in layout and directivity. In addition to single-number room acoustic parameters, time-frequency and spatiotemporal analyses of the cumulative sound energy are used to characterise the seat-dip effect. This approach reveals that the initial seat-dip attenuation 20 ms after the direct sound in existing concert halls is typically either narrow-band attenuation centred around 100 Hz or asymmetric wide-band attenuation centred around 200 Hz depending on the seat and floor design. The initial attenuation then levels off to varying degrees depending on the concert hall geometry, and spatially even distribution of the reflected incident sound energy at the listener position appears to be beneficial. Furthermore, seats with underpass enhance the low frequencies below the main attenuation frequency. Furthermore, listening tests with the auralised measurements in the concert halls were carried out to determine the audibility of the seat-dip effect and to study the perception of low frequencies. The results show that the single-number room acoustic parameters do not seem to capture the perception of low frequencies well. In addition, the results indicate that the seat-dip effect is not perceived in concert halls with sufficient reflected incident sound energy at the listener position. Consequently, to avoid hampering the low frequencies with the seat-dip effect, the main focus should be on designing adequate reflections, and the seating area design comes in second. Based on the results, it is recommended that seats with underpasses and flat or very moderately raked floors should be used.