Bass in Concert Halls Recent Studies on the Seat-Dip Effect

Abstract

The seat-dip effect is collectively referred to as the phenomena where the direct sound and its delayed copies starting after 5-7 ms interfere destructively leading to low frequency attenuation 8,15,16,18 . The delayed copies occur when the direct sound travels at grazing angles over the plane formed by the tops of the seat backs (or shoulders in occupied halls 18 ). The sound then bends between the seats, and reflects off the floor. At the receiver, this delayed copy joins the direct sound resulting in destructive interference being half-wavelength apart at certain frequency. In addition, delayed copies of the direct sound occur with diffraction from the tops of the seat backs. The main attenuation lies between 100-300 Hz, where its wavelength corresponds to four times the height of the seat backrest. The attenuation can extend up 1 kHz and it can be as deep as 15-20 dB.A rich bass and warmth are considered to be perceptually interchangeable terms 4,14 . In addition, bass is often clustered with fullness, depth and richness in listening tests using individual vocabulary profiling 11 . Hence, a loss of low frequencies in the direct sound caused by the seat-dip effect might be considered hazardous for the sound quality in concert halls. For example, Barron and Marshall observed that spatial impression is reduced if the early reflections also suffer from the seat-dip effect 3 .Most research on the seat-dip effect, for example related to the elimination of the effect, starts from the premise that such attenuation in the direct sound is detrimental to perception of sound quality in concert halls. Many methods focus on total elimination of the effect 1,7,8 or at least on shifting it to a lower frequency 15 -an approach often justified by the insensitivity of human hearing to low frequencies. However, for example intimacy seems to require pronounced bass below 100 Hz 12 meaning that this approach may not be beneficial after all. Furthermore, there is some evidence that if the later sound contains the low frequencies missing from the direct sound, the perception of bass might be enhanced 21 .