A Study of Acoustical Design of Under-Balcony Space in Auditoria

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The purpose of this study is to make clear the relation between the shape of balcony and acoustical properties under the overhang, and ultimately to derive an acoustic design criteria for balcony configuration is auditoria. On the assumption that a physical change of the sound field caused by building a balcony would be primarily a reduction of the energy of early reflections from upside, the directional characteristics of early reflections under the balcony is measured. The results show that the ratio of vertical component to total early reflection energy is very small under the overhang. Then psycho-acoustical experiments by using the measured data are perfoemed. The results show that auditory envelopment becomes stronger as the vertical component of reflected energy increases while lateral energy fraction is constant. Next. impulse responses are calculated in models of auditorium by means of computer simulation based on geometrical acoustics when the depth and the height of balcony-opening are varied. The evident effect of overhang can not be found in basic acoustical indices (SPL,C_<80>, TS and LE). This means that another index is necessary for evaluating the lack of auditory envelopment which is often perceived under the balcony. The directional characteristics of early reflections are examined. As a result, the relation between the configuration index of d/h and the ratio of vertical component to total early reflection energy ERv is obtained, where d and h are geometrically defined by the positions of sound source, receiving point and balcony edge. Applying the difference Iimen of ERv to this relation, it is found thet the maximum limit of d/h, in which the degree of auditory envelopment under the overhang is equivalent to that in main orchestra, is between 0.7 and 1.0. It can be thought that this is a design criteria of balcony in auditoria.