Late lateral energy fractions and the envelopment question in concert halls

In concert hall acoustics, spatial perception is a crucial element of the experience, yet several questions remain unresolved. In the 1960s, there was some work on what was then called ‘room impression’ caused by diffuse reverberation. The possible importance of early lateral reflections was proposed in 1967 by Marshall [Marshall AH. A note on the importance of room cross-section in concert halls. Journal of Sound and Vibration 1967;5:100–12] and until recently concern for the effect of early reflections has overshadowed study of the spatial effects linked to the later sound. Bradley and Soulodre [Bradley JS, Soulodre GA. The influence of late arriving energy on spatial impression. Journal of the Acoustical Society of America 1995;97:2263–71; Bradley JS, Soulodre GA. Objective measures of listener envelopment. Journal of the Acoustical Society of America 1995;98;2590–7] have now suggested that early reflections are predominantly responsible for creating a sense of source broadening [and apparent source width (ASW)], whereas a sense of envelopment, which had on occasions been linked to ASW, is almost solely produced by later lateral reflections. Bradley and Soulodre have proposed the late lateral energy level as a measure of listener envelopment (LEV). This paper considers some of the history of spatial perception in concert halls and reports on measured results made in 17 halls of the late lateral energy fraction (LLF) and the late lateral energy level (GLL). The spread of measured values of LLF turned out to be small and GLL was found to be predominantly determined by the total acoustic absorption of halls.