David H. Griesinger
Lexicon, 100 Beaver St., Waltham, MA 02154
This paper proposes a set of measures of musical acoustics derived from a study of binaural hearing and speech perception. The measures are tested on analyzing data from unoccupied and occupied halls, data from several halls which include electronically variable acoustics capable of a wide range of adjustment, and data from a binaural synthesizer of hall acoustics. The new measures include the 500-ms reverberant level (L500), early (0--60 ms) reverb time (ERT), middle (0.1--0.5 s) reverb time (MRT), late (0.5+) reverb time (LRT), early binaural fraction (EBF), and middle binaural fraction (MBF). L500, a measure of reverberant level, is related to early decay time (EDT). Optimum L500 is found to be high in the range of musical fundamentals, and low in the region of maximum speech information. ERT is in part a measure of the localizability of the sound, which is related to its clarity. MRT is particularly significant whenever the decay curve is not a simple exponential. It is found that when MRT is much longer that ERT the sound can be both clear and reverberant at the same time. The binaural measures are found to be similar to IACC, but have advantages at frequencies below 300 Hz. Some implications of this research on optimum hall shape and surface treatment are discussed.