The audibility of direct sound as a key to measuring the clarity of speech and music

Abstract

Human ear/brain systems evolved to decode the direction, timbre, and distance of multiple sounds in a complex and noisy environment. In a reverberant space, this information is only available at the onset of a sound, before reflections overwhelm it. But since the time of Sabine acoustic science has concentrated on the decay of sound in a reverberant field, not on the audibility of the onset information. In addition, it is well known that the ability to separate multiple sound sources depends critically on pitch, but acoustic research studies only noise and impulses. This paper proposes that clarity requires the ability to separately analyze multiple sounds (the cocktail party effect) and that the cocktail party effect depends on phase relationships between harmonics of complex tones. These phase relationships are scrambled in predicable ways by reflections and reverberation. Well known properties of human hearing are used to develop both a physical model for the neurology of onset detection and an impulse response measure for localization and clarity in a reverberant field. A C language implementation of the physical model is capable of predicting and perhaps measuring the localizability of individual musicians in a binaural recording of live music.