Auditory-Unit Formation

The previous chapters described how incoming acoustic information is distributed over a large array of auditory filters. This information generally does not originate from one sound source, but can come from multiple sound sources, such as at a cocktail party, in a bustling restaurant, or on a market place. In these circumstances, the auditory system must try to figure out which sound components come from which sound source, and reassemble the auditory information is such a way that the listener can meaningfully interpret the auditory environment in terms of auditory events such as footsteps, closing doors, human speech, musical melodies, etc., etc. This is called auditory scene analysis. In this chapter, the first stage of this process will be described, the formation of auditory units. An auditory unit is a sound to which the auditory system attributes one perceived moment of occurrence indicated by its beat. In spoken speech, the auditory units will in general be identified with the syllables that constitute the speech. It will be argued that their beats are formed by the clusters of onsets defined by the first consonants and the vowel of that syllable. In music, the separate tones of the melodies are the auditory units. Their beats are naturally formed by the onsets of the frequency components that make up the tones. Since every auditory unit has a beat, auditory units can be counted, which is why they are called units. This chapter describes the organizing principles operating in the process of auditory-unit formation. It is argued that the attributes of the auditory units such as their timbre, their loudness, and their pitch, emerge from the process of auditory unit formation, and are thus defined by the auditory information that contributes to the formation of these auditory units.