Let's consider the five features in more detail, with an eye toward understanding why they might be so widespread throughout Western and non-Western music. A preference for conjunct melodic motion likely derives from the features of the auditory system that create a three-dimensional "auditory scene."3See Bregman 1990, Narmour 1990, and Vos and Troost 1989. Huron 2007 contains data about statistical properties of Western melodies, including conjunct melodic motion. An eardrum, in effect, is a one-dimensional system that can only move back and forth. From this meager input our brains create a vivid three-dimensional sonic space consisting of individually localized sounds: the phone ringing in front of you, the honk of a car horn outside your window, and the sound of a droning music theorist off to your right. To accomplish this dazzling transfiguration, the brain relies on a number of computational tricks, one of which is to group sonic events that are nearby in pitch.4Wessel (1979) suggests that the relevant variable might be the "spectral centroid," which in normal listening circumstances is highly correlated with pitch. Thus, a sequence like Figure 1.1.1a tends to be heard as belonging to a single sound source, whereas Figure 1.1.1b creates the impression of multiple sources. In this sense, small melodic steps are intrinsic to the very notion of "melody."
Acoustic consonance, or intrinsic sonic restfulness, is another very widespread musical feature.5Izumi (2000) and Hulse et al. (1995) indicate that nonhuman animals such as monkeys and birds can distinguish consonance from dissonance. Many styles make heavy use of consonant intervals such as the octave and perfect fifth, assigning them privileged melodic and harmonic roles. Scales containing a large number of consonant intervals are found in seemingly independent musical cultures, and there is evidence from infant psychology that the preference for consonance is innate.6Crowder, Reznick, and Rosenkrantz 1991, Zentner and Kagan 1996 and 1998, Trainor and Heinmiller 1998, Trainor, Tsang and Cheung 2002, and McDermott and Hauser 2005. At present, however, we do not know for certain how universal or innate this preference is. Fortunately, this issue is largely irrelevant in the present context: what matters is just that many listeners, both Western and non-Western, do have a fairly deep-seated preference for consonant sonorities.