The tonotopic theory of sensory dissonance was first proposed by Greenwood (1961), and extended in work by Plomp and Levelt (1965) and by Kameoka and Kuriyagawa (1969). This tonotopic account attributes the phenomenal experience of dissonance to interactions along the basilar membrane where maximum sensory dissonance tends to occur when pure tone components are separated by roughly 0.4 mm (Greenwood, 1991).
One repercussion of this theory is that increasing the number of spectral components in a sonority will always result in increased dissonance. However, subjective experience does not accord with this view. For example, when asked to judge dissonance, listeners will characterize the dyad C4/B4 (major seventh interval) as more dissonant than the tetrad C4/E4/G4/B4 (major-major seventh chord). All tonotopic models of dissonance predict that the tetrad will sound more dissonant than the dyad because the spectral content of the dyad is a proper subset of the tetrad.
The aim of this research is to explain why perceived dissonance can be reduced despite an increase in spectral energy.
One confounding factor in previous dissonance experiments is that dyads, triads, etc. were not directly compared. An experiment was carried out to investigate the possible effect of numerosity on dissonance. Numerosity was manipulated and the effect on dissonance was observed.
The results show that numerosity affects sensory dissonance. In effect, the euphoniousness of a sonority decreases with additional spectral content but increases with perceived numerosity. Both peripheral and cognitive phenomena are implicated in dissonance perception.
This view of dissonance is able to account for several previously unexplained phenomena, such as the pleasantness associated with the chorus effect, the preference for reverberated sounds, and the effect of good voice-leading in the reducing perceived dissonance.