His Master's thesis (Diplomarbeit) was entitled "Ein Funktionsmodell des Gehörs" (A functional model of hearing).
Terhardt's Habilitation thesis (1972) was entitled "Ein Funktionsschema der Tonhöhenwahrnehmung von Klängen" (A model of pitch perception in complex sounds).
In the first stage, the inner ear (cochlea and basilar membrane) performs a running spectral analysis of the incoming signal.
A cornerstone of Terhardt's approach is the idea that because spectral pitches are subjective, we must not jump to conclusions about the relationship between them and their physiological (physical) foundations in the ear and brain.
But Terhardt and his colleagues also demonstrated that regular harmonic complex tones in speech and music are slightly ambiguous in pitch, which may be the ultimate origin of octave equivalence in music and the perceived tonal affinity of successive tones at octave or fifth intervals.
[6] Terhardt's approach to acoustic communication [7] is based on Karl Popper's theory of three worlds [8] according to which reality is either physical, experiential (perception, sensations, emotions) or abstract (thoughts, knowledge, information, culture).