Relative pitch representations and invariance to timbre

Information in speech and music is often conveyed through changes in fundamental frequency (f0), perceived by humans as “relative pitch”. Relative pitch judgments are complicated by two facts. First, sounds can simultaneously vary in timbre due to filtering imposed by a vocal tract or instrument body. Second, relative pitch can be extracted in two ways: by measuring changes in individual frequency components, or by estimating the f0 of different sounds and comparing the estimates. We examined the effects of timbral differences on relative pitch judgments, and whether such effects would depend on whether the judgments were based on frequency or f0. Listeners performed up/down and interval discrimination tasks with pairs of spoken vowels, instrument notes, or synthetic tones, synthesized to be either harmonic or inharmonic. Inharmonic sounds lack a well-defined f0, such that relative pitch must be extracted from changes in individual frequencies. Pitch judgments were biased by timbre differences, being less accurate when vowels/instruments were different than when they were the same. However, this bias was similar for harmonic and inharmonic sounds. In addition, the bias decreased for harmonic sounds when they were separated by delays, a setting known to cause reliance on f0 representations. This finding suggests that the representation of f0 is itself unbiased by timbre, but that pitch comparisons are influenced by timbre changes, the effect of which weakens over time. Relative pitch judgments are thus not invariant to timbre, even when timbral variation is naturalistic, and when such judgments are based on representations of f0. ### Competing Interest Statement The authors have declared no competing interest.