Gender Perception After Raising Vowel Fundamental and Formant Frequencies: Considerations for Oral Resonance Research

Objectives: Voice feminization therapy for male-to-female transgender women typically targets increasing fundamental frequency (F₀). Increasing vowel formant frequencies (FFs) has also been proposed. To better understand formant conditions that shift listeners' perception of gender from male to "not-male," individual and combined vowel FFs were incrementally raised, whereas F₀ was held constant at a gender-ambiguous level.

Methods: The study used a prospective, experimental group design. Using a customized MATLAB program, vowels (/i/, /æ/, /ɑ/, and /u/) spoken by an adult were manipulated by isolating and increasing FF_1-3 until they matched those of a woman. Listeners heard randomized samples and perceptually categorized each as male, female, or gender neutral. The latter two choices were combined and labeled not-male.

Results: Chi-square analyses revealed that listeners rated samples as not-male for /ɑ/ and /æ/ with all three formants shifted or individual formants shifted at >60%. Individual analysis of vowels, formants, and shifted FF using Kruskal-Wallis revealed a statistical significance for vowels only.

Conclusions: Results suggest that voice was convincingly perceived as not-male, for vowels characterized by a high F₁ frequency, and that raising FFs for all four vowels increased (in varying amounts) the perception of voice femininity beyond that of raising F₀ alone.