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Nasal-cavity structure is stable in speech and varied across speakers, which potentially gives rise to speaker characteristics. Many studies have reported the acoustic contribution of the nasal cavity for nasal and nasalized sounds with velopharyngeal port opening. However, nasal-cavity resonance does emerge in non-nasal vowels through transvelar nasal coupling, which results in non-negligible modifications to non-nasal vowel spectra. In this study, nasal and oral output sounds were separately recorded during non-nasal utterances, and spectral analysis was conducted. The results indicate clear inter-speaker variability in two spectral measures below 2 kHz: frequency location of double-peaked first nasal-cavity resonance and inconsistent distribution of minor dips above the first resonance. It was also observed that nostril outputs modulate oral output signals to lower the first formant frequency of naturally produced non-low vowels, which also exhibited varied degrees across speakers.