Amplitude variation of bone-conducted speech compared with air-conducted speech

We explore the phenomenon of amplitude variation of bone-conducted (BC) speech compared with that of air-conducted (AC) speech. During vocalization, in addition to the AC components emitted through the mouth, vibrations travel through the vocal tract wall and the skull bone before they arrive at the cochlea. A bone-conductive microphone placed on the talker's head can partly capture these vibrations and convert them to BC speech signals. The amplitude of this BC speech is influenced by the mechanical properties of the bone-conduction pathways. This influence is related to the vocal tract shape, which determines the resonances of the vocal tract filter. Referring to these resonances as formants of AC speech, we can describe the amplitude variation of BC speech with respect to the location of the formants of AC speech. In this work, the amplitude variation of BC speech of Japanese vowels, CV (consonant-vowel) syllables, and long utterances have been investigated in terms of the locations of the first two formants of AC speech. Our observation suggests that when the first formant is very low with a higher second formant, the relative amplitude of BC speech is amplified. On the other hand, a relatively high first formant and lower second formant of AC speech cause a reduction in the relative BC amplitude.