The physical nature of weak body-conducted vocal-tract resonance signals called non-audible murmur (NAM) were investigated using numerical simulation and acoustic analysis of the NAM signals. Computational fluid dynamics simulation reveals that a weak vortex flow occurs in the supraglottal region when uttering NAM; a source of NAM is a turbulent noise source produced due to a vortex flow. Furthermore, computational acoustics simulation reveals that NAM signals attenuate 50 dB at 1 kHz consisting of 30-dB full-range attenuation due to air-to-body transmission loss and –10-dB/octave spectral decay due to a sound propagation loss within the body, which roughly equals to the measurement results.
Bibliographic reference. Otani, Makoto / Hirahara, Tatsuya (2010): "Physics of body-conducted silent speech - production, propagation and representation of non-audible murmur", In INTERSPEECH-2010, 949-952.