  Several experiments have been performed to investigate the mechanical
vibrations associated with trachea and larynx when Italian vowels are emitted.
The mechanical measurements have been made by using two laser Doppler
vibrometers (based on the well-known not-invasive optical measurement
technique) coupled with the acoustic field measured by high-quality certified
microphones. The recorded signals are analyzed by using well-established
methods in time and frequency domains. The signals of the mechanical vibrations
along the trachea and the larynx are compared with those of the acoustic ones.
Focusing the attention of the signals' onsets, we can observe an upward
propagation of the mechanical vibrations for which it is possible to estimate a
delay between the traces. We observe that the mechanical oscillations at the
trachea start before the larynx and acoustic oscillations. Moreover these
tracheal oscillations are self-oscillations in time and are associated with the
pitch production, indicating a further hydrodynamic instability at trachea.
This leads to new insights in the mechanism controlling the pitch in the
speech.
