Acoustic Tube Endoscopy – Sonotubometry

In sonotubometry the acoustical features of the transmission link ear-nose are evaluated over time. For this an acoustical signal is emitted into the nose through a small loudspeaker and the system response is recorded at the ear by a miniature microphone. In the past a clear sinus signal with a frequency of 8 kHz was used as excitation signal and the variations of the energy received at the ear during activity of the eustachian tube were evaluated.

Our investigations showed, that the results are very unreliable if the transmission link is excited with only a single frequency. This is due to the fact, that the resonant frequencies of the acoustical system nose-ear vary considerably between subjects.

For this reason the introduction of broadband signals as excitation is a very important step to make sonotubometry much more reliable. Here the application of so-called perfect sequences as excitations signals is proposed. If repeated periodically they show an ideally flat power spectrum thus being the optimal excitation for system identification by means of NLMS.

To gain a reliable measure for detecting openings of the eustachian tube,  as indicated above, first a system identification is performed to gain an estimate of the impulse response of the measured system for every sampling instance, i.e. 48000 impulse reponses per second in this case. Second the power of every impulsereponse is calculated and plotted over time. The resulting curve is again a measure for the transmitted energy, this time, however, taking into account all frequencies in the measurable range.