ACOUSTIC COUPLING IN PHONATION AND ITS EFFECT ON ...

31recordings of normal vocal activities using a small-weight accelerometer on the skinsurface overlying the suprasternal notch. Many advantages of this approach havebeen exploited. The most attractive features are its non-invasive nature, miniaturesensor size, low sensitivity to the background noise, and reduced variability of therecorded signal compared with that of a microphone. The development of this continuousassessment has resulted in a relatively new and commercially available systemcalled the Ambulatory Phonation Monitor (APM) currently marketed by KayPEN-TAX [38]. However, the adoption of this technology into the clinical practice has beenslow due to technical limitations of current devices, the small number of parametersextracted from the signal, and the lack of statistically robust studies that evaluatethe true diagnostic capabilities of the ambulatory system.The ambulatory assessment of vocal function makes uses of measurements of skinvibration on soft tissue [36–38]. Analysis of speech sounds recorded via skin accelerationcontinues to be a topic of current research in applications such as monitoring ofsinging techniques [156,157], monitoring of vocal fatigue [158], evaluation of nasalization[159,160], improvement of voice intelligibility in noisy conditions [161], screeningof respiratory anomalies by means of pectoriloquy or egophony [162–166], and auditorybiofeedback for Parkinson disease patients [167].A number of challenges are inherent to this technique. Extraction clinical parametersrequires further understanding of underlying physical phenomena. Some potentialdifficulties that have been identified include the coupling between vocal tract andsubglottal system and the uncertainty of the tissue properties [35]. In addition, theaccelerometer specifications can vary when it is placed on soft tissue [168–170], andair-borne components can corrupt the signal [171]. These factors have limited theaccuracy of the clinical parameters extracted from the accelerometer signal. Therefore,the use of neck skin acceleration in voice monitoring requires not only a betterunderstanding of a number of related acoustical issues, but also the design of newinverse filtering approaches that take into account these multiple factors.