To summarize, a clinical investigation protocol should contain:
- A perceptual judgment of a trained speech-pathologist of the overall voice quality (good-reasonable-poor) and a judgment of the subset of the 8 perceptual semantic bipolar 7-point scales (deviant-normal, ugly-beautiful, breathy-not breathy, hypotonic-not hypotonic, low-high, deep-shrill, slow-quick, and dragging-brisk) based on read-aloud text;
- Acoustic signal typing based on a narrow-band spectrogram (100 ms analysis window) of a 2 s voice sample of a sustained /a/ at comfortable pitch and loudness level;
- Calculation by an experienced investigator of the following 6 acoustic parameters: median fundamental frequency, standard deviation of fundamental frequency, percentage of voiced, harmonics-to-noise ratio, glottal-to-noise excitation ratio, and band energy difference, on the same 2 s of a sustained /a/ at comfortable pitch and loudness;
- Measuring of maximum phonation time for a sustained /a/;
- Visual assessment by a clinician (ENT-specialist, radiologist, or speech-language pathologist) of the presence of a neoglottic bar during phonation, regurgitation of barium during phonation, and tonicity of the neoglottis during phonation in videofluoroscopy recordings and quantitative measurement of the minimal neoglottic distance at rest and during phonation, the surface area of the neoglottic bar at rest and during phonation, the prominence of the neoglottic bar at rest and during phonation and the increase of the maximal sub-neoglottic distance from rest to phonation in two representative digitalized images (rest and phonation) of videofluoroscopy recordings;
- Assessment of the visibility of the neoglottis, the visibility of the origin of the neoglottis, the amount of saliva interfering with neoglottic vibration, the shape of the neoglottis and the regularity of the vibration in digital high-speed imaging recordings (van As, et al. 1999).