Methods for Measuring Voice Production
Speech production mechanisms arise from the functions of the internal organs of the human body that are mostly invisible. Therefore, better understanding of speech production processes
relies on the development of observation techniques. The lung functions in speech can be assessed by the tools for aerodynamic measurements, while examination of the larynx functions during speech requires special techniques for imaging and signal recording.
Monitoring Respiratory Functions
Respiratory functions during speech are examined by recording aerodynamic measurements of lung volume, airflow, and pressure. Changes in lung volume are monitored
with several types of plethysmography (e.g., whole-body, induction, and magnetic). The airflow from the mouth is measured with pneumotachography using a mask with pressure probes (differential-pressure anemometry) or thermal probes (hot-wire anemometry). Measurements
of the subglottal pressure require a tracheal puncture of a needle with a pressure sensor or a thin catheter-type pressure transducer inserted from
the nostril to the trachea via the cartilaginous part of the glottis.
Laryngeal Endoscopy
Imaging of the vocal folds during speech has been conducted with a combination of an endoscope and video camera. A solid-type endoscope is capable of observing vocal
fold vibration with stroboscopic or real-time digital imaging techniques during sustained phonation. The flexible endoscope is beneficial for video recording of glottal movements during speech with a fiber optic bundle inserted into the pharynx through the
nostril via the velopharyngeal port. Recently, an electronic type of flexible endoscope with a built-in image sensor has become available.
Glottography
Glottography is a technique to monitor vocal fold vibration as a waveform. Figure
8
shows two types of glottographic techniques. Photoglottography (PGG) detects
light intensity modulated by the glottis using an optical sensor. The sensor is placed on the neck and a flexible endoscope is used as a light source. The signal from the sensor corresponds to the glottal aperture size, reflecting vocal fold vibration and glottal
adduction–abduction movement. Electroglottography (EGG) records the contact of the left and right vocal fold edges during vibration. Highfrequency current is applied to a pair of surface electrodes
placed on the skin above the thyroid lamina, which detect a varying induction current that
corresponds to the change in vocal fold contact area.
Figure 8a,b: Glottographic methods.
(a)
PGG
with fiberscopy uses a photodetector attached near the cricothyroid cartilage in two locations: one
attachment for measuring vibrations, and two attachment for glottal gestures.
(b)
EGG
uses a pair of electrodes on the skin above the thyroid lamina to form a induction circuit to record electrical currents passed through the vocal-fold edges
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