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Impedance Glottography

PostPosted: Wed Sep 11, 2013 6:49 pm
by Prasanth
Impedance Glottography is noninvasive measurement of the time variation of the degree of contact between the vibrating vocal folds during voice production. The aspect of contact being measured is called the vocal fold contact area (VFCA). To measure VFCA, the device used is called impedance glottograph. The device is also called electroglottograph or laryngograph. The principle of operation of device, the waveform obtained, an algorithm for determination of pitch period is discussed. EGG waveform for various voice qualities, drawbacks in EGG and various noises present are described. Concept of multichannel EGG and applications of electroglottography are discussed. Various commercial equipment available are compared alongwith the equipment developed by IIT Bombay.

Introduction Lungs, vocal tract, and larynx are the main organs related with generation of sound. The lungs are the source of airflow. The vocal tract is an accoustic enclosure and acts as acoustic filter shaping the spectrum of the generated sound. The source of most speech occurs in the larynx. There are two folds of the muscular bundle known as vocal chords inside the larynx. These vocal chords obstruct the airflow from the lungs and produce audible vibrations that make the speech. The mechanism of generation of sound is known as phonation. The vibrations consists of three phases namely contact phase, separation phase and open phase [1]. The impedance between the vocal chords is a function of tissue path length. When the vocal chords are open, the tissue path length is maximum and hence the impedance is maximum. When the vocal chords are closed, the tissue path length is minimum and hence the impedance is minimum. Impedance glottograph measures this impedance variation [1]. Impedance glottography is, thus, a non-invasive method of measuring vocal fold contact during voicing without affecting speech production. To measure VFCA, the device used is called impedance glottograph.

The device is also called electroglottograph (EGG) or laryngograph. The EGG measures the variation in impedance to a very small electrical current between the electrode pair placed across the neck as the area of vocal fold contact changes during voicing [2]. The method was first developed by Fabre (1957) and influential contributions are credited to Fourcin (1971) and Frokjaer-Jensen (1968). Commercially available devices are produced by Laryngograph Ltd., Glottal enterprises and F-J Electronics [3]. Development work for an electrogllotograph has been carried out at IIT Bombay as part of student projects by Bhagwat (1990), Sriram (1991), Thajudin (1994), Mahajan (1995), Chitnis (1998) and Patil (2000) [1].

Principle of Operation of Device

A high frequency electrical current of small voltage and amperage (physiologically safe) passes between two electrodes situated on the surface of the throat at the level of the thyroid cartilage. The basic configuration of the device is depicted in Fig.1. The electrodes are made of copper, silver or gold. They have the form of rings or rectangles covering an area ranging from 3 cm2 to 9 cm2. A third electrode is often used as a reference for impedance measurements. It may be designed as a separate electrode or as a ring electrode encircling each of the two other electrodes. The electrodes are usually mounted on a flexible band whose length may be adjusted to hold the electrodes in a steady position and to still allow the subject to comfortably speak and breathe naturally. Sometimes the electrodes are mounted on a small holder which is pressed against the throat by hand. A signal generator supplies the electrodes with an AC sinusoidal current of an alternating frequency usually ranging from 300 kHz to 5 MHz. This frequency is sufficiently high, so that the current capacitancively bypasses the less conductive skin layer without the use of additional conductive paste. The generator may produce constant voltage or constitute a constant current source. The supplied current is different for each particular device, but is not stronger than several milliamperes. The voltage between the electrodes depends on the tissue impedance but the typical value is about 0.5 V . In accordance a power dissipation of only several microwatts occurs at the level the subject's vocal folds