The Laryngograph is a device that monitors the vocal fold activity in the larynx without interfering with the processes of articulation. It does this by measuring the electrical impedance through the neck at the level of the larynx. To measure impedance (resistance to current flow) the Laryngograph has two guard-ring electrodes that are placed on the skin on either side of the larynx. A small high-frequency voltage is applied to the centre of one electrode and the circuit is completed using the centre of the other electrode. The use of a high frequency current, and the presence of earthed guard rings ensures that the current flows through the neck rather than across the skin.
The current flow has a (relatively!) large DC (direct current) component, which is not very informative, so the Laryngograph isolates the AC (alternating current) component at frequencies between about 20Hz and 2000Hz. These changes in current must be due to changes in the impedance of the neck at these frequencies. The only mechanism which causes such relatively high frequency changes in impedance is the vibration of the vocal folds in the larynx during phonation. The AC component is significantly amplified to a useful size, which may be recorded on a tape recorder or displayed on an oscilloscope.
Studies with the laryngograph and a simultaneous fibrescope imaging system have shown the relationship between the phases of the Laryngograph waveform (Lx) and the phases of vocal fold vibration. With the vocal folds apart, current flow is at a minimum. As the vocal folds snap together within a normal cycle, the current flow rises rapidly, indicating that it is degree of vocal fold contact that most affects the measured impedance. During the vocal fold closed phase, the current flow rises to a maximum, and as the vocal folds peel apart, the current slowly falls again.
Modal or typical voicing corresponds to a regular, non-breathy voice quality used in everyday communication situations. It is characterised by sharp vocal closures producing effective excitation of the supra-laryngeal resonances. It usually has regular closures giving a reliable indication of pitch. It usually has complete closures, along the full length of the folds, preventing air-escape and turbulence during the closed phase, and wide opening preventing turbulence in the open phase. The closed phase duration is often quite long, typically 60-70% of the total cycle. This long closed phase allows the vocal tract resonances to ring for a long time without damping.
Breathy voicing, associated with confidential or intimate communication situations, is characterised by air escape during voicing, usually by incomplete closure of the vocal folds. The vocal folds are connected to a single point at the front of the larynx, on the thyroid cartilage. However, at the back, they are connected to separate arytenoid cartilages. The arytenoids can rotate and swivel to draw the vocal folds across the air-way and to tension them. If the arytenoids fail to close off the air way properly, then a gap can occur at the back through which air can escape even in the vocal fold cycle closed phase. The presence of the gap creates a narrowing through which the air-flow can become turbulent, generating a noise signal, which in combination with a weaker closing pulse is indicative of breathy voice. A very short closed phase is also associated with breathy voice, perhaps 30-40% of the total voicing cycle duration.
Creaky voice, which is often found in phrase-final positions at the low end of a speaker's pitch range, is characterised by irregularity of cycle-to-cycle duration. Typically creaky voice occurs when the vocal folds are tightly approximated but weakly tensed. The tight approximation leads to very low air-flow, and very long closed phase durations, perhaps 80% or more. The slackness of the folds seems to disturb the normal mode of vibration of the folds, causing irregularities in cycle duration. One common style of creaky voice has alternating strong and weak cycles with long and short durations. This is called diplophonia, and perceptually appears as a kind of double pitch.
Falsetto voice is characterised by a very high fundamental frequency and a rather weak energy. The vocal folds are at a very high tension, and rather rigid, so that only the edges of the folds can actually vibrate. This leads to a much reduced mass of fold involved in vibrating and very short vibration cycles. The high tension also reduced the effectiveness of closure, and the closed phase and open phase are approximately equal.
| www.speechandhearing.net | © 2000 Mark Huckvale University College London |