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Damping of vocal fold oscillation at voice offset
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Damping of vocal fold oscillation at voice offset P.H. DeJonckere a,∗ , J. Lebacq b a b
Dept. Neurosciences, KULeuven; Federal Agency for Occupational Risks, Brussels, Belgium Neurosciences Institute, University of Louvain, B-1200 Brussels, Belgium
a r t i c l e
i n f o
Article history: Received 6 June 2016 Received in revised form 11 October 2016 Accepted 24 October 2016 Available online xxx Keywords: Damping High speed video EGG Photoglottography Flow glottography
a b s t r a c t Vocal folds show a damped oscillatory movement while abducting at the end of a vocal emission. The phenomenon can be observed with high-speed videoendoscopy and with different glottographic methods. It reflects important mechanical properties of the vocal oscillator, and cannot be voluntarily controlled. It could become a valuable clinical parameter, particularly in a medicolegal context, but its large variability in a same subject limits its use. First, possibilities and limitations of each recording method are reviewed. Second, the three main physiological factors accounting for the variability are analysed: (1) the timing dynamics of the expiratory pressure with respect to the opening of the glottis; (2) the speed at which vocal fold edges are abducted and glottal resistance drops, the combined effect of (1) and (2) determining the persisting transglottal flow, hence a persisting driving force; (3) the morphological change of the oscillator, whose lip-like shape becomes flattened depending on the degree of abduction. For clinical/medicolegal applications, additional research is required as to the recording protocol. A possible solution could be an entire recording with high speed transnasal videokymography of a standardised passage read by the subject, with a posteriori automatic extraction, by dedicated software, of all damping phases and computation of the average damping coefficient. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction At the end of a vocal emission, when the voicing is not interrupted by a laryngeal closure and the airway remains open, the vocal folds (VF) are abducting from the median phonatory position to the lateral respiratory position, the glottic mechanical impedance, and thus the subglottic pressure, suddenly drops [1,2]. With adequate instrumentation, it is possible to observe a damped oscillatory movement on each VF after the last contact phase of the two fold edges on the midline. This phenomenon results from combined internal (tissue viscosity) and external (air resistance) frictional forces, which cause a decrease of energy content of the oscillating system, reducing the amplitude of the oscillations as soon as the driving force disappears [1,3–6]. It is very brief and obviously occurs at a level beyond the scrutiny of traditional videolaryngostroboscopic examination. Nevertheless, the damping characteristics reflect important mechanical properties of the vocal oscillator, particularly related to the efficiency of the voice production. In concrete terms, the amplitude decrement from cycle to cycle reflects the energy input requested to maintain a steady
∗ Corresponding author. E-mail addresses: [email protected] (P.H. DeJonckere), [email protected] (J. Lebacq).
state oscillation, as in the steady state situation, the work input from the driving source (the lung pressure) exactly compensates the energy lost in damping. It means that several practical aspects appear directly linked:
(1) it has been shown that mechanical properties of the VF constitutionally differ between normal subjects [7]. Measurements of damping could help to clarify this concept, and to identify ‘robust’ (i.e. less fatigable) voices, and this are essential in the field of occupational voicing, or to investigate effects of e.g. training and ageing; (2) it is expected that, in several cases of organic VF pathology, the mechanical properties of the vocal oscillator become altered, due to physical changes in the layered structure of the VF [4,8]. Hence, damping characteristics could reflect the changes, resulting in e.g. a reduced vocal efficiency; (3) the damping of the vocal oscillator is an objective unconscious phenomenon that cannot be voluntarily controlled by the subject, thus escaping simulation. This aspect makes it particularly interesting in a medicolegal context for people claiming compensation for loss of vocal function in the case of injury or of occupational voice disease [9].
http://dx.doi.org/10.1016/j.bspc.2016.10.010 1746-8094/© 2016 Elsevier Ltd. All rights reserved.
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Fig. 1. VKG at four levels of the vibrating glottis obtained from high speed video. Healthy male subject. End of a/a:/at comfortable pitch (124 Hz) and loudness. Left halves of pictures corresponds to the more dorsal part of the vibrating glottis, and right halves to the more ventral part.
Fig. 2. Movements of the vocal fold edges, computed from the videokymograms of Fig. 1. The damping phase spans about 8 cycles. Upper traces correspond to the more dorsal part of the vibrating glottis, lower traces to the more ventral part.
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Fig. 3. VKG at four levels of the vibrating glottis obtained from high speed video. Left halves of pictures corresponds to the more dorsal part of the vibrating glottis, and right halves to the more ventral part. Healthy male subject. End of a somewhat breathy/a:/at comfortable pitch and loudness. Due to persistence of some airflow and slow vocal fold abduction, the damping phase spans at least 20 cycles, starting with a progressive shortening of the closed phase.
However, standardizing the recording methodology as well as avoiding biases appear to be a major issue. For current clinical and medicolegal applications, non-invasiveness is mandatory. The method should also interfere as little as possible with spontaneous phonation. The present study compares different techniques and approaches suitable for investigating and quantifying the damping phenomenon, discusses their advantages and drawbacks, and points out their respective pitfalls and limitations. Finally, directions for further research are suggested. 2. Methods The different adequate recording techniques currently available are: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Microphone Intraoral pressure transducer Flow glottograph (Rothenberg mask; FLOG) Pneumotachograph Electroglottograph (EGG) Photoglottograph (PGG) Mode 1 (tracheal transillumination) Photoglottograph (PGG) Mode 2 (pharyngeal illumination) Ultrasonic glottograph Videokymograph (VKG; single line scanner) High-speed videoendoscopic recorder
They are described in the following section. 3. Results Imaging techniques (high-speed video and VKG, or single line scanning) presently available make continuous digital recording possible. For VKG, the position of the scanned line needs to be controlled. Actually, only true high speed video with possibility to extract and display the kymograms (single line scans) of several
selected lines is suited (Figs. 1–4 ) [10]. Software is available for automatic computing of VKG-parameters [11–13]. Originally, high speed video and high speed videokymography required laryngoscopy with a rigid endoscope, able to deliver sufficiently powerful illumination (Xenon lamp 300 W) for frame rates above 2 kHz, with a resolution of more than 2000 × 2000 pixels. Rigid endoscopy is not invasive, but it is uncomfortable for the subject, and only sustained sounds can be investigated. Currently, improvements in light intensified digital imaging systems make it possible to acquire high-speed video recordings of the vocal folds through a flexible transnasal endoscope (diameter 3,4 mm) at rates of 4000 images per second or more [14,15]. The fiberscope can also be passed through a hole in a Rothenberg mask for simultaneous flowglottographic recording [16]. Image resolution is less than that obtained with rigid scopes, but it is sufficient if the tip of the scope is positioned close to the VF. Colour (implying a 4-times reduction of the maximum frame rate relative to monochrome for the same image quality) is not necessary in this scope. Compared to transoral rigid endoscopy, a flexible fiberscope makes it possible to observe a more natural laryngeal function over a wider range of speech and non-speech tasks. However, the sophistication and the high cost of the required material currently limit its use to research situations [15,16]. Methods measuring variations of acoustic pressure (microphone, intraoral pressure) or variations of airflow (flow glottograph, pneumotachograph) are patient-friendly and totally not invasive, but they actually measure the VF movements through an air buffer which has inertia and resonance characteristics, the latter depending on the vocal tract configuration [17]. To properly measure the damping of the VF oscillations, the signal needs to be corrected (inverse filtering); this is achieved by the flow glottography device (Rothenberg mask – a high speed wire screen pneumotachograph – combined with the MSIF2 inverse filtering system of Glottal Enterprises, Syracuse, NY, USA) (Fig. 5) [18]. An additional relevant information also provided by the flowglottogram is the end of vocal fold contact (closed plateau), clearly defined on the records. As a
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Fig. 4. Movements of the vocal fold edges, computed from the videokymograms of Fig. 3. Upper traces correspond to the more dorsal part of the vibrating glottis, lower traces to the more ventral part.
Fig. 5. Flow glottograph (Rothenberg’s mask): actually, a high speed wire screen pneumotachograph (time resolution of 0.5 ms).
drawback of the method, the mask to some extent limits speech movements. In photoglottography [19], the larynx is transilluminated and the amount of light through the glottis is continuously monitored. It is a direct measure of the glottal area, and thus effectively measures the true movements of the VF, both in mode 1 (photodiode in pharynx, light source on pretracheal skin) and in mode 2 (light source in pharynx, photodiode on pretracheal skin) [20]. Its time resolution is very high. But it cannot dissociate separate movements of the
two VF, as does high speed video. It is less invasive than imaging techniques, but more so than electroglottography. In practice, the magnitude of the photoglottographic signal is rather small, the skin acting like a strongly absorbing red/infrared filter. As a result, high power LEDs are inappropriate light sources because their emission spectrum is made of a limited number of peaks giving a global visual perception of white but with little red contents. Altogether, the amount of useful light emitted by LEDs is much less than that emitted by a heated tungsten filament, and a more appropriate light source is a tungsten filament light bulb driven by a constant ripple-free current source. For the sake of illustration, a small 15 W light bulb gives a much larger output signal than a 3000 lm high power white LED, without even requiring an IR anticaloric filter. As a receptor, large photodiodes should in principle give larger signals, as they collect more light, but in practice, they prove too large to be adequately positioned in the pharynx. We use a BP104 silicon photodiode (Vishay Precision Group, Malvern, PA, USA) glued onto a small laryngoscopic mirror (Nr. 3). This device can be used in combination with a Rothenberg mask; in this case, the handle of the mirror is introduced through the hermetically sealed hole intended for the hand piece of the mask. The current produced by the photodiode is processed by a currentto-voltage converter with a flat frequency response from DC to 2 kHz. The amplitude of the response is linear up to values ten times larger than the maximum signal due to the VF movements. When combined with flowglottography, it can validate the signal: Fig. 6 shows a quasi-perfect temporal correspondence between the damping on photoglottographic and flowglottographic signals.
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Fig. 6. Flowglottogram (upper trace in red, glottal area increasing upwards); PGG (lower trace in blue, glottal area increasing upwards) Healthy male subject. 130 Hz, 65 dBA. Horizontal scale in ms. The damping is similar. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Electroglottography [21] measures the transglottic electrical impedance using an AC current at a frequency above 100 kHz and monitors the changes in contact surface of the VF. The method is patient-friendly and does not interfere with vocalization. It allows precise phonetic tasks, with acoustic control. However, the sensitivity for detecting very small transglottic impedance variations (essential in this context) depends on the design of the electronic circuit. The original design of Fourcin and Abberton [22] has been superseded by more recent devices using a higher carrier-wave frequency, a more efficient feed-back control of the oscillator, multipole filters with sharper cut-off and flat bandwidth response (e.g. F-J Electronics, Denmark; Laryngograph, UK; Synchrovoice Research, USA; etc.). As a result, a better signal-to-noise ratio and a higher sensitivity are achieved with a larger bandwidth and better linearity [23]. As shown in Fig. 7, the EGG-signal can be as sensitive as the flowglottogram for detecting the smallest vocal fold oscillations, but, contrary to the flow signal, it fails to show the final phase of the damping, since there is no contact between the VF during this phase. The last ten sinusoidal EGG-cycles probably correspond to small (reduced amplitude) impedance fluctuations at the level of the ventral commissure. Imaging techniques, EGG, FLOG and PGG adequately identify the first part of damped oscillations, when the vocal folds still make contact on the midline. In this part, the damping phenomenon is characterized by a progressive reduction of the closed phase, possibly concomitant with a slight increase in amplitude of oscillation.
Ultrasonic glottography is still experimental and does not allow sufficient control of position/orientation of the probe.
4. Discussion Whichever method is used, an important issue is the apparent variability of the damping: this is clearly visible in Figs. 1 and 3, obtained in the same normal subject, during the same recording session. For phonation at comfortable pitch and loudness, three main physiological parameters seem to account for most of the observed variability in damping of the vocal fold oscillations at voice offset. Two of them are actually related to some persistence of the driving force: (1) the timing dynamics of the expiratory pressure (muscular/elastic, depending on lung volume) with respect to the opening of the glottis; (2) the speed at which the vocal fold edges are abducted and the glottal resistance drops; the combined effect of (1) and (2) actually determines the persisting transglottal flow, hence a persisting driving force; (3) the third important parameter is of morphological nature: ideally, the morphology of the oscillator should remain constant during the damping phase. Actually, from a certain degree of abduction, the morphology of the oscillating masses changes considerably, as can be seen in Fig. 8. In the abducted position, the lip-like shape of the VF disappears and the VF is ‘flattened’ laterally. The extent of this non-linear change depends on the degree of abduction. Creating an abrupt airflow interruption at a subglottic level is practically ruled out in vivo. The airflow can be interrupted
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Fig. 7. Flowglottogram (lower trace in red, glottal area increasing upwards) and EGG (upper trace in blue, glottal closing upwards). Healthy male subject, 125 Hz, 65 dB. Horizontal scale in ms. The EGG-signal is as sensitive as the flowglottogram for detecting the smallest vocal fold oscillations, but fails to show the final phase of the damping. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Frontal RX-laryngography with contrast. Left: VF in phonatory position; Right: VF in abducted (inspiratory) position. In the abducted position, the lip-like shape of the VF disappears and the VF is ‘flattened’ laterally.
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Fig. 9. Flowglottogram (upper trace, in red, with large oscillations, glottal area increasing upwards) and EGG (lower trace in blue, glottal closing upwards). Horizontal scale in ms. Three repetitions of/uk/while lowering the rate. Between the first two repetitions, no interruption of vocal fold oscillation occurs, while it just occurs between the second and the third repetition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
downstream, either artificially by an inflatable balloon within the pneumotachograph, or physiologically by linguo-palatal occlusion (e.g. in/uk/). In the case of an artificial abrupt interruption during a/a:/, some transglottal airflow (and vocal fold vibration) can persist by limited inflation of the upper vocal tract, upstream of the occlusion. In the case of a/uk/(or another denasal vowel followed by a voiceless occlusive), the vocal fold vibration usually stops (in a variable way) before the articulatory movement of the/k/. When the occlusive is voiced (/ug/) the vocal fold vibration persists after the linguo-palatal occlusion, but again, as for an artificial airflow interruption, in a very variable way. A persisting transglottal flow after the last VF contact clearly slows the damping, as can be seen by comparing Figs. 1 and 2 with Figs. 3 and 4, obtained from the same (healthy) subject at a few seconds interval. In fact, this illustrates the main problem and limitation encountered with all imaging methods, as well as with PGG. The search for a critical repetition rate of a denasal vowel followed by a voiceless occlusive was attempted: this means the rate (e.g. 6,2 s−1 ) at which the oscillation is actually interrupted (Fig. 9). In such a task, repetition contributes to standardizing several parameters. This protocol could be of some interest, but it requires a trained vocalist, and seems unsuitable for clinical or medicolegal application. All observations so far deal with phonation at comfortable pitch and loudness. Changes in intensity and fundamental frequency also influence the damping characteristics, but this effect seems to be far smaller than that of the other considered factors. Recently, damping
has also been observed in inspiratory phonation [24]: the characteristics are comparable with those of expiratory voicing. Yet another possibility for observing the damping phenomenon would be an external mechanical impulsion on the larynx, without any phonation. However it would require an adequate positioning of the VF at the moment of impulsion, and any recording would be perturbed by the mechanical artifact. Furthermore, an external impulsion will elicit a laryngeal reflex (with a delay in the order of 10–40 ms [25,26]) which will interfere with the damping. For these reasons, this procedure is very unlikely to produce a reliable result. Our limited experience with organic pathology mainly shows irregularity during the damping phase, with chaotic patterns (Fig. 10). This can probably be attributed to the fact that the absence of midline collision and the progressively reduced vibrating mass act as sensitizing factors for asynchronisms due to superficial lesions, which are mostly unilateral or asymmetrical. This particularity could be of some use in objectifying micro-organic vocal fold pathology. On this particular point, an important advantage of high-speed videoendoscopy lies in the fact that damping can be analyzed on each VF separately.
5. Conclusion The damping phenomenon is probably an important issue in voice pathophysiology and perhaps a valuable clinical parameter for various applications. Several methods make it possible to record the damping phase of the vocal folds at the end of a vocal emis-
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Fig. 10. Photoglottogram (glottal area as a function of time; glottal area increases downwards) at voice offset in a female patient with moderate hoarseness and a slight chronic laryngitis. The damping phase has an irregular pattern.
sion. Currently available non invasive techniques (as FLOG and PGG) appear to be well suited. High speed video and high speed videokymography (together with the software for image processing and analysis) have an obvious superiority but are not currently widespread due to high cost. Further, most recent developments (via a transnasal flexible scope) have made it much more comfortable for the subject/patient. However the main problem for a reliable measure of the damping is not the technique, but the variability of damping characteristics that are related to the laryngeal and respiratory behaviour at the end of the vocal emission. Defining a simple protocol for standardising the vocal emission seems unsuccessful. Hence the only solution seems the entire recording, using high speed video, of a standardised passage read by the subject, with a posteriori automatic extraction by dedicated software of all damping phases and computation of the average damping coefficient. The question of course remains whether the provided information makes this sophisticated approach worthwile for the clinician. References [1] P.H. Dejonckere, J. Lebacq, Mechanical study of the damping of the phonatory oscillator, Arch. Int. Physiol. Biochim. 88 (1980) 31–32. [2] P.H. Dejonckere, Damping biomechanics of vocal fold oscillation, in: J. Gauffin, B. Hammarberg (Eds.), Vocal Fold Physiology: Acoustic, Perceptual and Physiological Aspects of Voice Mechanisms, Singular Publishing Group, Inc., San Diego, 1991, pp. 105–111. [3] I.R. Titze, The physics of small-amplitude oscillation of the vocal folds, J. Acoust. Soc. Am. 83 (1988) 1536–1552. [4] P. Dejonckere, J. Lebacq, Relation de phase entre la dynamique de la pression sous-glottique et le mouvement oscillatoire des cordes vocales: II. Attaque vocalique et fin d’émission, Arch. Physiol. Biochem. 88 (1980) 343–355. [5] P.H. Dejonckere, Lebacq J. Damping, coefficient of oscillating vocal folds in relation with pitch perturbations, Speech Commun. 3 (1984) 89–92. [6] B.K. Finkelhor, I.R. Titze, P.L. Durham, The effect of viscosity changes in the vocal folds on the range of oscillation, J. Voice 6 (1988) 320–325.
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Damping of vocal fold oscillation at voice offset P.H. DeJonckere a,∗ , J. Lebacq b a b
Dept. Neurosciences, KULeuven; Federal Agency for Occupational Risks, Brussels, Belgium Neurosciences Institute, University of Louvain, B-1200 Brussels, Belgium
a r t i c l e
i n f o
Article history: Received 6 June 2016 Received in revised form 11 October 2016 Accepted 24 October 2016 Available online xxx Keywords: Damping High speed video EGG Photoglottography Flow glottography
a b s t r a c t Vocal folds show a damped oscillatory movement while abducting at the end of a vocal emission. The phenomenon can be observed with high-speed videoendoscopy and with different glottographic methods. It reflects important mechanical properties of the vocal oscillator, and cannot be voluntarily controlled. It could become a valuable clinical parameter, particularly in a medicolegal context, but its large variability in a same subject limits its use. First, possibilities and limitations of each recording method are reviewed. Second, the three main physiological factors accounting for the variability are analysed: (1) the timing dynamics of the expiratory pressure with respect to the opening of the glottis; (2) the speed at which vocal fold edges are abducted and glottal resistance drops, the combined effect of (1) and (2) determining the persisting transglottal flow, hence a persisting driving force; (3) the morphological change of the oscillator, whose lip-like shape becomes flattened depending on the degree of abduction. For clinical/medicolegal applications, additional research is required as to the recording protocol. A possible solution could be an entire recording with high speed transnasal videokymography of a standardised passage read by the subject, with a posteriori automatic extraction, by dedicated software, of all damping phases and computation of the average damping coefficient. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction At the end of a vocal emission, when the voicing is not interrupted by a laryngeal closure and the airway remains open, the vocal folds (VF) are abducting from the median phonatory position to the lateral respiratory position, the glottic mechanical impedance, and thus the subglottic pressure, suddenly drops [1,2]. With adequate instrumentation, it is possible to observe a damped oscillatory movement on each VF after the last contact phase of the two fold edges on the midline. This phenomenon results from combined internal (tissue viscosity) and external (air resistance) frictional forces, which cause a decrease of energy content of the oscillating system, reducing the amplitude of the oscillations as soon as the driving force disappears [1,3–6]. It is very brief and obviously occurs at a level beyond the scrutiny of traditional videolaryngostroboscopic examination. Nevertheless, the damping characteristics reflect important mechanical properties of the vocal oscillator, particularly related to the efficiency of the voice production. In concrete terms, the amplitude decrement from cycle to cycle reflects the energy input requested to maintain a steady
∗ Corresponding author. E-mail addresses: [email protected] (P.H. DeJonckere), [email protected] (J. Lebacq).
state oscillation, as in the steady state situation, the work input from the driving source (the lung pressure) exactly compensates the energy lost in damping. It means that several practical aspects appear directly linked:
(1) it has been shown that mechanical properties of the VF constitutionally differ between normal subjects [7]. Measurements of damping could help to clarify this concept, and to identify ‘robust’ (i.e. less fatigable) voices, and this are essential in the field of occupational voicing, or to investigate effects of e.g. training and ageing; (2) it is expected that, in several cases of organic VF pathology, the mechanical properties of the vocal oscillator become altered, due to physical changes in the layered structure of the VF [4,8]. Hence, damping characteristics could reflect the changes, resulting in e.g. a reduced vocal efficiency; (3) the damping of the vocal oscillator is an objective unconscious phenomenon that cannot be voluntarily controlled by the subject, thus escaping simulation. This aspect makes it particularly interesting in a medicolegal context for people claiming compensation for loss of vocal function in the case of injury or of occupational voice disease [9].
http://dx.doi.org/10.1016/j.bspc.2016.10.010 1746-8094/© 2016 Elsevier Ltd. All rights reserved.
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Fig. 1. VKG at four levels of the vibrating glottis obtained from high speed video. Healthy male subject. End of a/a:/at comfortable pitch (124 Hz) and loudness. Left halves of pictures corresponds to the more dorsal part of the vibrating glottis, and right halves to the more ventral part.
Fig. 2. Movements of the vocal fold edges, computed from the videokymograms of Fig. 1. The damping phase spans about 8 cycles. Upper traces correspond to the more dorsal part of the vibrating glottis, lower traces to the more ventral part.
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Fig. 3. VKG at four levels of the vibrating glottis obtained from high speed video. Left halves of pictures corresponds to the more dorsal part of the vibrating glottis, and right halves to the more ventral part. Healthy male subject. End of a somewhat breathy/a:/at comfortable pitch and loudness. Due to persistence of some airflow and slow vocal fold abduction, the damping phase spans at least 20 cycles, starting with a progressive shortening of the closed phase.
However, standardizing the recording methodology as well as avoiding biases appear to be a major issue. For current clinical and medicolegal applications, non-invasiveness is mandatory. The method should also interfere as little as possible with spontaneous phonation. The present study compares different techniques and approaches suitable for investigating and quantifying the damping phenomenon, discusses their advantages and drawbacks, and points out their respective pitfalls and limitations. Finally, directions for further research are suggested. 2. Methods The different adequate recording techniques currently available are: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Microphone Intraoral pressure transducer Flow glottograph (Rothenberg mask; FLOG) Pneumotachograph Electroglottograph (EGG) Photoglottograph (PGG) Mode 1 (tracheal transillumination) Photoglottograph (PGG) Mode 2 (pharyngeal illumination) Ultrasonic glottograph Videokymograph (VKG; single line scanner) High-speed videoendoscopic recorder
They are described in the following section. 3. Results Imaging techniques (high-speed video and VKG, or single line scanning) presently available make continuous digital recording possible. For VKG, the position of the scanned line needs to be controlled. Actually, only true high speed video with possibility to extract and display the kymograms (single line scans) of several
selected lines is suited (Figs. 1–4 ) [10]. Software is available for automatic computing of VKG-parameters [11–13]. Originally, high speed video and high speed videokymography required laryngoscopy with a rigid endoscope, able to deliver sufficiently powerful illumination (Xenon lamp 300 W) for frame rates above 2 kHz, with a resolution of more than 2000 × 2000 pixels. Rigid endoscopy is not invasive, but it is uncomfortable for the subject, and only sustained sounds can be investigated. Currently, improvements in light intensified digital imaging systems make it possible to acquire high-speed video recordings of the vocal folds through a flexible transnasal endoscope (diameter 3,4 mm) at rates of 4000 images per second or more [14,15]. The fiberscope can also be passed through a hole in a Rothenberg mask for simultaneous flowglottographic recording [16]. Image resolution is less than that obtained with rigid scopes, but it is sufficient if the tip of the scope is positioned close to the VF. Colour (implying a 4-times reduction of the maximum frame rate relative to monochrome for the same image quality) is not necessary in this scope. Compared to transoral rigid endoscopy, a flexible fiberscope makes it possible to observe a more natural laryngeal function over a wider range of speech and non-speech tasks. However, the sophistication and the high cost of the required material currently limit its use to research situations [15,16]. Methods measuring variations of acoustic pressure (microphone, intraoral pressure) or variations of airflow (flow glottograph, pneumotachograph) are patient-friendly and totally not invasive, but they actually measure the VF movements through an air buffer which has inertia and resonance characteristics, the latter depending on the vocal tract configuration [17]. To properly measure the damping of the VF oscillations, the signal needs to be corrected (inverse filtering); this is achieved by the flow glottography device (Rothenberg mask – a high speed wire screen pneumotachograph – combined with the MSIF2 inverse filtering system of Glottal Enterprises, Syracuse, NY, USA) (Fig. 5) [18]. An additional relevant information also provided by the flowglottogram is the end of vocal fold contact (closed plateau), clearly defined on the records. As a
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Fig. 4. Movements of the vocal fold edges, computed from the videokymograms of Fig. 3. Upper traces correspond to the more dorsal part of the vibrating glottis, lower traces to the more ventral part.
Fig. 5. Flow glottograph (Rothenberg’s mask): actually, a high speed wire screen pneumotachograph (time resolution of 0.5 ms).
drawback of the method, the mask to some extent limits speech movements. In photoglottography [19], the larynx is transilluminated and the amount of light through the glottis is continuously monitored. It is a direct measure of the glottal area, and thus effectively measures the true movements of the VF, both in mode 1 (photodiode in pharynx, light source on pretracheal skin) and in mode 2 (light source in pharynx, photodiode on pretracheal skin) [20]. Its time resolution is very high. But it cannot dissociate separate movements of the
two VF, as does high speed video. It is less invasive than imaging techniques, but more so than electroglottography. In practice, the magnitude of the photoglottographic signal is rather small, the skin acting like a strongly absorbing red/infrared filter. As a result, high power LEDs are inappropriate light sources because their emission spectrum is made of a limited number of peaks giving a global visual perception of white but with little red contents. Altogether, the amount of useful light emitted by LEDs is much less than that emitted by a heated tungsten filament, and a more appropriate light source is a tungsten filament light bulb driven by a constant ripple-free current source. For the sake of illustration, a small 15 W light bulb gives a much larger output signal than a 3000 lm high power white LED, without even requiring an IR anticaloric filter. As a receptor, large photodiodes should in principle give larger signals, as they collect more light, but in practice, they prove too large to be adequately positioned in the pharynx. We use a BP104 silicon photodiode (Vishay Precision Group, Malvern, PA, USA) glued onto a small laryngoscopic mirror (Nr. 3). This device can be used in combination with a Rothenberg mask; in this case, the handle of the mirror is introduced through the hermetically sealed hole intended for the hand piece of the mask. The current produced by the photodiode is processed by a currentto-voltage converter with a flat frequency response from DC to 2 kHz. The amplitude of the response is linear up to values ten times larger than the maximum signal due to the VF movements. When combined with flowglottography, it can validate the signal: Fig. 6 shows a quasi-perfect temporal correspondence between the damping on photoglottographic and flowglottographic signals.
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Fig. 6. Flowglottogram (upper trace in red, glottal area increasing upwards); PGG (lower trace in blue, glottal area increasing upwards) Healthy male subject. 130 Hz, 65 dBA. Horizontal scale in ms. The damping is similar. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Electroglottography [21] measures the transglottic electrical impedance using an AC current at a frequency above 100 kHz and monitors the changes in contact surface of the VF. The method is patient-friendly and does not interfere with vocalization. It allows precise phonetic tasks, with acoustic control. However, the sensitivity for detecting very small transglottic impedance variations (essential in this context) depends on the design of the electronic circuit. The original design of Fourcin and Abberton [22] has been superseded by more recent devices using a higher carrier-wave frequency, a more efficient feed-back control of the oscillator, multipole filters with sharper cut-off and flat bandwidth response (e.g. F-J Electronics, Denmark; Laryngograph, UK; Synchrovoice Research, USA; etc.). As a result, a better signal-to-noise ratio and a higher sensitivity are achieved with a larger bandwidth and better linearity [23]. As shown in Fig. 7, the EGG-signal can be as sensitive as the flowglottogram for detecting the smallest vocal fold oscillations, but, contrary to the flow signal, it fails to show the final phase of the damping, since there is no contact between the VF during this phase. The last ten sinusoidal EGG-cycles probably correspond to small (reduced amplitude) impedance fluctuations at the level of the ventral commissure. Imaging techniques, EGG, FLOG and PGG adequately identify the first part of damped oscillations, when the vocal folds still make contact on the midline. In this part, the damping phenomenon is characterized by a progressive reduction of the closed phase, possibly concomitant with a slight increase in amplitude of oscillation.
Ultrasonic glottography is still experimental and does not allow sufficient control of position/orientation of the probe.
4. Discussion Whichever method is used, an important issue is the apparent variability of the damping: this is clearly visible in Figs. 1 and 3, obtained in the same normal subject, during the same recording session. For phonation at comfortable pitch and loudness, three main physiological parameters seem to account for most of the observed variability in damping of the vocal fold oscillations at voice offset. Two of them are actually related to some persistence of the driving force: (1) the timing dynamics of the expiratory pressure (muscular/elastic, depending on lung volume) with respect to the opening of the glottis; (2) the speed at which the vocal fold edges are abducted and the glottal resistance drops; the combined effect of (1) and (2) actually determines the persisting transglottal flow, hence a persisting driving force; (3) the third important parameter is of morphological nature: ideally, the morphology of the oscillator should remain constant during the damping phase. Actually, from a certain degree of abduction, the morphology of the oscillating masses changes considerably, as can be seen in Fig. 8. In the abducted position, the lip-like shape of the VF disappears and the VF is ‘flattened’ laterally. The extent of this non-linear change depends on the degree of abduction. Creating an abrupt airflow interruption at a subglottic level is practically ruled out in vivo. The airflow can be interrupted
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Fig. 7. Flowglottogram (lower trace in red, glottal area increasing upwards) and EGG (upper trace in blue, glottal closing upwards). Healthy male subject, 125 Hz, 65 dB. Horizontal scale in ms. The EGG-signal is as sensitive as the flowglottogram for detecting the smallest vocal fold oscillations, but fails to show the final phase of the damping. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Frontal RX-laryngography with contrast. Left: VF in phonatory position; Right: VF in abducted (inspiratory) position. In the abducted position, the lip-like shape of the VF disappears and the VF is ‘flattened’ laterally.
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Fig. 9. Flowglottogram (upper trace, in red, with large oscillations, glottal area increasing upwards) and EGG (lower trace in blue, glottal closing upwards). Horizontal scale in ms. Three repetitions of/uk/while lowering the rate. Between the first two repetitions, no interruption of vocal fold oscillation occurs, while it just occurs between the second and the third repetition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
downstream, either artificially by an inflatable balloon within the pneumotachograph, or physiologically by linguo-palatal occlusion (e.g. in/uk/). In the case of an artificial abrupt interruption during a/a:/, some transglottal airflow (and vocal fold vibration) can persist by limited inflation of the upper vocal tract, upstream of the occlusion. In the case of a/uk/(or another denasal vowel followed by a voiceless occlusive), the vocal fold vibration usually stops (in a variable way) before the articulatory movement of the/k/. When the occlusive is voiced (/ug/) the vocal fold vibration persists after the linguo-palatal occlusion, but again, as for an artificial airflow interruption, in a very variable way. A persisting transglottal flow after the last VF contact clearly slows the damping, as can be seen by comparing Figs. 1 and 2 with Figs. 3 and 4, obtained from the same (healthy) subject at a few seconds interval. In fact, this illustrates the main problem and limitation encountered with all imaging methods, as well as with PGG. The search for a critical repetition rate of a denasal vowel followed by a voiceless occlusive was attempted: this means the rate (e.g. 6,2 s−1 ) at which the oscillation is actually interrupted (Fig. 9). In such a task, repetition contributes to standardizing several parameters. This protocol could be of some interest, but it requires a trained vocalist, and seems unsuitable for clinical or medicolegal application. All observations so far deal with phonation at comfortable pitch and loudness. Changes in intensity and fundamental frequency also influence the damping characteristics, but this effect seems to be far smaller than that of the other considered factors. Recently, damping
has also been observed in inspiratory phonation [24]: the characteristics are comparable with those of expiratory voicing. Yet another possibility for observing the damping phenomenon would be an external mechanical impulsion on the larynx, without any phonation. However it would require an adequate positioning of the VF at the moment of impulsion, and any recording would be perturbed by the mechanical artifact. Furthermore, an external impulsion will elicit a laryngeal reflex (with a delay in the order of 10–40 ms [25,26]) which will interfere with the damping. For these reasons, this procedure is very unlikely to produce a reliable result. Our limited experience with organic pathology mainly shows irregularity during the damping phase, with chaotic patterns (Fig. 10). This can probably be attributed to the fact that the absence of midline collision and the progressively reduced vibrating mass act as sensitizing factors for asynchronisms due to superficial lesions, which are mostly unilateral or asymmetrical. This particularity could be of some use in objectifying micro-organic vocal fold pathology. On this particular point, an important advantage of high-speed videoendoscopy lies in the fact that damping can be analyzed on each VF separately.
5. Conclusion The damping phenomenon is probably an important issue in voice pathophysiology and perhaps a valuable clinical parameter for various applications. Several methods make it possible to record the damping phase of the vocal folds at the end of a vocal emis-
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Fig. 10. Photoglottogram (glottal area as a function of time; glottal area increases downwards) at voice offset in a female patient with moderate hoarseness and a slight chronic laryngitis. The damping phase has an irregular pattern.
sion. Currently available non invasive techniques (as FLOG and PGG) appear to be well suited. High speed video and high speed videokymography (together with the software for image processing and analysis) have an obvious superiority but are not currently widespread due to high cost. Further, most recent developments (via a transnasal flexible scope) have made it much more comfortable for the subject/patient. However the main problem for a reliable measure of the damping is not the technique, but the variability of damping characteristics that are related to the laryngeal and respiratory behaviour at the end of the vocal emission. Defining a simple protocol for standardising the vocal emission seems unsuccessful. Hence the only solution seems the entire recording, using high speed video, of a standardised passage read by the subject, with a posteriori automatic extraction by dedicated software of all damping phases and computation of the average damping coefficient. The question of course remains whether the provided information makes this sophisticated approach worthwile for the clinician. References [1] P.H. Dejonckere, J. Lebacq, Mechanical study of the damping of the phonatory oscillator, Arch. Int. Physiol. Biochim. 88 (1980) 31–32. [2] P.H. Dejonckere, Damping biomechanics of vocal fold oscillation, in: J. Gauffin, B. Hammarberg (Eds.), Vocal Fold Physiology: Acoustic, Perceptual and Physiological Aspects of Voice Mechanisms, Singular Publishing Group, Inc., San Diego, 1991, pp. 105–111. [3] I.R. Titze, The physics of small-amplitude oscillation of the vocal folds, J. Acoust. Soc. Am. 83 (1988) 1536–1552. [4] P. Dejonckere, J. Lebacq, Relation de phase entre la dynamique de la pression sous-glottique et le mouvement oscillatoire des cordes vocales: II. Attaque vocalique et fin d’émission, Arch. Physiol. Biochem. 88 (1980) 343–355. [5] P.H. Dejonckere, Lebacq J. Damping, coefficient of oscillating vocal folds in relation with pitch perturbations, Speech Commun. 3 (1984) 89–92. [6] B.K. Finkelhor, I.R. Titze, P.L. Durham, The effect of viscosity changes in the vocal folds on the range of oscillation, J. Voice 6 (1988) 320–325.
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