A STUDY OF SIMULTANEOUS MEASUREMENT OF VOCAL FOLD VIBRATIONS AND FLOW VELOCITY VARIATIONS JUST ABOVE THE GLOTTIS

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1 ISTP-16, 2005, PRAGUE 16 TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA A STUDY OF SIMULTANEOUS MEASUREMENT OF VOCAL FOLD VIBRATIONS AND FLOW VELOCITY VARIATIONS JUST ABOVE THE GLOTTIS Shiro ARII Λ, Hideyuki KATAOKA ΛΛ, Yoshitaka OCHIAI Λ, Kensaku HASEGAWA ΛΛ, Hiroya KITANO ΛΛ, Toyohiko SUZUKI Λ Λ Faculty of Engineering, Tottori University, JAPAN ΛΛ Faculty of Medicine, Tottori University, JAPAN Toyohiko SUZUKI: suzuki@mech.tottori-u.ac.jp, , Keywords: Vocal fold, Flow velocity, Sound, Vibration, Glottis Abstract Human voices are originated by vibration of vocal folds in the larynx. Vocal folds are mainly composed of internal (muscle) bands and mucous tissue. It is considered that this tissue vibrates responding to breath airflow from the lungs and the resulting oscillation makes the origin of vocal sound, i.e., phonation. The vocal fold tissue is relatively flexible and an easily deformable substance. However its movement cannot be controlled at will, but the vocal fold may be either tensed or relaxed by controlling the internal laryngeal muscles. Such control may change the vibrating part and its frequency so that the radiated voice tone can be changed. The oscillation of the human vocal fold during phonation can be directly observed in a stroboscopic way or by direct picturing with the help of a high-speed video-scope. Further, the oscillation frequency can be determined by analyzing a sequence of pictures so obtained. However it has been unsuccessful to show clearly the general belief that human phonation originates in consequence of the wave-like motion of the vocal fold tissue induced by the interaction of the tissue and the expiratory airflow (simply, airflow, hereafter). In other words, the complex feature of this phenomenon may not be sufficiently captured by planar observation. Detailed study of this airflow is believed to be important to elucidate mechanism of human phonation with the help of simultaneous observation of vocal fold movement, more specifically, glottal opening by means of ultra-high-speed digital camera. Thus in the present study experimental analysis of this airflow just above glottis was made to clarify phonation mechanism and seek better modeling of vocal folds, focusing on direct measurement of airflow velocity by means of a tiny hot wire probe. Voice was also recorded simultaneously with a microphone located near the mouth of the subject. 1 Introduction Various methods were applied to directly observe the motion of the vocal folds during phonation. Those are the Method of Electromyogram (1), in which the actions of both nerves and muscles (i.e., chords) are probed by measuring the fibrillation voltage in the internal laryngeal muscle, the Method of Trans-glottal Transducer Insertion (2), in which piezo- electric transducer is inserted to measure the pressure variation accompanied by phonation, and the Method of Laryngeal Stroboscope or the like, in which video pictures are taken to directry observe vo- 1

2 cal folds oscillation. Regarding the modeling of phonation, Two-Mass- Model as it is known, which was proposed to simulate the vocal folds as a two- freedom system consisting of two masses and two springs (3). Based on these studies, it is generally considered that temporal variation of the glottal opening during phonation is fairly well approximated by a periodic change shaped by a triangle. Such waveforms are commonly used to represent those of volume flow of air rate passing glottal constriction. And thus in previous theoretical researches concerning phonation, pressure variations of the vocal folds are assumed to have a similar waveform to those of the volume flow rate. In most previous researches on voice generation, the planar sound source was assumed for the laryngeal sound source and the effects of mean airflow in the larynx were neglected. Some experimental researches have reported on measuring airflow velocity near the glottis, utilizing animal vocal folds. In research on in-vivo canine vocal folds the glottal volume flow rate was determined experimentally with the usage of sequential pictures taken with a high-speed videoscope (4). It is said that vocal folds oscillation is generated in consequence by the interaction between the vocal fold tissue and the airflow passing glottis and then transmitted through the tissue layer as an internal wave. It is well amplified in resonance to flow oscillation. Thus for better understanding of the phonation mechanism, the influence of the airflow back to the glottis opening must be considered for the analysis of phonation, in contrast, the traditional treatment of the theme in which airflow variation was considered simply as a consequence of the glottal opening area change controlled by the subject at will. Considering these facts, in the present study more detailed experimental analysis of the airflow above the glottis paid special attention to the mutual interaction between tissue and the airflow. In this way attention was focused not only on the basic period of the glottis opening movement, but also on the cross correlation between the glottis opening and the airflow velocity, and on the high-frequency spectral contents contained in the airflow velocity variation. 2 Experimental Method 2.1 Principle of phonation Humans produce sounds by oscillating the vocal folds in the larynx. The larynx is located at the inlet of the tracheal where it branches into the esophagus. Thus its essential effect is to avoid foreign matter to be swallowed in the tracheal. Vocal folds are composed mainly of an internal vocal chord and a mucous membrane coat. This membrane coat oscillates synchronously to the airflow and makes the origin of the human voice, that is, phonation. The glottis is almost closed during phonation while it is completely open during breathing without phonation. The opening and closing of the glottis, that is, switching of the vocal chords movement is made possible by the actions of the internal laryngeal muscles which consist mainly of cricothyroid and vocal chords, and move the arthrosis of laryngeal cartilage. Airflow from the lungs opens the glottis effectively by its pressure, then the air starts to flow through the larynx. At the instant the sub-glottis pressure decreases, the glottis is closed because of the removal of high sub-glottal pressure. Vocal folds oscillation is considered to be a sequence of repetition of the opening-closing movement of the vocal folds. A time-wise variation of the open area of the glottis has a significant correlation to generation of voice sound. In this way the complex structure of the vocal folds and the oscillation frequency mainly determine the particular feature of each individual vocal sound in consequence of the interactions with the airflow. It is said that the mean airflow rate required for phonation is 100 to 200 ml per second in the case of a human adult and the dynamic pressure of the airflow is 5 to 10 mm H 2 O. Pressure of air in the lungs may not oscillate itself, and thus vocal folds oscillation is essentially a consequence of the dynamic interaction between the elastic nature of vocal folds and the airflow through it, that is, a self-resonating oscillation of fluid-structure interaction. 2

3 2.2 Principle of measurement of vocal cords oscillation Since vocal folds oscillate by repeating the opening and closing of the glottis, the airflow is stopped intermittently so that the oscillation is measurable as a signal of airflow variation most clearly in the supra-glottis region. In addition, the airflow variation should contain interactive components induced by the internal wave motion of the mucous membrane of the vocal folds. Furthermore it is reported that the length of the oscillating part of the vocal folds is about 1 cm, and the amplitude of the oscillation is around 1 mm. Thus the air flows through this narrow slit so that it creates a strong shear flow accompanying vortex generation. The features of such complex flows, which may change the shape of the mucous membrane and influence the deformation of the membrane, may be caught only through measuring the velocity variation, not by direct observation of the membrane with the help of highspeed camera. cilloscope, a personal computer and a condenser microphone (abbreviated to MIC). The outputs from the CTA and MIC are monitored through the experiment via the digital oscilloscope and stored in the personal computer after an A-D conversion with a 12 bit conversion rate. The CTA and tungsten probes used (dia: 5 micron ) have a frequency response good enough to measure air velocity within an accuracy of 1.00 % in a frequency range of up to 10 khz. Digital data converted from output voltages were used for the conversion to velocity and/or further analyses such as Fourier analysis. Small hot-wire probes were designed and 3 Measuring Method of Airflow in Larynx 3.1 Measuring system for airflow velocity The schematic diagram of the measuring system for the airflow velocity is shown in Fig. 1. The system consists of a hot-wire probe, a constant temperature anemometer (KANOMAX MODEL 1011, abbreviated to CTA hereafter), a digital os- (a) Hot-wire probe High Speed Camera Photoron FASTCAM MAX I 2 Endoscope (Video Scope) OLYMPUS ENF TypeP4 Video Monitor Constant Temperature Anemometer KANOMAX MODEL 1011 Digital Oscilloscope YOKOGAWA DL1200E Microphone B&K 4165 Hot-wire Probe Personal Computer Pentium III 500MHz Vocal fold Esophagus Fig. 1 Measurement system Trachea (b) Endoscope Fig. 2 Hot-wire probe and endoscope 3

4 fabricated in the laboratory for this particular application in the present research so that it could be inserted into the larynx through an endoscope tube. Fig. 2 shows a picture of one of the probes and the endoscopes. The probe was manually manipulated for proper placement for the measurement by monitoring its location in the larynx through the endoscope. The other end of the probe is attached to a BNC adapter, which was connected to a coaxial output cable. In addition, vocal sound was also investigated simultaneously with an airflow velocity measurement and/or high-speed camera study. Sound pressure measurement was made with a microphone placed approximately 10 to 15 cm from the mouth of the subject. No particular treatment was made to avoid any other possible reflections of sound in the room used at the clinic. 3.2 Calibration of hot-wire probes Calibration curves were obtained for all the hotwire probes and then utilized to convert output voltage to the airflow velocity. A small wind tunnel shown in Fig. 3 was used for the calibration. It consists of a diverging nozzle and a flow straightener section of a honeycomb sheet. At the test section in the downstream of the straightener section static and pitot pressure were measured, from which the airflow velocity at the section was calculated. At the same time the output of the hot-wire probe was recorded via the CTA. The airflow velocity and the hot-wire voltage output produced a calibration curve for the probe. The test was performed in a range of velocity lower than 20 m/s taking into consideration the airflow velocity range that is generally found in the larynx during phonation. 3.3 Measuring method of airflow velocity In order to avoid or suppress any reaction of the larynx responding to the insertion of a probe, the probe was inserted via the nasal cavity so that it could be placed directly just above the glottis. The subject was sufficiently anesthetized locally using 4 % xylocaine in advance. Next, an insertion tube(5 mm O.D.), which is commonly used for endoscopy, was inserted and fixed at the proper position where the tip of the tube comes across the epiglottis to the point just above his glottis. The position of the probe was carefully monitored and checked with endoscope CRT screen. The inner tube, through which the probe tube was inserted, was cleaned with alcohol each time before the probe tube was inserted. Such treatment was necessary because any adherent such as a mucous substance may damage the 3 Output voltage V Air flow velocity m/s Fig. 3 Calibration system of hot-wire probe Fig. 4 probe A typical calibration curve of hot-wire 4

5 probe. In this way any degradation of the probe was carefully avoided. In addition to the airflow velocity in the larynx, the sound pressure was measured during phonation. Furthermore, the images of the vibrationg vocal folds taken with a high speed camera were stored simultaneously. Velocity m/s 10 4 Results and Discussion Several young men and women in their early and mid 20 s cooperated as subjects for the present experiment. Almost all experiments were conducted with the phonation of the vowels ah or eh. In every case, the subject produced the sounds in a relaxed attitude. Fig. 5 (a), (b), (c) are given to show the location of the hot-wire probe in relation to the vocal folds. These figures represent the typical phases of the glottal opening, that is, one phase of a relatively opened glottis during phonation, another at a almost closed phase during phonation, and finally a completely opened phase during no phonation. 4.1 Results for airflow velocity passing glottis Fig. 6 (a) shows a typical temporal variation of the airflow velocity, and Fig. 6 (b) shows the corresponding Fourier spectrum. It is seen that the average velocity is around 2.5 m/s. Naturally, the average velocity increases when a louder sound is pronounced. It was noticed that the fundamental frequency component and its several harmonics mainly comprise the lower frequency part of the signal. The fundamental frequency was identified at 159 Hz which is in accordance to that of the opening of the glottis. It should be pointed out that a very high frequency component of a significant level was found at khz. Thus the Probe Probe (a) (b) (c) Fig. 5 The location of hot-wire probe Probe Fourier spectrum Time ms (a) Airflow velocity (b) Fourier spectrum of airflow velocity Fig. 6 Typical temporal variation of airflow velocity airflow velocity was seen to change mainly with the fundamental frequency of 159 Hz and some harmonics, but it was also superposed with a significant level of a khz component. Similar features were also found for other subjects. However it was discovered that the anesthetization on the vocal folds might change this tendency. Fig. 7 (a) shows a typical temporal variation of the airflow velocity after the anesthetization, and Fig. 7 (b) shows the corresponding Fourier spectrum. Fig. 8 shows comparison of the Fourier spectra of the airflow velocity obtained with and without anesthetization performed on the same subject. The influence of anesthetization upon the produced sound as well as the airflow velocity near the outlet of the vocal folds is clear. Naturally the reseachers noticed this change during the experiment. Considering these changes, the possible effect of the anesthetization was hardening of the elasticity of the vocal folds. Note that the anesthetization was applied not only to the area of nasal cavity but also 5

6 Velocity m/s 2 1 Sound pressure V Time ms (a) Airflow velocity Time ms Fig. 9 Typical temporal sound pressure Fourier spectrum Fourier spectrum (b) Fourier spectrum of airflow velocity Fig. 7 Typical temporal variation of airflow velocity after anesthetization Fourier spectrum Before anesthetized After anesthetized Fig. 8 Comparison of Fourie spectrum between before and after anesthetization to the whole vocal folds in this experiment shown Fig. 7. It is inferred that such an influence of the anesthetization on voiced tone comes through relaxation of the vocal folds or the related tissues. In the others of the present series of experiment, anesthetization was applied only to the nasal cavity Fig. 10 Fourier spectrum of sound pressure 4.2 Sound pressure measured just outside of mouth A typical time variation of sound pressure measured near the mouth is shown in Fig. 9 and the corresponding Fourier spectrum is in Fig. 10. It should be pointed out here that the series of the experiments to be shown hereafter was performed with the slightly different arrangement of the hot-wire probe in relation to the endoscope tube. In these cases, the hot-wire probe was fixed to the endoscope tube with minimum protrusion from the tube exit so that it would not obstruct the field of the view. As a result some influence of the tube on the hot-wire probe output was suspected. Relatively large difference was found in the airflow velocity spectra between the former series of the experiment and the latter series. It is supposed to be due to this influence. It is seen that the high frequency components of significant level found in the former is much lowered in the latter. It is also noted that the major spectral contents found in the former tend to be still significant but be found at slightly higher frequencies in the latter. In the figure the output for the pres- 6

7 Fourier spectrum Air velocity Sound pressure Cross correlation (a) Frequency range of 0 to 10kHz Time ms (a) Cross-correlation Fourier spectrum 10 1 Air velocity Sound pressure Cross spectral density (b) Frequency range of 0 to 1kHz Fig. 11 Comparison of Fourier spectrum of airflow velocity and sound pressure (b) Cross spectral density Fig. 12 Cross correlation and spectrum density of airflow velocity and sound pressure sure signal was not converted to Pascal in unit. It was noted that the sound pressure consists mainly of lower frequency spectral components and very high frequency components. These lower frequency components were easily found consisting of the fundamental frequency component and its harmonics. On the other hand, high frequency components were not directly related to the fundamental one, but supposedly produced by other mechanisms. However either lower or higher frequency contents have a strong correlation with those of the airflow velocity variation, as shown in next section. 4.3 Cross-correlation between airflow velocity and sound pressure The two Fourier spectra, each of which was airflow velocity and sound pressure respectively, are compared in Fig. 11 (a) and (b). Both the airflow velocity and the sound pressure have major components at common frequencies particularly in the frequency range of 0 to 1kHz. To see the causality further, cross correlation of the two quantities and their spectrum density were computed as shown in Fig. 12. Although the contribution of the fundamental frequency components and the harmonics of the airflow velocity to produce sound is obvious, other components contained in the velocity spectra was also seen to make some contribution to phonation at certain combination frequencies such as the sum or the difference. 4.4 Correlation between opening of glottis and the airflow velocity passing the glottis It is important to elucidate the correlation between the glottal opening and the airflow velocity exiting the glottis since this velocity variation comprises the major part of the human voice as the monopole-type sound source. Because of the extreme complexity of the structure and the dynamic movement of the vocal folds, quantitative measurement of its deformation and motion are formidable. And thus a qualitative descrip- 7

8 0 ms 0.5 ms 1.0 ms 1.5 ms 2.0 ms 2.5 ms 3.0 ms 3.5 ms 4.0 ms 4.5 ms 5.0 ms 5.5 ms Fig. 13 The series of pictures taken with the high speed camera tion was made to explain how the airflow velocity changes with the opening and closing timing of the glottis with the help of Fig. 13, 14 and 15. A series of pictures taken with the high speed camera were shown in Fig. 13 and 15. The high speed motion pictures were taken at a rate of 2000 frame/sec. The timing of each picture in the series corresponds to the timing shown in Fig. 14 as a solid circle on the airflow velocity vresus time curve. From this series of pictures it can be seen that a new closing phase in a periodic cycle starts roughly at 3 ms, and ends at around 7 ms. An opening phase in the same cycle starts about 7.5 ms. These timings can be compared with the temporal variation of the airflow velocity as follows. During the closing phase of the glottis the airflow velocity was at its highest in the cycle. The fact clearly explains that the closing phase of the glottis observed is not a complete closing of the glottis, but a fully constricted phase, so Velocity m/s Time ms Fig. 14 Temporal variation of airflow velocity 8

9 6.0 ms 6.5 ms 7.0 ms 7.5 ms 8.0 ms 8.5 ms 9.0 ms 9.5 ms 10 ms 10.5 ms 11.0 ms 11.5 ms Fig. 15 The series of pictures taken with the high speed camera that the air flows like a jet from the glottis at the almost highest velocity. 5 Conclusions The airflow velocity in the larynx during phonation was experimentally studied. Direct observation of the vocal fold motion was also performed simultaneously with the measurement of produced sound pressure as well as airflow velocity. The following conclusions may be drawn from the study. 1) The periodic change of the airflow velocity occurs out of the phase of opening of glottis. though both have the same fundamental frequency. This fact is important since it gives a crucial hint for better simulation model of phonation mechanism. 2) The airflow velocity in the larynx has highfrequency components of significant level, though it seems to contribute small to produced sound. Further studies are needed to clarify the role of these high frequency components played in the produced and radiated sound. 3) Anesthetization on the vocal folds could influence to change the property of vocal folds such that voiced sound changes in tone. References (1) Hirano, M. and Ohala, J., J. Speech Res. 12 (1969) (2) Koike Y. and Perkins, W., Folia Phoniat., 20 (1968),

10 (3) Ishizuka, K. and Flanagan, J. L., The Bell System Techical Journal 51-6 (1972) (4) Verneuil, A. et al, Ann Otol Rhinol Laryngol, 112 (2003), (5) INGO R. TITZE, PRINCIPLES OF VOICE PRODUCTION (1994) Prentice-Hall. 10