Subglottal System Research Articles

Measurements of subglottal resonances and their influence on vowel spectra

Data collected from several speakers are being studied to estimate subglottal resonant frequencies. The utterances were ten vowels of American English embedded in the nonsense syllable /■hVd/. Sound-pressure signals were recorded by a microphone, and signals transduced by an accelerometer placed on the neck below the larynx were recorded simultaneously. In the accelerometer signal the subglottal resonances appear as well-defined spectral peaks, whereas in the microphone signal these resonances are manifested as small perturbations in the spectra. These perturbations are more pronounced for a speaker who, based on other spectral data, phonates with a significant glottal chink [H. M. Hanson and K. N. Stevens, Proc. ICPhS 95, Stockholm]. Comparison of spectra of the accelerometer and sound-pressure signals for two female speakers has resulted in estimated subglottal resonant frequencies of 625, 1540, and 2110 Hz. The bandwidths of the first subglottal resonances have also been estimated for the two speakers at 110 and 210 Hz, respectively. These results are in keeping with previous reports. Discontinuities in apparent formant movements have been observed in the vicinity of the estimated subglottal resonances, suggesting that resonances of the subglottal system can influence the acoustic characteristics of vowels. [Work supported by NIH Grant Nos. DC00205-02, DC00075.]

Transfer function of sound transmission in subglottal human respiratory system at low frequencies

The amplitude of sound transmission from the mouth to a site overlying the extrathoracic trachea and two sites on the posterior chest wall was measured in eight healthy adult male subjects at resting lung volume over the 100- to 600-Hz frequency range. The ratios of the estimated magnitude spectra of transmission of each of the chest wall sites to the tracheal site were determined, with the resulting spectra representing effective transfer functions of transmission in the subglottal system. For the group, the transfer functions exhibited a single peak, which occurred at 143 +/- 13 Hz (mean +/- SD) with a quality factor (Q) of 2.0 +/- 0.2 for the upper chest wall site and at 129 +/- 6 Hz with a Q of 2.2 +/- 0.4 for the lower site. The trend of decreasing spectral energy with increasing frequency was indicated by roll-offs of -10 +/- 4 and -17 +/- 5 dB/octave from 300 to 600 Hz at the two sites, respectively. The fundamental radial mode of a model thoracic cavity, which is a large rigid cylinder filled with lossless lung tissue, provides a good estimate of the observed low-frequency resonance. This agreement suggests that thoracic cavity resonances may have particularly important effects on sound transmission at frequencies below approximately 250 Hz, where the magnitude of parenchymal attenuation appears to be small.

Sonar pulse radiation and filtering in the mustached bat, P t e r o n o t u s p a r n e l l i i r u b i g i n o s u s

Parnell’s mustached bat, Pteronotus parnellii, uses echolocation to hunt for insects in the New World tropics. The sonar signals, which are emitted from the mouth, are long, constant-frequency types with a ‘‘missing’’ fundamental, so that the second and third harmonics contain most of the pulse energy. This pulse structure implies that the supraglottal vocal tract acts as an acoustic notch filter for the fundamental. To investigate this possibility, light and heavy gases were used to obtain the vocal tract transfer function. The results show that a fundamental notch filter is present, and anatomical manipulations of the vocal tract suggest that this filtering is achieved by a combination of antiphasic reflections from the nasal passages and an area-function filter in the throat. During these experiments, tracheal sound was also monitored to assess the acoustical role of the tracheal sac. This is an elastic section of the trachea, situated immediately posterior to the larynx, which inflates during vocalization. While demonstrating that this structure plays a significant role in determining the acoustics of the subglottal system, our results neither confirm nor dismiss the hypothesis that it reflects sound back to the larynx to improve vocal efficiency. As part of the investigation, the radiation pattern for the mustached bat was also obtained and was compared with various model sources. At the second and third harmonics, sound radiation is highly directional and consistent with the idea that the mouth acts as a conical horn.

On subglottal formant analysis.

When subglottal pressure signals which are recorded during normal speech production are spectrally analyzed, the frequency of the first spectral maximum appears to deviate appreciably from the first resonance frequency which has been reported in the literature and which stems from measurements of the acoustic impedance of the subglottal system. It is postulated that this is caused by the spectrum of the excitation function. This hypothesis is corroborated by a modeling study. Using an extended version of the well-known two-mass model of the vocal folds that can account for a glottal leak, it is shown that under realistic physiological assumptions glottal flow waveforms are generated whose spectral properties cause a downward shift of the location of the first spectral maximum in the subglottal pressure signals. The order of magnitude of this effect is investigated for different glottal settings and with a subglottal system that is modeled according to the impedance measurements reported in the literature. The outcomes of this modeling study show that the location of the first spectral maximum of the subglottal pressure may deviate appreciably from the natural frequency of the subglottal system. As a consequence, however, the comfortable assumption that in normal speech the glottal excitation function is constant and zero during the "closed glottis interval" has to be called into question.

Acoustic properties of the subglottal system us determined from recordings of subglottal pressure

A complete model of the speech production system should include a model of the subglottal part. For the construction of such a model measurement data are needed. The most extensive and comprehensive data available today are the input impedance measurements on five laryngectomized Japanese subjects made by Ishizaka et al. [J. Acoust. Soc. Am. 60, 190–197 (1976)]. We have made wideband recordings of the subglottal pressure variations during speech production in five normal adult Dutch males. It appears that the resonances of the subglottal system are excited by the glottal source in much the same way as the supraglottal resonances. Thus, it seems feasible to determine the frequencies and bandwidths of the subglottal resonances from the subglottal pressure signals. Using this procedure we have consistently found a frequency of the lowest resonance which is considerably lower than the one measured by Ishizaka et al. We will discuss a number of possible explanations for the different measurement results and their implications for improving the model of the subglottal system. [Work supported by the Dutch Organization for the Advancement of Pure Research ZWO.]

Acoustic effects of longitudinal displacement in vocal‐cord vibration

Previously we modelled vocal‐cord vibration by a nonlinear mechanical oscillator composed of two stiffness‐coupled masses. The oscillator is driven by a function of intraglottal pressure and subglottal pressure, and the coupled masses are permitted one‐dimensional displacement transverse to the direction of glottal flow. This formulation produces realistic self‐oscillatory behavior, and duplicates characteristic phase‐differences in cord motion and acoustic interactions with vocal tract and subglottal system. Through approximating calculations, we estimated that the longitudinal component of real cord motion (along the direction of glottal flow) has second‐order influence upon glottal flow. In this paper we examine more rigorously this earlier estimate. We modify the two‐mass model for an additional (longitudinal) degree of freedom in cord displacement. Oscillator and coupling parameters remain as before, but the oscillator is driven by functions of both transglottal pressure and intraglottal pressure. As a result of interaction with the subglottal system, we observe a microstructure in subglottal pressure variation. The microstructure is excited mainly by vocal‐cord closure, and is consistent with measurements on human phonation. Also, we compute dynamic two‐dimensional trajectories for the cords (masses), and find that they correspond with observations made on natural larynxes. We conclude that the longitudinal component of displacement is not essential for realistic self‐oscillation of the cord model. Finally, we synthesize voiced sounds with the model, and find that the glottal flow component deriving from longitudinal displacement is physically and perceptually of second‐order importance.

Input acoustic-impedance measurement of the subglottal system

The input acoustic impedance looking into the trachea was measured on several laryngectomized subjects. The results show remarkable inconsistency with van den Berg’s data. The subglottal resonances are found at approximately 640, 1400, and 2100 Hz. The acoustic input impedance at the first peak ranges between 25 and 50 cgs acoustic ohms. The results are analyzed by means of a digital simulation based on anatomical studies, and the tract wall impedance necessary to produce the measured subglottal impedance is calculated. Subject Classification: [43]70.20, [43]70.40, [43]35.20.

Spectral Effects of Subglottal Cavity System

Aspirated sounds display shifts of formant frequencies and new formants are added. These can typically be observed in the aspirative part of stop consonant bursts. A quantitative modeling of this influence has been performed on the basis of subglottal impedance data and simple geometry approximations of the subglottal system. The influence of subglottal coupling on voice-source characteristics is discussed.

Dynamic Constraints in the Variation of Subglottal Pressure

Using a circuit-type model for the subglottal system similar to that originally proposed by van den Berg [Acta Physiol. Pharm. Neerl. 9, 361–385 (1960)], it is shown that the physical dynamic constraints of the subglottal system do not significantly limit the ability of the respiratory musculature to produce changes in subglottal pressure. The primary limitations appear to come from the dynamic limitations inherent in the mechanism of muscle contraction, with the physical system between the muscles and the pressure in the trachea adding a delay of about 15 msec. The model discussed can also be used to predict the variations of subglottal pressure resulting from changes in glottal-supraglottal air flow resistance.