Vocal Tract Transfer Function Research Articles

Subglottal resonances in healthy older adults

Resonances of the subglottal airways impact the vocal tract transfer function and voice production during speech through linear and non-linear acoustic coupling. It has been posited that subglottal resonances form acoustic boundaries between contrasting sets of vowels and consonants based on their coupling effects on formant frequencies. Thus far, studies of subglottal resonances have reported data almost exclusively from children and young adults; data from older adults are rare. This study investigates subglottal resonances in eight healthy older adults between 50 and 80 years of age (5 males, 3 females). Initial analyses indicate that the first and especially the second subglottal resonances are at lower frequencies than expected when compared against height-matched young adults but that the subglottal resonance frequencies are not dependent on posture, vowel acoustics, or standard measures of pulmonary function.

Evaluation of Glottal Inverse Filtering Techniques on OPENGLOT Synthetic Male and Female Vowels

Current articulatory-based three-dimensional source–filter models, which allow the production of vowels and diphtongs, still present very limited expressiveness. Glottal inverse filtering (GIF) techniques can become instrumental to identify specific characteristics of both the glottal source signal and the vocal tract transfer function to resemble expressive speech. Several GIF methods have been proposed in the literature; however, their comparison becomes difficult due to the lack of common and exhaustive experimental settings. In this work, first, a two-phase analysis methodology for the comparison of GIF techniques based on a reference dataset is introduced. Next, state-of-the-art GIF techniques based on iterative adaptive inverse filtering (IAIF) and quasi closed phase (QCP) approaches are thoroughly evaluated on OPENGLOT, an open database specifically designed to evaluate GIF, computing well-established GIF error measures after extending male vowels with their female counterparts. The results show that GIF methods obtain better results on male vowels. The QCP-based techniques significantly outperform IAIF-based methods for almost all error metrics and scenarios and are, at the same time, more stable across sex, phonation type, F0, and vowels. The IAIF variants improve the original technique for most error metrics on male vowels, while QCP with spectral tilt compensation achieves a lower spectral tilt error for male vowels than the original QCP.

Voice efficiency for different voice qualities combining experimentally derived sound signals and numerical modeling of the vocal tract.

Purpose: Concerning voice efficiency considerations of different singing styles, from western classical singing to contemporary commercial music, only limited data is available to date. This single-subject study attempts to quantify the acoustic sound intensity within the human glottis depending on different vocal tract configurations and vocal fold vibration. Methods: Combining Finite-Element-Models derived from 3D-MRI data, audio recordings, and electroglottography (EGG) we analyzed vocal tract transfer functions, particle velocity and acoustic pressure at the glottis, and EGG-related quantities to evaluate voice efficiency at the glottal level and resonance characteristics of different voice qualities according to Estill Voice Training®. Results: Voice qualities Opera and Belting represent highly efficient strategies but apply different vowel strategies and should thus be capable of predominate orchestral sounds. Twang and Belting use similar vowels, but the twang vocal tract configuration enabled the occurrence of anti-resonances and was associated with reduced vocal fold contact but still partially comparable energy transfer from the glottis to the vocal tract. Speech was associated with highly efficient glottal to vocal tract energy transfer, but with the absence of psychoactive strategies makes it more susceptible to noise interference. Falsetto and Sobbing apply less efficiently. Falsetto mainly due to its voice source characteristics, Sobbing due to energy loss in the vocal tract. Thus technical amplification might be appropriate here. Conclusion: Differences exist between voice qualities regarding the sound intensity, caused by different vocal tract morphologies and oscillation characteristics of the vocal folds. The combination of numerical analysis of geometries inside the human body and experimentally determined data outside sheds light on acoustical quantities at the glottal level.

Computer simulation of vocal tract resonance tuning strategies with respect to fundamental frequency and voice source spectral slope in singing.

A well-known concept of singing voice pedagogy is "formant tuning," where the lowest two vocal tract resonances ( fR1, fR2) are systematically tuned to harmonics of the laryngeal voice source to maximize the level of radiated sound. A comprehensive evaluation of this resonance tuning concept is still needed. Here, the effect of fR1, fR2 variation was systematically evaluated in silico across the entire fundamental frequency range of classical singing for three voice source characteristics with spectral slopes of -6, -12, and -18 dB/octave. Respective vocal tract transfer functions were generated with a previously introduced low-dimensional computational model, and resultant radiated sound levels were expressed in dB(A). Two distinct strategies for optimized sound output emerged for low vs high voices. At low pitches, spectral slope was the predominant factor for sound level increase, and resonance tuning only had a marginal effect. In contrast, resonance tuning strategies became more prevalent and voice source strength played an increasingly marginal role as fundamental frequency increased to the upper limits of the soprano range. This suggests that different voice classes (e.g., low male vs high female) likely have fundamentally different strategies for optimizing sound output, which has fundamental implications for pedagogical practice.

Effects of Nasalization on Vocal Tract Response Curve

Earlier studies have shown that nasalization affects the radiated spectrum by modifying the vocal tract transfer function in a complex manner. Here we study this phenomenon by measuring sine-sweep response of 3-D models of the vowels /u, a, ᴂ, i/, derived from volumetric MR imaging, coupled by means of tubes of different lengths and diameters to a 3-D model of a nasal tract. The coupling introduced a dip into the vocal tract transfer function. The dip frequency was close to the main resonance of the nasal tract, a result in agreement with the Fujimura & Lindqvist in vivo sweep tone measurements [Fujimura & Lindqvist, 1972]. With increasing size of the coupling tube the depth of the dip increased and the first formant peak either changed in frequency or was split by the dip. Only marginal effects were observed of the paranasal sinuses. For certain coupling tube sizes, the spectrum balance was changed, boosting the formant peaks in the 2 - 4 kHz range. A velopharyngeal opening introduces a dip in the transfer function at the main resonance of the nasal tract. Its depth increases with the area of the opening and its frequency rises in some vowels.

Vocal Tract Parameter Estimation and Modeling with Application in Diagnosis of Voice Disorders

Vocal Tract modelling plays an important role in acoustic phonetics that gives a speech language therapist (SLP) information about the shape of the vocal tract. Vocal tract transfer function estimation gives important insights in diagnosing voice disorders. This paper presents a survey of vocal tract modelling methods employed in literature. Vocal tract parameter estimation using Vagmi and Praat are detailed in this paper, followed by an investigation into a method to measure the resonance of the vocal tract using frequency sweeps at the lips of the human subject. The design and implementation of the vocal tract resonance measurement device has been tested and has potential for application in diagnosis of voice disorders.

Validating a psychoacoustic model of voice quality.

No agreed-upon method currently exists for objective measurement of perceived voice quality. This paper describes validation of a psychoacoustic model designed to fill this gap. This model includes parameters to characterize the harmonic and inharmonic voice sources, vocal tract transfer function, fundamental frequency, and amplitude of the voice, which together serve to completely quantify the integral sound of a target voice sample. In experiment 1, 200 voices with and without diagnosed vocal pathology were fit with the model using analysis-by-synthesis. The resulting synthetic voice samples were not distinguishable from the original voice tokens, suggesting that the model has all the parameters it needs to fully quantify voice quality. In experiment 2 parameters that model the harmonic voice source were removed one by one, and the voice tokens were re-synthesized with the reduced model. In every case the lower-dimensional models provided worse perceptual matches to the quality of the natural tokens than did the original set, indicating that the psychoacoustic model cannot be reduced in dimensionality without loss of fit to the data. Results confirm that this model can be validly applied to quantify voice quality in clinical and research applications.

Effect of the vocal tract filtering on the measurement of vocal cycle perturbations

Jitter and shimmer appear as symptoms of disordered voices and they are defined as the cycle-to-cycle variation of the cycle length and cycle amplitude, respectively. The frequency modulation of the phonatory excitation, i.e. vocal jitter, causes amplitude modulation (shimmer) of the speech signal due to the action of the vocal tract transfer function. In this Letter, the authors investigate the extent to which the vocal tract filtering affects the accuracy of measurement of vocal cycle perturbations of the phonatory excitation. To achieve this goal, the direct digital synthesiser is used to generate a series of glottal excitation cycles with the desired amount of vocal perturbations that crosses the vocal tract model to produce a set of 231 stimuli of sustained vowels [a]. Praat is used to measure the amount of perturbations. Results show that the values of the input shimmer inserted in the glottal excitation are smaller than the values measured by Praat. An undesired amount of shimmer is generated in the speech signal when the glottal excitation signal perturbed with jitter crosses through the vocal tract.

Multidimensional Timbre Spaces of Cochlear Implant Vocoded and Non-vocoded Synthetic Female Singing Voices.

Many post-lingually deafened cochlear implant (CI) users report that they no longer enjoy listening to music, which could possibly contribute to a perceived reduction in quality of life. One aspect of music perception, vocal timbre perception, may be difficult for CI users because they may not be able to use the same timbral cues available to normal hearing listeners. Vocal tract resonance frequencies have been shown to provide perceptual cues to voice categories such as baritone, tenor, mezzo-soprano, and soprano, while changes in glottal source spectral slope are believed to be related to perception of vocal quality dimensions such as fluty vs. brassy. As a first step toward understanding vocal timbre perception in CI users, we employed an 8-channel noise-band vocoder to test how vocoding can alter the timbral perception of female synthetic sung vowels across pitches. Non-vocoded and vocoded stimuli were synthesized with vibrato using 3 excitation source spectral slopes and 3 vocal tract transfer functions (mezzo-soprano, intermediate, soprano) at the pitches C4, B4, and F5. Six multi-dimensional scaling experiments were conducted: C4 not vocoded, C4 vocoded, B4 not vocoded, B4 vocoded, F5 not vocoded, and F5 vocoded. At the pitch C4, for both non-vocoded and vocoded conditions, dimension 1 grouped stimuli according to voice category and was most strongly predicted by spectral centroid from 0 to 2 kHz. While dimension 2 grouped stimuli according to excitation source spectral slope, it was organized slightly differently and predicted by different acoustic parameters in the non-vocoded and vocoded conditions. For pitches B4 and F5 spectral centroid from 0 to 2 kHz most strongly predicted dimension 1. However, while dimension 1 separated all 3 voice categories in the vocoded condition, dimension 1 only separated the soprano stimuli from the intermediate and mezzo-soprano stimuli in the non-vocoded condition. While it is unclear how these results predict timbre perception in CI listeners, in general, these results suggest that perhaps some aspects of vocal timbre may remain.

Reconstruction of Mandarin Electrolaryngeal Fricatives With Hybrid Noise Source

The Mandarin electrolaryngeal (EL) speech is suffering from severe fricative confusion due to improper EL source in EL speech production and abnormal physiological structure of vocal tract in the laryngectomized condition. To reduce the fricative confusions, this paper proposes a hybrid noise source by combining the typical natural fricative sources and compensation sources that consider the acoustic defects in the frequency domain caused by the truncated vocal tract and abnormal source location in EL speech production. All parameters of the model are fricative-specific and the parameters of the compensation sources are determined by analyzing the vocal tract transfer functions before and after the laryngectomy. All five Mandarin fricatives are produced by laryngectomized subjects with an experimental EL system loading the hybrid noise source and the wideband noise source. The acoustic and perceptual features of these reconstructed EL fricatives are analyzed and evaluated by comparing with the conventional EL fricatives and normal fricatives. The results indicate that the hybrid noise source successfully improves the acoustic properties of the EL fricatives by forming better spectral shapes, raising the frequencies of average energy concentration, and producing better spectral skewness and kurtosis. Finally, due to these improvements of acoustic properties, the hybrid noise sources achieve much larger intelligibility for EL fricatives than the wideband noise source and the conventional EL source. Thus, the hybrid noise source is an effective, feasible, and promising method of reducing the severe fricative confusions and improving the intelligibility of EL speech.

Multitaper harmonic analysis of infant vocalizations

Infant vocalizations exhibit many special complexities relative to adult speech that make acoustic analysis difficult, even though the basic physics of sound production in the vocal tract is identical. Fundamental frequency is raised, resulting in wider-spaced harmonics that make it hard to accurately estimate the spectral envelope or locate formant resonances, which are higher than in adult voices, due to anatomical scaling effects. Irregular vocal fold vibratory regimes are often observed, involving period doubling and other nonlinear phenomena not seen in adult speech, along with atypical patterns of turbulent noise production. Unsurprisingly, traditional analysis techniques that depend on estimating the spectral envelope from the speech signal do not work well when applied to infant vocalizations. An alternative approach is proposed, using multitaper analysis to calculate the time-varying amplitude and phase of every harmonic component of the voice, along with the residual noise component, recovering the vocal tract transfer function from the results. The new technique is compared against linear prediction and cepstral analysis using home audio recordings of 20 infants collected from 0 to 24 months using LENA technology. Developmental progressions in the acoustic structure of the infant voice are identified that cannot be found using traditional methods.

How to precisely measure the volume velocity transfer function of physical vocal tract models by external excitation.

Recently, 3D printing has been increasingly used to create physical models of the vocal tract with geometries obtained from magnetic resonance imaging. These printed models allow measuring the vocal tract transfer function, which is not reliably possible in vivo for the vocal tract of living humans. The transfer functions enable the detailed examination of the acoustic effects of specific articulatory strategies in speaking and singing, and the validation of acoustic plane-wave models for realistic vocal tract geometries in articulatory speech synthesis. To measure the acoustic transfer function of 3D-printed models, two techniques have been described: (1) excitation of the models with a broadband sound source at the glottis and measurement of the sound pressure radiated from the lips, and (2) excitation of the models with an external source in front of the lips and measurement of the sound pressure inside the models at the glottal end. The former method is more frequently used and more intuitive due to its similarity to speech production. However, the latter method avoids the intricate problem of constructing a suitable broadband glottal source and is therefore more effective. It has been shown to yield a transfer function similar, but not exactly equal to the volume velocity transfer function between the glottis and the lips, which is usually used to characterize vocal tract acoustics. Here, we revisit this method and show both, theoretically and experimentally, how it can be extended to yield the precise volume velocity transfer function of the vocal tract.

Mapping vowel categories at high fundamental frequencies using multidimensional scaling of cochlea-scaled spectra

Recent studies have shown that accurate vowel category perception can be maintained at fundamental frequencies (f o) up to at least 880 Hz. In such cases, the typical first formant frequency (F 1) of most vowels is exceeded by f o and the vocal tract transfer function is to a high degree undersampled. It seems therefore unlikely that common formant patterns are the main acoustic features used by listeners to recognize vowels at high f os. Here, we present results from multidimensional scaling (MDS) analyses calculated by averaging cochlea-scaled spectra across multiple steady-state vowels (N = 324; all 250 ms) produced by a female native German speaker at nine f os within a range of 220-880 Hz. All vowels (/i, y, e, ø, ɛ, a, o, u/) were recognized accurately by listeners (N = 20) in a previous study [Friedrichs et al., The phonological function of vowels is maintained at fundamental frequencies up to 880 Hz, J. Acoust. Soc. Am. 138, EL36-EL42 (2015)]. MDS reveals that with increasing f o, the vowel height dimension partially collapses, but the front/back distinction expands, thus allowing the vowels to be distinguished. This indicates that the perceptual space is reorganized and vowel height and frontness are being combined in a correlated way at higher f os.

Analysis of children’s high front vowel area function using three-dimensional ultrasound imaging

Understanding children’s speech production can follow from an analysis of children’s vocal tract area functions. While some studies have used imaging techniques on children to measure tongue contours, to our knowledge, no vocal tract area functions of children have been examined. We attempt to measure the oral cavity vocal tract area functions of children’s vowels using three-dimensional ultrasound imaging recordings of the tongue and palate. In this investigation, we model the oral cavity area function of the vowel [i] of children and compare it to that of an adult oral cavity area function measured using the same ultrasound technique. We show evidence that the tongue positioning and articulation of the vowel [i] is dramatically different for children and adults. Specifically, we focus on how the different tongue positioning of children and adults affects the vocal tract transfer function of [i]. Some theories of children’s speech, either mechanically or perceptually based, can be inferred from these findings and will be compared to previous studies. [Work supported in part by NSF Grant 1551113, NSF Grant 1551131, NSF Grant 1514544, and NIH Grant R01DC007124.]

The Identification of High-pitched Sung Vowels in Sense and Nonsense Words by Professional Singers and Untrained Listeners

High-pitched sung vowels may be considered phonetically "underspecified" because of (i) the tuning of the F1 to the f0 accompanying pitch raising and (ii) the wide harmonic spacing of the voice source resulting in the undersampling of the vocal tract transfer function. Therefore, sung vowel intelligibility is expected to decrease as the f0 increases. Based on the literature of speech perception, it is often suggested that sung vowels are better perceived if uttered in consonantal (CVC) context than in isolation even at high f0. The results for singing, however, are contradictory. In the present study, we further investigate this question. We compare vowel identification in sense and nonsense CVC sequences and show that the positive effect of the context disappears if the number of legal choices in a perception test is similar in both conditions, meaning that any positive effect of the CVC context may only stem from the smaller number of possible responses, i.e., from higher probabilities. Additionally, it is also tested whether the training in production (i.e., singing training) may also lead to a perceptual advantage of the singers over nonsingers in the identification of high-pitched sung vowels. The results show no advantage of this kind.

The acoustical role of vocal tract in the horseshoe bat, Rhinolophus pusillus.

The sound field distribution in the vocal tract with a single sound source in the glottis and the transfer function of the supraglottal vocal tract of the horseshoe bat, Rhinolophus pusillus, have been obtained using the finite-element method (FEM) technique. The models of vocal tracts used for FEM calculation are constructed by tomography scanning. These models are used to set up a finite-element model for calculating the sound field distribution by loading the unit sound source in the glottis. By changing the frequency of the unit sound source, the frequency response was figured out and the acoustic role of vocal tract chambers was examined by obtaining the transfer function and sound pressure distribution before and after filling the chambers using voxels. Sound pressures in the trachea and nostrils are recorded and some analysis of the acoustics of the subglottal and vocal tract was made to find the function of the construction in the vocal tract and subglottal parts. The results show nasal chambers can effectively improve the Q (quality factor) value near the second harmonic, and alternate the sound distribution in the supraglottal part. Whereas the tracheal chambers can reduce the amplitude second harmonic in the subglottal part, its function is like a notch filter which can block the second harmonic component of the back propagation sound under the glottis.

Formant measurement in children’s speech based on spectral filtering

Children’s speech presents a challenging problem for formant frequency measurement. In part, this is because high fundamental frequencies, typical of a children’s speech production, generate widely spaced harmonic components that may undersample the spectral shape of the vocal tract transfer function. In addition, there is often a weakening of upper harmonic energy and a noise component due to glottal turbulence. The purpose of this study was to develop a formant measurement technique based on cepstral analysis that does not require modification of the cepstrum itself or transformation back to the spectral domain. Instead, a narrow-band spectrum is low-pass filtered with a cutoff point (i.e., cutoff “quefrency” in the terminology of cepstral analysis) to preserve only the spectral envelope. To test the method, speech representative of a 2–3 year-old child was simulated with an airway modulation model of speech production. The model, which includes physiologically-scaled vocal folds and vocal tract, generates sound output analogous to a microphone signal. The vocal tract resonance frequencies can be calculated independently of the output signal and thus provide test cases that allow for assessing the accuracy of the formant tracking algorithm. When applied to the simulated child-like speech, the spectral filtering approach was shown to provide a clear spectrographic representation of formant change over the time course of the signal, and facilitates tracking formant frequencies for further analysis.

Formant frequencies and bandwidths of the vocal tract transfer function are affected by the mechanical impedance of the vocal tract wall.

The acoustical properties of the vocal tract, the air-filled cavity between the vocal folds and the mouth opening, are determined by its individual geometry, the physical properties of the air and of its boundaries. In this article, we address the necessity of complex impedance boundary conditions at the mouth opening and at the border of the acoustical domain inside the human vocal tract. Using finite element models based on MRI data for spoken and sung vowels /a/, /i/ and // and comparison of the transfer characteristics by analysis of acoustical data using an inverse filtering method, the global wall impedance showed a frequency-dependent behaviour and depends on the produced vowel and therefore on the individual vocal tract geometry. The values of the normalised inertial component (represented by the imaginary part of the impedance) ranged from 250,hbox {g}/hbox {m}^{2} at frequencies higher than about 3 kHz up to about 2.5times 10^{5},hbox {g}/hbox {m}^{2} in the mid-frequency range around 1.5–3 kHz. In contrast, the normalised dissipation (represented by the real part of the impedance) ranged from 65 to 4.5times 10^{5},hbox {Ns}/hbox {m}^{3}. These results indicate that structures enclosing the vocal tract (e.g. oral and pharyngeal mucosa and muscle tissues), especially their mechanical properties, influence the transfer of the acoustical energy and the position and bandwidth of the formant frequencies. It implies that the timbre characteristics of vowel sounds are likely to be tuned by specific control of relaxation and strain of the surrounding structures of the vocal tract.

Glottal Spectral Separation for Speech Synthesis

This paper proposes an analysis method to separate the glottal source and vocal tract components of speech that is called Glottal Spectral Separation (GSS). This method can produce high-quality synthetic speech using an acoustic glottal source model. In the source-filter models commonly used in speech technology applications it is assumed the source is a spectrally flat excitation signal and the vocal tract filter can be represented by the spectral envelope of speech. Although this model can produce high-quality speech, it has limitations for voice transformation because it does not allow control over glottal parameters which are correlated with voice quality. The main problem with using a speech model that better represents the glottal source and the vocal tract filter is that current analysis methods for separating these components are not robust enough to produce the same speech quality as using a model based on the spectral envelope of speech. The proposed GSS method is an attempt to overcome this problem, and consists of the following three steps. Initially, the glottal source signal is estimated from the speech signal. Then, the speech spectrum is divided by the spectral envelope of the glottal source signal in order to remove the glottal source effects from the speech signal. Finally, the vocal tract transfer function is obtained by computing the spectral envelope of the resulting signal. In this work, the glottal source signal is represented using the Liljencrants-Fant model (LF-model). The experiments we present here show that the analysis-synthesis technique based on GSS can produce speech comparable to that of a high-quality vocoder that is based on the spectral envelope representation. However, it also permit control over voice qualities, namely to transform a modal voice into breathy and tense, by modifying the glottal parameters.

Modeling the Vocal Tract Transfer Function Using a 3D Digital Waveguide Mesh

The digital waveguide mesh has been shown to be capable of reproducing the acoustic impulse response of cylindrical vocal tract analogs. This study extends the same methodology to three-dimensional simulation of the acoustic response of graphical models of the vocal tract obtained from magnetic resonance imaging for a group of trained subjects. By such simulation of the vocal tract transfer function and convolution with an appropriate source waveform, basic phonemes are resynthesized and compared with benchmark audio recordings. The technologies and techniques used for simulation are described, alongside the protocol for image capture and the process for collection of benchmark audio. The results of simulation and acoustic recording are then evaluated and compared. The value of three-dimensional simulation in comparison to existing lower-dimensionality equivalents is assessed. It is found that while three-dimensional simulation provides a strong representation of the low frequency vocal tract transfer function, at higher frequencies its performance becomes geometry-dependent. MRI imaging and benchmark audio is provided for future studies and to permit comparison with comparable means of acoustic simulation.