Interference between normal vibrato and artificial stimulation of laryngeal muscles at near-vibrato rates

Vocal Vibrato Characteristics in Historical and Contemporary Opera, Operetta, and Schlager

OBJECTIVES/HYPOTHESIS

Vibrato is a core aesthetic element in singing. It varies considerably by both genre and era. Though studied extensively in Western classical singing over the years, there is a dearth of studies on vibrato in contemporary commercial music. In addressing this research gap, the objective of this study was to find and investigate common crossover song material from the opera, operetta, and Schlager singing styles from the historical early 20th to the contemporary 21st century epochs.

STUDY DESIGN/METHODS

A total of 51 commercial recordings of two songs, "Es muss was Wunderbares sein" by Ralph Benatzky, and "Die ganze Welt ist himmelblau" by Robert Stolz, from "The White Horse Inn" ("Im weißen Rößl") were collected from opera, operetta, and Schlager singers. Each sample was annotated using Praat and analyzed in a custom Matlab- and Python-based algorithmic approach of singing voice separation and sine wave fitting novel to vibrato research.

RESULTS

With respect to vibrato rate and extent, the three most notable findings were that (1) f_o and vibrato were inherently connected; (2) Schlager, as a historical aesthetic category, has unique vibrato characteristics, with higher overall rate and lower overall extent; and (3) f_o and vibrato extent varied over time based on the historical or contemporary recording year for each genre.

CONCLUSIONS

Though these results should be interpreted with caution due to the limited sample size, conducting such acoustical analysis is relevant for voice pedagogy. This study sheds light on the complexity of vocal vibrato production physiology and acoustics while providing insight into various aesthetic choices when performing music of different genres and stylistic time periods. In the age of crossover singing training and commercially available recordings, this investigation reveals important distinctions regarding vocal vibrato across genres and eras that bear beneficial implications for singers and teachers of singing.

The Role of Vibrato in Group Singing: A Systematic Review

INTRODUCTION

Arguably one of the most controversial features of the singing voice, especially in a choral context, is the phenomenon of vibrato. Numerous pedagogical articles, mostly written by experienced choral singers or directors, discuss the importance of vibrato, sharing anecdotal insights about its control (often advocating reduction) in choral blend and vocal health.

OBJECTIVES

This systematic review aims to identify the main questions posed in the empirical study of vibrato during group singing, and how they have been addressed to date through empirical investigations.

METHODS

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses was employed for this review. The review team consisted of the two authors. A data extraction form was designed to capture information about the participants (including age, gender, music and singing experience), study methods, experimental procedures, outcome measures, and statistical findings.

RESULTS

Seventeen studies were included in the review and revealed that (a) relatively few empirical investigations have studied vibrato in vocal ensembles; (b) the majority focused on vibrato production rather than perception; (c) methodological approaches include Synthesis, Multi-track recordings, Stereo/Binaural recordings, and Lx/Contact recordings; and (d) the studies focused on the main themes of Adjustment, Identification, Interaction, Intonation, and Style.

CONCLUSION

With the current body of literature, it is not possible to draw general conclusions around vibrato behavior during group singing. However, the review highlights the main subareas of interest and valuable methods which have been established and through which future research can collectively shed further light on the role of vibrato in choral singing.

The Influence of Voice Training on Vocal Learner's Objective Acoustic Voice Components

OBJECTIVE

Acoustic parameters of voice were studied in music majors throughout 18 months of training to understand the influence of voice training on voice.

METHODS

Twenty-three students from Xiamen Music School between 12 and 15 years old were enrolled. Acoustic examination was performed three times- every 6 months for 18 months. Various traditional acoustic parameters were measured, including dysphonia severity index (DSI), jitter, and D-value of vocal range. Nonlinear dynamic measures were also measured, including diffusive chaos to construct voice type component profiles (VTCPs), spectrum convergence ratio, and nonlinear energy difference ratio. The results were analyzed by multivariate analysis of variance.

RESULTS

Over the study duration, there was an improvement of DSI (P = 0.002), and D-value of vocal range (P = 0.000). Among nonlinear parameters, only voice type component data demonstrated significant changes during the study duration. Both Voice Type Component 1(VTC1) and VTC3 values differed from Time 1 to Time 2 as well as from Time 1 to Time 3. The proportion of VTC1 in samples generally decreased, while VTC3, representative of aperiodicity, increased. Both nonlinear energy difference ratio and spectrum convergence ratio exhibited no significant changes throughout the study.

CONCLUSION

Professional voice training can improve DSI and D-value of vocal range in singers' voices. These parameters have potential to be used for voice training evaluation and screening. Many nonlinear parameters did not detect differences in the healthy voices studied, but VTCPs created using intrinsic dimension present a valuable new method, visualizing increases in aperiodicity of the speaking voices after professional voice training.

Delay-induced low-frequency modulation of the voice during sustained phonation

An important property of negative feedback systems is the tendency to oscillate when feedback is delayed. This paper evaluated this phenomenon in a sustained phonation task, where subjects prolonged a vowel with 0-600 ms delays in auditory feedback. This resulted in a delay-dependent vocal wow: from 0.4 to 1 Hz fluctuations in fundamental frequency and intensity that increased in period and amplitude as the delay increased. A similar modulation in low-frequency oscillations was not observed in the first two formant frequencies, although some subjects did display increased variability. Results suggest that delayed auditory feedback enhances an existing periodic fluctuation in the voice, with a more complex, possibly indirect, influence on supraglottal articulation. These findings have important implications for understanding how speech may be affected by artificially applied or disease-based delays in sensory feedback.

Strain modulations as a mechanism to reduce stress relaxation in laryngeal tissues

Vocal fold tissues in animal and human species undergo deformation processes at several types of loading rates: a slow strain involved in vocal fold posturing (on the order of 1 Hz or so), cyclic and faster posturing often found in speech tasks or vocal embellishment (1–10 Hz), and shear strain associated with vocal fold vibration during phonation (100 Hz and higher). Relevant to these deformation patterns are the viscous properties of laryngeal tissues, which exhibit non-linear stress relaxation and recovery. In the current study, a large strain time-dependent constitutive model of human vocal fold tissue is used to investigate effects of phonatory posturing cyclic strain in the range of 1 Hz to 10 Hz. Tissue data for two subjects are considered and used to contrast the potential effects of age. Results suggest that modulation frequency and extent (amplitude), as well as the amount of vocal fold overall strain, all affect the change in stress relaxation with modulation added. Generally, the vocal fold cover reduces the rate of relaxation while the opposite is true for the vocal ligament. Further, higher modulation frequencies appear to reduce the rate of relaxation, primarily affecting the ligament. The potential benefits of cyclic strain, often found in vibrato (around 5 Hz modulation) and intonational inflection, are discussed in terms of vocal effort and vocal pitch maintenance. Additionally, elderly tissue appears to not exhibit these benefits to modulation. The exacerbating effect such modulations may have on certain voice disorders, such as muscle tension dysphonia, are explored.

Vocal indices of stress: a review

OBJECTIVE

Identification of stress patterns in the voice has multiple potential applications. The objective was to review literature pertaining to the effects of various forms of stress upon the healthy voice.

STUDY DESIGN

Literature review, discussion of results, and direction for further study.

METHODS

This review article offers a model of stress and a review of the historical and recent research into the effects of stress on the voice. Electronic databases were searched using the key words. No studies were excluded on the basis of design; however, an attempt was made to include in the discussion studies which primarily address physiological and acoustic vocal parameters. The results of greater than 50 studies examining the effect of stressors ranging from lie and guilt to high altitude and space flight upon the voice were included in the review.

RESULTS

Increase in fundamental frequency is the most commonly reported effect of stress in well-controlled trials. The trend, however, is not universal. A reduction in noise as reflected by the diminished vocal jitter is reported, but less frequently.

CONCLUSIONS

Stress types, gender, and individual differences in baseline autonomic tone may explain the primarily equivocal findings of effects of stressor exposure or perceived stress on voice; and as such, the article concludes with a discussion of directions for future study.

Change in vibrato rate and extent during tertiary training in classical singing students

OBJECTIVES

Vibrato is an integral and desirable feature of the classical singing voice. For elite singing students, achieving and developing vibrato may constitute one of the essential elements of their vocal training, although it is not necessarily a focus of that training.

STUDY DESIGN

In this longitudinal study, we measured vibrato rate (VR) and vibrato extent (VE) and regularity (SD) of VR and VE in student singers over the course of four semesters of tertiary level voice training at a conservatorium of music to determine how these parameters changed during training.

METHOD

Fifteen singers completed four semesters (2 years) of training. Singers performed four sustained pitches across their vocal range. Peaks and troughs of vibrato were isolated from the fundamental frequency trace to calculate VR in hertz and VE in semitones.

RESULTS

Analysis using linear mixed models revealed significant increases in VE and decreases in VRSD over time. VR was within expected limits for classical singers in all semesters, and small VR reductions were not statistically significant between semesters over 2 years of training. VE showed significant increases between the start of year 1 and year 2. Periodicity of singers' VR (SD) improved over training, with significant decreases to VRSD over time. There was no significant change to VESD.

CONCLUSIONS

Future studies will ascertain whether further changes to VR and VE occur over longer training periods, or whether the major changes occur early in tertiary training.

The impact of ‘open throat’ technique on vibrato rate, extent and onset in classical singing

Mitchell, Kenny et al. (2003) identified 'open throat' as integral to the production of an even and consistent sound in classical singing. In this study, we compared vibrato rate, extent and onset of six advanced singing students under three conditions: 'optimal' (O), representing maximal open throat; 'sub-optimal' (SO), using reduced open throat; and loud sub-optimal (LSO), using reduced open throat but controlling for the effect of loudness. Fifteen expert judges correctly identified the sound produced when singers used open throat with 85% accuracy. Having verified the technique perceptually, we used a series of univariate repeated measures ANOVAs with planned orthogonal contrasts to test the hypotheses that frequency modulations associated with vibrato rate, extent and onset would vary outside acceptable or desirable parameters for SO and LSO. Hypotheses were confirmed for vibrato extent and onset but not for rate. There were no significant differences between SO and LSO on any of the vibrato parameters. As vibrato is considered a key indicator of good singing, these findings suggest that open throat is important to the production of a good sound in classical singing.

Laryngeal-Level Amplitude Modulation in Vibrato

The goal of this investigation was to test a new methodology for measuring amplitude modulation (AM) at the level of the vocal folds during vibrato in trained singers, because previous research has suggested that AM arises in large part as an acoustic epiphenomenon through an interaction of the harmonics in the laryngeal source with the resonances of the vocal tract as the fundamental frequency oscillates. A within-subjects model was used to compare vocal activity across three pitch and three loudness conditions. Seventeen female singers with a range of training and experience were recorded with a microphone and an electroglottograph (EGG). Fluctuations in the ratio of closing to opening peaks in the first derivative of the EGG signal were used as an index of laryngeal-level AM. Evidence of laryngeal AM was found to a greater or lesser extent in all the singers, and its extent was not related to the degree of training. Across singers and pitch conditions, it was more prominent at lower intensities. The differentiated EGG signal lends itself to the measurement of AM at the level of the larynx, and the extent of the modulation appears more related to the level of vocal effort than to individual singer characteristics.

Patterns of Laryngeal Electromyography and the Activity of the Respiratory System During Spontaneous Laughter

Laryngeal muscle electromyography (EMG) and measures of the behavior of the respiratory system have been made during spontaneous laughter in two groups of subjects. The smaller group also had a direct measure of tracheal pressure during this behavior. Laryngeal adductors such as the thyroarytenoid (TA) and lateral cricoarytenoid (LCA) exhibited brief high-amplitude bursts of activity, at a rate of approximately 5 Hz, which were usually associated on a 1 : 1 basis with the sound bursts (ha ha ha) of laughter. The laryngeal abductor, posterior cricoarytenoid (PCA), also showed bursts of activity that were out of phase with TA and LCA. The cricothyroid (CT) was only weakly, if at all, modulated during the bursting activity of the other laryngeal muscles. Tracheal pressure usually exhibited positive pressure pulses during laughter that were often, but not always, temporally correlated to the bursts of laryngeal adductor EMG activity. Such pressure modulations appeared to precisely determine when—and if—phonation was produced during the laugh. During laughter, laryngeal EMG is highly stereotyped both within and between subjects. In most instances, this activity appears to be supported by coordinated pulses of tracheal pressure. The periaqueductal gray (PAG) has been shown in animal studies to produce emotionally indicative vocalizations, in which the laryngeal and respiratory system are coordinated. Therefore, it is suggested that the PAG is involved with the production of laughter.

The role of auditory feedback in sustaining vocal vibrato

Vocal vibrato and tremor are characterized by oscillations in voice fundamental frequency (F0). These oscillations may be sustained by a control loop within the auditory system. One component of the control loop is the pitch-shift reflex (PSR). The PSR is a closed loop negative feedback reflex that is triggered in response to discrepancies between intended and perceived pitch with a latency of approximately 100 ms. Consecutive compensatory reflexive responses lead to oscillations in pitch every approximately 200 ms, resulting in approximately 5-Hz modulation of F0. Pitch-shift reflexes were elicited experimentally in six subjects while they sustained /u/ vowels at a comfortable pitch and loudness. Auditory feedback was sinusoidally modulated at discrete integer frequencies (1 to 10 Hz) with +/- 25 cents amplitude. Modulated auditory feedback induced oscillations in voice F0 output of all subjects at rates consistent with vocal vibrato and tremor. Transfer functions revealed peak gains at 4 to 7 Hz in all subjects, with an average peak gain at 5 Hz. These gains occurred in the modulation frequency region where the voice output and auditory feedback signals were in phase. A control loop in the auditory system may sustain vocal vibrato and tremorlike oscillations in voice F0.

Vibrato rate adjustment

The goal of the present study was to document the acoustic changes that occur as singers attempt to increase or decrease their vibrato rate to match target stimuli. Eight advanced singing students produced vowels with vibrato in three registers, both naturally and while attempting to match faster or slower rate stimuli. Slower rates were associated with lower intensity and less steady vibrato. Faster rates involved increased vibrato extent in the chest register and increased intensity in the head register. Singers whose spontaneous vibrato rates were naturally either slower or faster tended to also be relatively slower or faster when matching target rates. This ability to modify rate may have beneficial effects on the artistic quality of the voice for performance.

The influence of pitch and loudness changes on the acoustics of vocal tremor

The effect of tremor on phonation is to modulate an otherwise steady sound source in its amplitude, fundamental frequency, or both. The severity of untreated vocal tremor has been reported to change under certain conditions that may be related to muscle tension. In order to better understand the phenomenon of vocal tremor, its acoustic properties were examined as individuals volitionally altered their pitch and loudness. These voice conditions were anticipated to alter the tension of the intrinsic laryngeal muscles. The voices of 10 individuals with a diagnosis of vocal tremor were recorded before participating in a longitudinal treatment study. They produced vowels at low and high pitch and loudness levels as well as in a comfortable voice condition. Acoustic analyses quantified the amplitude and frequency modulations of the speakers' voices across the various conditions. Individual speakers varied in the way the pitch and loudness changes affected their tremor, but the following statistically significant effects for the speakers as a group were observed: Higher pitch phonation was associated with a more rapid rate for both amplitude and frequency modulations. Amplitude modulation become faster for louder phonation. Low-pitched phonotion led to decreases in the extent of amplitude tremor. Varying pitch led to dramatic changes in the phase relationship between amplitude and frequency modulation in some of the speakers, whereas this effect was not apparent in other speakers.

A reflex resonance model of vocal vibrato

A reflex mechanism with a long latency (>40 ms) is implicated as a plausible cause of vocal vibrato. At least one pair of agonist-antagonist muscles that can change vocal-fold length is needed, such as the cricothyroid muscle paired with the thyroarytenoid muscle, or the cricothyroid muscle paired with the lateral cricoarytenoid muscle or a strap muscle. Such an agonist-antagonist muscle pair can produce negative feedback instability in vocal-fold length with this long reflex latency, producing oscillations on the order of 5-7 Hz. It is shown that singers appear to increase the gain in the reflex loop to cultivate the vibrato, which grows out of a spectrum of 0-15-Hz physiologic tremors in raw form.

Application of singing techniques for the treatment of dysphonia

Management of voice disorders requires careful examination of vocal control and compensation, with exercises designed to improve efficient compensation while reducing effects of pathology on vocal production. Specific singing exercises have been proven to be beneficial in the resolution and compensation for pathology. Detailed assessment and intervention strategies are outlined from the perspective of the vocologist, who specializes in combining vocal arts with voice therapy.

Botulinum toxin injections for essential voice tremor

Fifteen patients, 13 women and 2 men, with a mean age of 72.7 years (56 to 86 years) and a clinical diagnosis of essential voice tremor, were treated with botulinum injections to the thyroarytenoid muscles, and in some cases, to the cricothyroid or thyrohyoid muscles. Evaluations were based on subjective judgments by the patients, and on perceptual and acoustic analysis of voice recordings. Subjective evaluations indicated that the treatment had a beneficial effect in 67% of the patients. Perceptual evaluations showed a significant decrease in voice tremor during connected speech (p < .05). Acoustic analysis showed a nearly significant decrease in the fundamental frequency variations (p = .06) and a significant decrease in fundamental frequency during sustained vowel phonation (p < .01 ). The results of perceptual evaluation coincided most closely with the subjective judgments. It was concluded that the treatment was successful in 50% to 65% of the patients, depending on the method of evaluation.

Modulation of fundamental frequency by laryngeal muscles during vibrato

The variations in voice fundamental frequency (F0) that occur during vibrato production may be produced, at least in part, by modulation of laryngeal muscle activity. We have quantified this relation by using a cross-correlation analysis of the changes in F0 during vibrato and the changes either in motor unit firing rate or in gross electromyographic activity from the cricothyroid (CT) and the thyroarytenoid (TA) muscles. Two trained amateur tenors provided the data. Correlations were generally quite strong (mean r for the CT was 0.72 for singer 1 and 0.50 for singer 2; mean r for the TA was 0.31 for singer 2), thus providing support for previous evidence that fundamental frequency modulation in vibrato involves active modulation of the laryngeal motoneuron pool, especially by the CT muscle. In addition, phase delays between muscle modulation and changes in fundamental frequency were substantial (averaging approximately 130 degrees for the CT and 140 degrees for the TA). This finding may help provide insight regarding the mechanisms responsible for the production of vibrato.

A clearer view of singing voice production: 25 years of progress

Voice research has enjoyed its most productive period of history during the past 25 years. Many of the enigmas related to the biomechanics and acoustics of the singing voice have been resolved. This paper presents state-of-the-art understanding regarding the following topics: vibrato, the singer's formant, formant tracking, voice registers, subglottal pressure, voice classification, modes of vocal fold vibration, laryngeal position during singing, flow glottography, and singing synthesis. In addition to these topics, the people who have made the most significant contributions to the advancement of singing research are recognized.