Dissimilarity and the Classification of Male Singing Voices

Traditionally, timbre has been defined as that perceptual attribute that differentiates two sounds when pitch and loudness are equal, and thus is a measure of dissimilarity. By such a definition, each voice possesses a set of timbres, and the ability to identify any voice or voice category across different pitch-loudness-vowel combinations must be due to an ability to "link" these timbres by abstracting the "timbre transformation," the manner in which timbre subtly changes across pitch and loudness for a specific voice or voice category. Using stimuli produced across the singing range by singers from different voice categories, this study sought to examine how timbre and pitch interact in the perception of dissimilarity in male singing voices. This study also investigated whether or not listener experience affects the perception of timbre as a function of pitch. The resulting multidimensional scaling (MDS) representations showed that for all stimuli and listeners, dimension 1 correlated with pitch, while dimension 2 correlated with spectral centroid and separated vocal stimuli into the categories baritone and tenor. Dimension 3 appeared highly idiosyncratic depending on the nature of the stimuli and on the experience of the listener. Inexperienced listeners appeared to rely more heavily on pitch in making dissimilarity judgments than did experienced listeners. The resulting MDS representations of dissimilarity across pitch provide a glimpse of the timbre transformation of voice categories across pitch.     

Dissimilarity and the Classification of Female Singing Voices: A Preliminary Study

Traditionally, timbre has been defined as that perceptual attribute that differentiates two sounds when pitch and loudness are equal and thus is a measure of dissimilarity. By such a definition, each voice possesses a set of timbres, and the identity of any voice or voice category across different pitch-loudness-vowel combinations must be due to an abstraction of the pattern of timbre transformation. Using stimuli produced across the singing range by singers from different voice categories, this study sought to examine how timbre and pitch interact in the perception of dissimilarity. This study also investigated whether listener experience affects the perception of timbre as a function of pitch. The resulting multidimensional scaling (MDS) representations showed that for all stimuli and listeners, dimension 1 correlated with pitch, whereas dimension 2 correlated with spectral centroid and separated vocal stimuli into the categories mezzo-soprano and soprano. Dimension 3 appeared highly idiosyncratic depending on the nature of the stimuli and on the experience of the listener. Inexperienced listeners appeared to rely more heavily on pitch in making dissimilarity judgments than did experienced listeners. The resulting MDS representations of dissimilarity across pitch provide a glimpse of the timbre transformation of voice categories across pitch.     

Effect of Dysphonia and Cognitive-Perceptual Listener Strategies on Speech Intelligibility

There is a high prevalence of dysphonia among professional voice users and the impact of the disordered voice on the speaker is well documented. However, there is minimal research on the impact of the disordered voice on the listener. Considering that professional voice users include teachers and air-traffic controllers, among others, it is imperative to determine the impact of a disordered voice on the listener. To address this, the objectives of the current study included: (1) determine whether there are differences in speech intelligibility between individuals with healthy voices and those with dysphonia; (2) understand whether cognitive-perceptual strategies increase speech intelligibility for dysphonic speakers; and (3) determine the relationship between subjective voice quality ratings and speech intelligibility. Sentence stimuli were recorded from 12 speakers with dysphonia and four age- and gender-matched typical, healthy speakers and presented to 129 healthy listeners divided into one of three strategy groups (ie, control, acknowledgement, and listener strategies). Four expert raters also completed a perceptual voice assessment using the Consensus Assessment Perceptual Evaluation of Voice for each speaker. Results indicated that dysphonic voices were significantly less intelligible than healthy voices (P ≤ 0.001) and the use of cognitive-perceptual strategies provided to the listener did not significantly improve speech intelligibility scores (P = 0.602). Using the subjective voice quality ratings, regression analysis found that breathiness was able to predict 41% of the variance associated with number of errors (P = 0.008). Overall results of the study suggest that speakers with dysphonia demonstrate reduced speech intelligibility and that providing the listener with specific strategies may not result in improved intelligibility.     

Sources of listener disagreement in voice quality assessment

Traditional interval or ordinal rating scale protocols appear to be poorly suited to measuring vocal quality. To investigate why this might be so, listeners were asked to classify pathological voices as having or not having different voice qualities. It was reasoned that this simple task would allow listeners to focus on the kind of quality a voice had, rather than how much of a quality it possessed, and thus might provide evidence for the validity of traditional vocal qualities. In experiment 1, listeners judged whether natural pathological voice samples were or were not primarily breathy and rough. Listener agreement in both tasks was above chance, but listeners agreed poorly that individual voices belonged in particular perceptual classes. To determine whether these results reflect listeners' difficulty agreeing about single perceptual attributes of complex stimuli, listeners in experiment 2 classified natural pathological voices and synthetic stimuli (varying in f0 only) as low pitched or not low pitched. If disagreements derive from difficulties dividing an auditory continuum consistently, then patterns of agreement should be similar for both kinds of stimuli. In fact, listener agreement was significantly better for the synthetic stimuli than for the natural voices. Difficulty isolating single perceptual dimensions of complex stimuli thus appears to be one reason why traditional unidimensional rating protocols are unsuited to measuring pathologic voice quality. Listeners did agree that a few aphonic voices were breathy, and that a few voices with prominent vocal fry and/or interharmonics were rough. These few cases of agreement may have occurred because the acoustic characteristics of the voices in question corresponded to the limiting case of the quality being judged. Values of f0 that generated listener agreement in experiment 2 were more extreme for natural than for synthetic stimuli, consistent with this interpretation.     

The use of an auditory model in predicting perceptual ratings of breathy voice quality

Despite much research, the relationship between vocal acoustic signals and perceived voice quality is not well understood. The present study used an auditory model proposed by Moore et al¹⁰ to study how changes in the acoustic spectrum may relate to changes in perceptual ratings of breathiness. Perceptual ratings of breathiness were obtained using a multidimensional scaling (MDS) design. The stimulus distances on the dominant MDS dimension were correlated with several commonly used acoustic measures for voice quality. These distances were also compared with measures obtained from the output of the auditory model. Results show that the partial loudness of the harmonic energy obtained with the aspiration noise acting as a masker was the most important predictor of perceptual ratings of breathiness. Results also demonstrate that measures obtained from the auditory spectrum were better predictors of perceptual ratings of breathiness than were commonly used acoustic spectral measures.     

Perceptual sensitivity to first harmonic amplitude in the voice source

Little is known about the perceptual importance of changes in the shape of the source spectrum, although many measures have been proposed and correlations with different vocal qualities (breathiness, roughness, nasality, strain...) have frequently been reported. This study investigated just-noticeable differences in the relative amplitudes of the first two harmonics (H1-H2) for speakers of Mandarin and English. Listeners heard pairs of vowels that differed only in the amplitude of the first harmonic and judged whether or not the voice tokens were identical in voice quality. Across voices and listeners, just-noticeable-differences averaged 3.18 dB. This value is small relative to the range of values across voices, indicating that H1-H2 is a perceptually valid acoustic measure of vocal quality. For both groups of listeners, differences in the amplitude of the first harmonic were easier to detect when the source spectral slope was steeply falling so that F0 dominated the spectrum. Mandarin speakers were significantly more sensitive (by about 1 dB) to differences in first harmonic amplitudes than were English speakers. Two explanations for these results are possible: Mandarin speakers may have learned to hear changes in harmonic amplitudes due to changes in voice quality that are correlated with the tones of Mandarin; or Mandarin speakers' experience with tonal contrasts may increase their sensitivity to small differences in the amplitude of F0 (which is also the first harmonic).     

When and why listeners disagree in voice quality assessment tasks

Modeling sources of listener variability in voice quality assessment is the first step in developing reliable, valid protocols for measuring quality, and provides insight into the reasons that listeners disagree in their quality assessments. This study examined the adequacy of one such model by quantifying the contributions of four factors to interrater variability: instability of listeners' internal standards for different qualities, difficulties isolating individual attributes in voice patterns, scale resolution, and the magnitude of the attribute being measured. One hundred twenty listeners in six experiments assessed vocal quality in tasks that differed in scale resolution, in the presence/absence of comparison stimuli, and in the extent to which the comparison stimuli (if present) matched the target voices. These factors accounted for 84.2% of the variance in the likelihood that listeners would agree exactly in their assessments. Providing listeners with comparison stimuli that matched the target voices doubled the likelihood that they would agree exactly. Listeners also agreed significantly better when assessing quality on continuous versus six-point scales. These results indicate that interrater variability is an issue of task design, not of listener unreliability.     

Voice Perceptions and Quality of Life of Transgender People

Despite the plethora of research documenting that the voice and quality of life (QoL) are related, the exact nature of this relationship is vague. Studies have not addressed people who consider their voice to influence their life and identity, but would not be considered to have a voice “disorder” (e.g., transgender individuals). Individuals seeking vocal feminization may or may not have vocal pathology and often have concerns not addressed on the standard psychosocial measures of voice impact. Recent development of a voice-related QoL measure specific to the needs of transgender care (Transgender Self-Evaluation Questionnaire [TSEQ]) affords opportunity to explore relationships between self-perceived QoL and perceptions of femininity and likability associated with transgender voice. Twenty male-to-female transgender individuals living as a female 100% of the time completed the TSEQ and contributed a speech sample describing Norman Rockwell’s “The Waiting Room” picture. Twenty-five undergraduate listeners rated voice femininity and voice likability after audio-only presentation of each speech sample. Speakers also self-rated their voices on these parameters. For male-to-female transgender clients, QoL is moderately correlated with how others perceive their voice. QoL ratings correlate more strongly with speaker’s self-rated perception of voice compared with others’ perceptions, more so for likability than femininity. This study complements previous research reports that subjective measures from clients and listeners may be valuable for evaluating the effectiveness of treatment in terms of how treatment influences voice-related QoL issues for transgender people.     

Communicative Suitability of Voice Following Radiotherapy for T1 Glottic Carcinoma: Testing the Reliability of a Rating Instrument

The objective of the study was to determine whether a communicative suitability rating instrument could be used in a meaningful way to assess functionality of voice following radiotherapy for T1 glottic cancer. Seventeen naive listeners judged the suitability of voice of a patient group with T1 glottic carcinoma (n = 20) just before treatment, a group of patients (n = 40) after radiotherapy, and a matched control group (n = 20) of normal speakers. Listeners rated suitability on a 10-point scale for 10 speaking situations, which supposedly make different demands. In order to validate scores on communicative suitability, ratings were related to perceptual voice quality evaluations and videolaryngostroboscopic evaluations. Results indicate that the concept of measuring listener judgments of communicative suitability of voice is basically sound. Raters are reliable and can discriminate between groups of normal and pathological voices. Patients with T1 glottic carcinoma (assessed before the start of treatment) have on average the least suitable voices. Following radiotherapy suitability is, on average, improved, but does not approach the suitability of normal voices. Ratings on communicative suitability were clearly related to perceptual voice quality aspects and videolaryngostroboscopic evaluations. A subset of three communicative suitability rating scales is recommended as part of the protocol for evaluating voice outcome after radiotherapy for early glottic cancer, besides perceptual evaluation of voice quality by trained and naive raters, videolaryngostroboscopy, acoustical analyses, and self-ratings of vocal performance.     

Some difference limens for the perception of breathiness

Perception of breathy voice quality appears to be cued by changes in the vowel spectrum. These changes are related to alterations in the intensity of aspiration noise and spectral slope of the harmonic energy [Shrivastav and Sapienza, J. Acoust. Soc. Am., 114 (4), 2217-2224 (2003)]. Ten young-adult listeners with normal hearing were tested using an adaptive listening task to determine the smallest change in signal-to-noise ratio that resulted in a change in breathiness. Six vowel continua, three female and three male, were generated using a Klatt synthesizer and served as stimuli. Results showed that listeners needed as much as 20-dB increase in aspiration noise to perceive a change in breathiness against a relatively normal voice. In contrast, listeners needed approximately an 11-dB increase in aspiration noise to discriminate breathiness against a severely breathy voice. The difference limens for breathiness were observed to vary across the six talkers. Voices having aspiration noise that was predominantly in the high frequencies had smaller difference limens. No significant differences for male and female voice were observed.