Vocal Tract Length Differences Research Articles

Effects of age and musical expertise on perception of speech in speech maskers in adults

Perceiving “cocktail party” speech, or speech-on-speech (SoS), requires perceptual mechanisms such as segregating target from masking speech using voice cues, and on cognitive mechanisms such as selective attention and inhibition. Both aging and musical expertise have been shown to affect these mechanisms. Voice cues that help distinguish different speakers include mean fundamental frequency (F0), related to voice pitch, and the vocal-tract length (VTL), related to speaker size. Some studies reported older adults’ decreased sensitivity to F0 differences, possibly affecting their ability to discriminate speakers. Furthermore, age-related cognitive changes may lead to difficulties in attention direction and inhibition. Compared to non-musicians, musicians are reported to show enhanced processing of acoustic features such as F0, as well as enhanced cognitive abilities such as auditory attention skills and working memory. While this intuitively could lead to a musician advantage for SoS, reports of musicians outperforming non-musicians are inconsistent across both younger and older adults. Differences across previous SoS paradigms have made comparison of musicianship advantages in SoS in younger and older adults—and underlying mechanisms—difficult. Therefore, we investigated the extent to which older compared to younger adults benefit from either musical expertise or F0 and VTL differences between target and masker voices.

Voice cue sensitivity in children with cochlear implants and with hearing aids

Voice cues, such as fundamental frequency (F0) and vocal-tract length (VTL), allow listeners to distinguish speakers, which can facilitate speech perception in challenging listening conditions. Children with typical hearing continue to develop their sensitivity to F0 and VTL differences throughout childhood, but this is not as clear for children with hearing loss. In prelingually deaf implanted children with cochlear implants (CIs), reduced spectrotemporal details may cause a delay or even a plateau in the development of voice cue sensitivity, while neuroplasticity may compensate for these factors. For children with hearing aids, it is unclear how voice cue perception could be affected by combined factors of neuroplasticity, physiological aspects of hearing loss, and compensatory front-end processing of hearing aids. Here, we will present recent work on F0 and VTL sensitivity in children with CIs and with hearing aids (4–18 years). Our results show a large variability in both CI and hearing-aided children, with some children performing at the level of age-matched normal-hearing children, while others performing lower. Both groups show trends for development as a function of age, but these seem to differ for F0 and VTL. Potential factors that may be contributing to these results will be discussed during the presentation.

School-age children benefit from voice gender cue differences for the perception of speech in competing speech.

Differences in speakers' voice characteristics, such as mean fundamental frequency (F0) and vocal-tract length (VTL), that primarily define speakers' so-called perceived voice gender facilitate the perception of speech in competing speech. Perceiving speech in competing speech is particularly challenging for children, which may relate to their lower sensitivity to differences in voice characteristics than adults. This study investigated the development of the benefit from F0 and VTL differences in school-age children (4-12 years) for separating two competing speakers while tasked with comprehending one of them and also the relationship between this benefit and their corresponding voice discrimination thresholds. Children benefited from differences in F0, VTL, or both cues at all ages tested. This benefit proportionally remained the same across age, although overall accuracy continued to differ from that of adults. Additionally, children's benefit from F0 and VTL differences and their overall accuracy were not related to their discrimination thresholds. Hence, although children's voice discrimination thresholds and speech in competing speech perception abilities develop throughout the school-age years, children already show a benefit from voice gender cue differences early on. Factors other than children's discrimination thresholds seem to relate more closely to their developing speech in competing speech perception abilities.

Independent and Combined Effects of Fundamental Frequency and Vocal Tract Length Differences for School-Age Children's Sentence Recognition in a Two-Talker Masker.

Purpose The purpose of this study was to examine the independent and combined contributions of fundamental frequency (F0) and vocal tract length (VTL) differences on children's speech-in-speech recognition in the presence of a competing two-talker masker. Method Participants were 64 children (5-17 years old) and 25 adults (18-39 years old). Sentence recognition thresholds were measured in a two-talker masker. Target sentences had either the same mean F0 and VTL of the masker or were digitally altered so that the target and masker differed in F0 (Experiment 1), differed in VTL (Experiment 2), or differed in both F0 and VTL (Experiment 3). To determine the benefit, masking release was computed by subtracting thresholds in each shifted condition from the threshold in the unshifted condition. Results Results demonstrate that children's ability to benefit from either F0 or VTL differences (Experiments 1 and 2) depended on listener age, with younger children showing less improvement in speech reception thresholds compared to older children and adults. Age effects were also evident in the combined-cue conditions (Experiment 3), but children showed greater improvements compared to F0-only or VTL-only manipulations. Conclusions There was a prolonged pattern of development in children's ability to benefit from F0 or VTL differences between target and masker speech. Young children failed to capitalize on F0 and VTL differences to the same extent as older children and adults but did show a robust benefit when the cues were combined, supporting the hypothesis that younger children rely more heavily on redundant cues compared to older children and adults.

Effect of Spectral Contrast Enhancement on Speech-on-Speech Intelligibility and Voice Cue Sensitivity in Cochlear Implant Users.

Speech intelligibility in the presence of a competing talker (speech-on-speech; SoS) presents more difficulties for cochlear implant (CI) users compared with normal-hearing listeners. A recent study implied that these difficulties may be related to CI users' low sensitivity to two fundamental voice cues, namely, the fundamental frequency (F0) and the vocal tract length (VTL) of the speaker. Because of the limited spectral resolution in the implant, important spectral cues carrying F0 and VTL information are expected to be distorted. This study aims to address two questions: (1) whether spectral contrast enhancement (SCE), previously shown to enhance CI users' speech intelligibility in the presence of steady state background noise, could also improve CI users' SoS intelligibility, and (2) whether such improvements in SoS from SCE processing are due to enhancements in CI users' sensitivity to F0 and VTL differences between the competing talkers. The effect of SCE on SoS intelligibility and comprehension was measured in two separate tasks in a sample of 14 CI users with Cochlear devices. In the first task, the CI users were asked to repeat the sentence spoken by the target speaker in the presence of a single competing talker. The competing talker was the same target speaker whose F0 and VTL were parametrically manipulated to obtain the different experimental conditions. SoS intelligibility, in terms of the percentage of correctly repeated words from the target sentence, was assessed using the standard advanced combination encoder (ACE) strategy and SCE for each voice condition. In the second task, SoS comprehension accuracy and response times were measured using the same experimental setup as in the first task, but with a different corpus. In the final task, CI users' sensitivity to F0 and VTL differences were measured for the ACE and SCE strategies. The benefit in F0 and VTL discrimination from SCE processing was evaluated with respect to the improvement in SoS perception from SCE. While SCE demonstrated the potential of improving SoS intelligibility in CI users, this effect appeared to stem from SCE improving the overall signal to noise ratio in SoS rather than improving the sensitivity to the underlying F0 and VTL differences. A second key finding of this study was that, contrary to what has been observed in a previous study for childlike voice manipulations, F0 and VTL manipulations of a reference female speaker (target speaker) toward male-like voices provided a small but significant release from masking for the CI users tested. The present findings, together with those previously reported in the literature, indicate that SCE could serve as a possible background-noise-reduction strategy in commercial CI speech processors that could enhance speech intelligibility especially in the presence of background talkers that have longer VTLs compared with the target speaker.

The ΔF method of vocal tract length normalization for vowels

Given the acoustic consequences of physiological differences between talkers, there is a practical need for effective and theoretically motivated procedures of vowel normalization to facilitate comparison of speech produced by people who differ by dialect or language. In addition, there is a question whether listeners might utilize a normalization procedure during speech perception. This paper reports the results of two studies that explore these questions—with particular focus on vocal tract length normalization. Drawing on research in speech engineering, where accurate estimates of vocal tract length are needed in some approaches to automatic speech recognition and speaker verification, a new model of vowel normalization is introduced. The model uses a direct measure of average formant spacing (the ΔF) which can be used to measure vocal tract length. The acoustic consequences of vocal tract length differences are removed from vowel measurements by scaling vowel formant measurements by ΔF. Study 1 found that this method is comparable to Nearey’s (1978) uniform normalization method, while providing an explicit vocal tract length interpretation, and a rationalized unit of measure. Study 2 found that uniform normalization measures (which let each formant serve as a noisy estimator of ΔF) improve vowel classification even with only a couple of randomly selected vowel tokens. This suggests that vocal tract length normalization could be involved in speech perception.

Using Phonetic Posteriorgram Based Frame Pairing for Segmental Accent Conversion

Accent conversion AC aims to transform non-native utterances to sound as if the speaker had a native accent. This can be achieved by mapping source speech spectra from a native speaker into the acoustic space of the target non-native speaker. In prior work, we proposed an AC approach that matches frames between the two speakers based on their acoustic similarity after compensating for differences in vocal tract length. In this paper, we propose a new approach that matches frames between the two speakers based on their phonetic rather than acoustic similarity. Namely, we map frames from the two speakers into a phonetic posteriorgram using speaker-independent acoustic models trained on native speech. We thoroughly evaluate the approach on a speech corpus containing multiple native and non-native speakers. The proposed algorithm outperforms the prior approach, improving ratings of acoustic quality 22% increase in mean opinion score and native accent 69% preference while retaining the voice quality of the non-native speaker. Furthermore, we show that the approach can be used in the reverse conversion direction, i.e., generating speech with a native speaker's voice quality and a non-native accent. Finally, we show that this approach can be applied to non-parallel training data, achieving the same accent conversion performance.

Children’s ability to benefit from fundamental frequency and vocal tract length differences during speech-in-speech recognition

The present study evaluated whether children’s speech-in-speech recognition benefits from differences in fundamental frequency (F0) or vocal tract length (VTL) between the target and masker talkers’ voices. Children, like adults, can benefit from a sex mismatch between competing talkers, but the relative contribution of individual voice characteristics to children’s improved speech-in-speech understanding is unknown. In this study, we first tested children’s ability to use differences in either F0 or VTL between target and masker speech to evaluate the independent influence of these cues on speech-in-speech recognition. Then F0 and VTL differences were combined to determine whether cue redundancy would reduce the child/adult differences observed in these contexts. Sentence recognition thresholds were measured in a two-talker speech masker. All stimuli were recorded by the same female talker. F0 and VTL of the target sentences were manipulated using the pitch-synchronous overlap-add method. Preliminary results suggest a prolonged developmental trajectory in the ability to use F0 or VTL in isolation, but indicate that the combination of these cues benefits children at an earlier age compared to when either cue is presented in isolation. Adults showed the greatest benefit from the F0-only manipulation, showing no additional benefit when F0 and VTL were combined.

A perceptual and acoustic study of voice gender in prepubertal children

Purpose: The present study attempted to investigate the sexual dimorphism in prepubertal children's voice and their implicit knowledge on voice gender.Methods: Fundamental frequency (F0) and the first two formant frequencies (F1 and F2) of voice samples produced by 25 male and 26 female Cantonese-speaking prepubertal children produced under two speaking conditions: (1) natural condition, and (2) imitation condition when mimicking the opposite gender voice were examined.Results: Accuracy of gender identification by adult listeners for voices associated with the natural and imitation conditions was 81.7% and 59.5% respectively. In addition to the lack of difference in F0 between genders under the natural condition, F1 and F2 associated with boys were lower than those with girls, suggesting differences in vocal tract length and possible sex-specific articulatory behaviors between boys and girls, in an attempt to enhance sexual dimorphism for gender voice identification. Under the imitation condition, boys exhibited significantly higher F0 and F1 than girls.Conclusion: Prepubertal children possessed the implicit knowledge of sexually dimorphic acoustic correlates (F0 and F1) and were capable of altering the rate of vocal fold vibration and the effective vocal tract length at will to render vocal characteristics of the opposite sex.

On the color of voices: Does good perception of vocal differences relate to better speech intelligibility in cocktail-party settings?

Understanding speech in cocktail-party settings poses a challenge for cochlear implant (CI) users. Distortions of vocal cues as they are perceived through the implant have been hypothesized to partially contribute to this difficulty. Vocal cues can be defined along two orthogonal dimensions: the fundamental frequency, F0, related to the pitch, and the vocal tract length (VTL), correlated with speaker size. While deficiency in F0 perception in CI users has been long known, recent research has shown a large deficit in VTL perception. How this contributes to speech intelligibility in the presence of a competing talker (speech-on-speech; SOS) remains unknown. In this study, we investigated the relationship between SOS intelligibility and the sensitivity to F0 and VTL differences (JNDs) in 18 CI listeners using two separate speech tasks. Results indicate a strong correlation between voice cue sensitivity and SOS intelligibility: participants who are more sensitive to small differences in F0 and VTL tend to perform better on the two SOS tasks compared to those who are less sensitive. These results are consistent with the hypothesis that degraded voice cue sensitivity contributes to SOS perception deficits, but other potential explanations, like differences in cognitive processing and attention, cannot yet be ruled out. [Funding: The University Medical Center Groningen (UMCG), Advanced Bionics, Netherlands Organization for Scientific Research (NWO).]

Investigating the use of formant frequencies in listener judgments of speaker size

The formant-pattern present in a given vowel sound will be determined by the vocal-tract length (VTL) of the speaker as well as by phoneme-specific information. Although human listeners tend to associate lower formant-frequencies with larger speakers, it is unclear whether they are responding to VTL information in speech sounds, or simply responding to the formant-pattern present in the sound. In this experiment listeners were presented with pairs of synthetic vowels from the set of (/i æ ʊ/), which could differ on the basis of simulated VTL and vowel category, within-pair. Listeners were divided into groups based on the number of formants contained by stimulus vowels (2, 3, 4, and 5-formant vowel groups). For each trial, listeners were asked to indicate which vowel sounded like it had been produced by a taller speaker. Results indicate that listeners do not rely solely on VTL cues when making speaker-size judgments, and that they exhibit biases towards selecting given phonemes as taller, even when contrary to the VTL differences between the voices. Furthermore, the higher formants (up to F5) are used by listeners when making speaker-size judgments, though not in a manner consistent with VTL-based speaker-size judgments.

Factors limiting vocal-tract length discrimination in cochlear implant simulations.

Perception of voice characteristics allows normal hearing listeners to identify the gender of a speaker, and to better segregate speakers from each other in cocktail party situations. This benefit is largely driven by the perception of two vocal characteristics of the speaker: The fundamental frequency (F0) and the vocal-tract length (VTL). Previous studies have suggested that cochlear implant (CI) users have difficulties in perceiving these cues. The aim of the present study was to investigate possible causes for limited sensitivity to VTL differences in CI users. Different acoustic simulations of CI stimulation were implemented to characterize the role of spectral resolution on VTL, both in terms of number of channels and amount of channel interaction. The results indicate that with 12 channels, channel interaction caused by current spread is likely to prevent CI users from perceiving VTL differences typically found between male and female speakers.

On the non-acoustic nature of acoustic invariants

Various proposals have attempted to put speech targets into invariant “acoustic” form, sometimes with an additional transformation into “auditory” space. These targets, however, are not strictly acoustic. Targets for vowels, for example, are transformed so that vocal tract length differences (between talkers, especially across men, women, and children) are taken into account. Such a transformation makes the resultant targets combinations of articulatory and acoustic information. The auditory transformation improves automatic speech recognition, but the theoretical underpinnings for this result have been unclear. Ghosh, Goldstein, and Narayanan [J. Acoust. Soc. Am. 129, 4014–4022] shows that the articulatory information is maximized by the auditory transform, indicating that this transform is not solely in the acoustic domain. Moreover, a lowered F3 has been proposed as the production target for American English /r/ [e.g., Nieto-Castanon et al., J. Acoust. Soc. Am. 117, 3196–3212], but synthesis that retains an exemplary /r/ F3 while altering F1 and F2 results in other percepts, such as /w/ or a pharyngeal glide. F3 as an acoustic target is insufficient by itself and must incorporate articulatory dynamics implied by the other formants. Thus, “acoustic” invariants, to the extent they work at all, do so by incorporating articulation.

Effects of fundamental frequency and vocal-tract length cues on sentence segregation by listeners with hearing loss

The purpose was to determine the effect of hearing loss on the ability to separate competing talkers using talker differences in fundamental frequency (F0) and apparent vocal-tract length (VTL). Performance of 13 adults with hearing loss and 6 adults with normal hearing was measured using the Coordinate Response Measure. For listeners with hearing loss, the speech was amplified and filtered according to the NAL-RP hearing aid prescription. Target-to-competition ratios varied from 0 to 9 dB. The target sentence was randomly assigned to the higher or lower values of F0 or VTL on each trial. Performance improved for F0 differences up to 9 and 6 semitones for people with normal hearing and hearing loss, respectively, but only when the target talker had the higher F0. Recognition for the lower F0 target improved when trial-to-trial uncertainty was removed (9-semitone condition). Scores improved with increasing differences in VTL for the normal-hearing group. On average, hearing-impaired listeners did not benefit from VTL cues, but substantial inter-subject variability was observed. The amount of benefit from VTL cues was related to the average hearing loss in the 1-3-kHz region when the target talker had the shorter VTL.

The interaction of vocal characteristics and audibility in the recognition of concurrent syllables.

In concurrent-speech recognition, performance is enhanced when either the glottal pulse rate (GPR) or the vocal tract length (VTL) of the target speaker differs from that of the distracter, but relatively little is known about the trading relationship between the two variables, or how they interact with other cues such as signal-to-noise ratio (SNR). This paper presents a study in which listeners were asked to identify a target syllable in the presence of a distracter syllable, with carefully matched temporal envelopes. The syllables varied in GPR and VTL over a large range, and they were presented at different SNRs. The results showed that performance is particularly sensitive to the combination of GPR and VTL when the SNR is 0 dB. Equal-performance contours showed that when there are no other cues, a two-semitone difference in GPR produced the same advantage in performance as a 20% difference in VTL. This corresponds to a trading relationship between GPR and VTL of 1.6. The results illustrate that the auditory system can use any combination of differences in GPR, VTL, and SNR to segregate competing speech signals.

Vowel normalization using stop consonant loci.

Vowel normalization techniques used in sociophonetics aim to eliminate the influence of vocal tract length differences on resonant frequencies so that speaker‐specific vowel configurations can be compared more precisely. In vowel‐extrinsic methods, a speaker’s F1‐F2 range is scaled up or down, and vowel spaces are aligned using the geometrical F1‐F2 mean. However, a speaker’s F1‐F2 mean may be systematically biased if his or her vowel configuration is affected by a set of parallel vowel shifts, e.g., fronting or raising of several vowels in a chain shift. To address this problem, an alternative method of deriving speaker‐specific scaling factors was tested based on a vowel‐independent acoustic measure: the locus (virtual origin) of the second formant transition following the /d/‐release. Forty adult speakers of American English (20 male, 20 female) were recorded reading ten repetitions of 16 /dVd/‐words. From these data, individual F1‐F2 means and locus equations were calculated, including each speaker’s F2 locus frequency for /d/. Preliminary results based on the corner vowels /i, a, u/ show that F2 locus and F1‐F2 mean are strongly correlated (r‐squared=0.84). Statistical modeling is underway to determine whether scaling factors derived from locus analysis are superior to scaling factors derived from F1‐F2 means.

Unattended speech processing: Effect of vocal-tract length

Rivenez et al. [J. Acoust. Soc. Am. 119 (6), 4027-4040 (2006)] recently demonstrated that an unattended message is able to prime by 28 ms a simultaneously presented attended message when the two messages have a different F0 range. This study asks whether a difference in vocal-tract length between the two messages rather than a difference in F0 can also produce such priming. A priming effect of 13 ms was found when messages were in the same F0 range but had different (15%-30%) vocal-tract length, suggesting that the processing of unattended speech strongly relies on the presence of perceptual grouping cues.

Effects of fundamental frequency and vocal-tract length changes on attention to one of two simultaneous talkers.

Three experiments used the Coordinated Response Measure task to examine the roles that differences in F0 and differences in vocal-tract length have on the ability to attend to one of two simultaneous speech signals. The first experiment asked how increases in the natural F0 difference between two sentences (originally spoken by the same talker) affected listeners' ability to attend to one of the sentences. The second experiment used differences in vocal-tract length, and the third used both F0 and vocal-tract length differences. Differences in F0 greater than 2 semitones produced systematic improvements in performance. Differences in vocal-tract length produced systematic improvements in performance when the ratio of lengths was 1.08 or greater, particularly when the shorter vocal tract belonged to the target talker. Neither of these manipulations produced improvements in performance as great as those produced by a different-sex talker. Systematic changes in both F0 and vocal-tract length that simulated an incremental shift in gender produced substantially larger improvements in performance than did differences in F0 or vocal-tract length alone. In general, shifting one of two utterances spoken by a female voice towards a male voice produces a greater improvement in performance than shifting male towards female. The increase in performance varied with the intonation patterns of individual talkers, being smallest for those talkers who showed most variability in their intonation patterns between different utterances.

A novel feature transformation for vocal tract length normalization in automatic speech recognition

This paper proposes a method to transform acoustic models that have been trained with a certain group of speakers for use on different speech in hidden Markov model based (HMM-based) automatic speech recognition. Features are transformed on the basis of assumptions regarding the difference in vocal tract length between the groups of speakers. First, the vocal tract length (VTL) of these groups has been estimated based on the average third formant F/sub 3/. Second, the linear acoustic theory of speech production has been applied to warp the spectral characteristics of the existing models so as to match the incoming speech. The mapping is composed of subsequent nonlinear submappings. By locally linearizing it and comparing results in the output, a linear approximation for the exact mapping was obtained which is accurate as long as the warping is reasonably small. The feature vector, which is computed from a speech frame, consists of the mel scale cepstral coefficients (MFCC) along with delta and delta/sup 2/-cepstra as well as delta and delta/sup 2/ energy. The method has been tested for TI digits data base, containing adult and children speech, consisting of isolated digits and digit strings of different length. The word error rate when trained on adults and tested on children with transformed adult models is decreased by more than a factor of two compared to the nontransformed case.

Perceptual evaluations of voice scatter in unison choir sounds

The preferences of experiences listerners for pitch and formant frequency dispersion in unison choir sounds were explored using synthesized stimuli. Two types of dispersion were investigated: (a) pitch scatter, which arises when voices in an ensemble exhibit small differences in mean fundamental frequency, and (b) spectral smear, defined as such dispersion of formants 3 to 5 as arises from differences in vocal tract length. Each stimulus represented a choir section of five bass, tenor, alto, or soprano voices, producing the vowel [u], [a], or [ae]. Subjects chose one dispersion level out of six available, selecting the "maximum tolerable" in a first run and the "preferred" in a second run. The listeners were very different in their tolerance for dispersion. Typical scatter choices were 14 cent standard deviation for "tolerable" and 0 or 5 cent for "preferred." The smear choices were less consistent; the standard deviations were 12 and 7%, respectively. In all modes of assessment, the largest dispersion was chosen for the vowel [u] on a bass tone. There was a vowel effect on the smear choices. The effects of voice category were not significant.