An Acoustic Framework for Detecting Fatigue in Speech Based Human-Computer-Interaction.
ABSTRACT This article describes a general framework for detecting accident-prone fatigue states based on prosody, articulation and
speech quality related speech characteristics. The advantages of this real-time measurement approach are that obtaining speech
data is non obtrusive, and free from sensor application and calibration efforts. The main part of the feature computation
is the combination of frame level based speech features and high level contour descriptors resulting in over 8,500 features
per speech sample. In general the measurement process follows the speech adapted steps of pattern recognition: (a) recording
speech, (b) preprocessing (segmenting speech units of interest), (c) feature computation (using perceptual and signal processing
related features, as e.g. fundamental frequency, intensity, pause patterns, formants, cepstral coefficients), (d) dimensionality
reduction (filter and wrapper based feature subset selection, (un-)supervised feature transformation), (e) classification
(e.g. SVM, K-NN classifier), and (f) evaluation (e.g. 10-fold cross validation). The validity of this approach is briefly
discussed by summarizing the empirical results of a sleep deprivation study.
Article: [Sleepiness and driving].Laeknabladid 94(7-8):521. · 0.46 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Only a few studies have examined the possible association between excessive daytime sleepiness (EDS) and risk of occupational injuries, and most of them were based on self-reports. This study tested this association in daytime workers using injury data taken from organizational archives. A retrospective and prospective study. It covered injury occurrence during two years prior to a sleep disorder assessment/education procedure and injury occurrence in the following year. The workers were given the assessment results and, when applicable, a letter to the treating physician. Eight industrial plants. Lectures and discussions on sleep disorders, treatment, and implications to safety and quality of life were conducted with small groups who completed the sleep assessment questionnaire beforehand. The workers completed the sleep assessment questionnaire prior to the lecture/discussion. 532 non-shift daytime workers. N/A. A battery of questionnaires to assess EDS (by the Epworth Sleepiness Scale), suspected sleep disorders, sleep habits, and job and environmental conditions. Of the workers studied 22.6% had EDS. Most of those (96.3%) indicated that they had experienced this propensity for the past two years or more and 56% of them had experienced it for 10 years or more. Logistic regression analysis indicated that during the two-year period prior to the procedure, EDS was associated with an increased risk of sustaining a work injury (OR=2.23, 95% CI 1.30-3.81), even after controlling for possible confounders, including factory category, job and environmental conditions. In the year after the procedure, the injury rate decreased by one-third in the workers with EDS but remained unchanged in the workers without EDS. Consequently, the association between EDS and injury was no longer significant (OR=1.42, 95% CI 0.71-2.85). EDS is a prevalent phenomenon in non-shift daytime workers. Workers with EDS had over two-fold higher risk of sustaining an occupational injury. Providing workers with the assessment results and of the implications of EDS for safety may explain the decrease in occupational injuries upon follow-up. This decrease might have occurred either because of workers taking steps to reduce EDS and/or adopting safety behaviors.Sleep 06/2002; 25(3):315-22. · 5.10 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The study investigated sleepiness and sleep in aircrew during long-haul flights. The objectives were to identify loss of alertness and to recommend a practical approach to the design of an alerting system to be used by aircrew to prevent involuntary sleep. The flights were between London and Miami, covering both day- and night-time sectors, each with a duration of approximately 9 h. The subjects were 12 British Airways pilots. Various physiological variables were measured that could potentially be used to indicate the presence of drowsiness and involuntary sleep: brain electrical activity (electroencephalogram, EEG), eye movements via the electro-oculogram (EOG), wrist activity, head movements and galvanic skin resistance. The EEG and EOG identified sleepiness and sleep, as well as being potential measures on which to base an alarm system. Ten pilots either slept or showed evidence of sleepiness as assessed by the EEG and EOG. Many of the episodes of sleepiness lasted < 20 s, which could mean that the subjects were unaware of their occurrence and of the potential consequences on performance and vigilance. All physiological parameters showed changes during sleep, although only the EEG and EOG were modified by sleepiness. During sleep, skin resistance was increased, and wrist activity and head movements were absent for long periods. The study indicated that the measurement of eye movements (either alone or in combination with the EEG), wrist activity or head movement may be used as the basis of an alarm system to prevent involuntary sleep. Skin resistance is considered to be unsuitable, however, being related in a more general way to fatigue rather than to sleep episodes. The optimal way to monitor the onset of sleep would be to measure eye movements; however, this is not feasible in the flight deck environment at the present time due to the intrusive nature of the recording methodology. Wrist activity is therefore recommended as the basis of an alertness alarm. Such a device would alert the pilot after approximately 4-5 min of wrist inactivity, since this duration has been shown by the present study to be associated with sleep. The possibility that sleep inertia (reduced alertness immediately after awakening from sleep) could follow periods of sleep lasting 5 min needs to be considered. The findings reported here might be applicable to other occupational environments where fatigue and sleepiness are known to occur.Ergonomics 02/2001; 44(1):82-106. · 1.67 Impact Factor
An Acoustic Framework for Detecting Fatigue in
Speech Based Human-Computer-Interaction
Jarek Krajewski1, Rainer Wieland1, Anton Batliner2
1 University of Wuppertal, 42097 Wuppertal, Germany
Work and Organizational Psychology
2 University of Erlangen-Nuremberg, 91058 Erlangen, Germany
Lehrstuhl fuer Mustererkennung
Abstract. This article describes a general framework for detecting accident-
prone fatigue states based on prosody, articulation and speech quality related
speech characteristics. The advantages of this real-time measurement approach
are that obtaining speech data is non obtrusive, and free from sensor application
and calibration efforts. The main part of the feature computation is the
combination of frame level based speech features and high level contour
descriptors resulting in over 8,500 features per speech sample. In general the
measurement process follows the speech adapted steps of pattern recognition:
(a) recording speech, (b) preprocessing (segmenting speech units of interest),
(c) feature computation (using perceptual and signal processing related features,
as e.g. fundamental frequency, intensity, pause patterns, formants, cepstral
coefficients), (d) dimensionality reduction (filter and wrapper based feature
subset selection, (un-)supervised feature transformation), (e) classification (e.g.
SVM, K-NN classifier), and (f) evaluation (e.g. 10-fold cross validation). The
validity of this approach is briefly discussed by summarizing the empirical
results of a sleep deprivation study.
Keywords: Acoustic Features, Assistive Technologies, Pattern Recognition,
Fatigue, Accident Prevention, Affective Computing
1 Measuring Fatigue in Human-Computer-Interaction
Fatigue has been widely accepted as a significant cause in a variety of traffic
accidents [1-3]. Due to their slightly reduced cognitive processing speed, especially
elderly persons might be vulnerable to the additional fatigue driven impairment of
cognitive functions [4,5]. Similar risk factor constellations can be found in private
application contexts (e.g. Telecare systems identifying critical fatigue states in senior
people) as well as in general work contexts (e.g. monitoring tasks, air traffic control),
too. Hence, detecting fatigue states and reacting to them is an important issue for
accident prevention in elderly persons.
Side applications of detecting fatigue in Human-Computer-Interaction (HCI) can be
identified within the field of Assistive Technologies for elderly and disabled persons
[e.g. 6], making HCI more natural, e.g. in speech interfaces of assistive domotics.
Adapting the system output to the actual emotional and fatigue related user states
might enhance the acceptance of these systems due to their improved naturalism. The
emotional-intelligent, user state sensitive communication could improve comfort.
Furthermore it may result in better comprehensiveness if the system output is adapted
to the user's actual fatigue-impaired attentional and cognitive resources. In addition to
this, speech recognition or speaker verification systems itself might be improved by
taking the fatigue related speech changes into account. As a results speech based
Assistive Technologies could benefit from these progresses.
Many efforts have been reported in the literature for measuring biosignal based
fatigue states . These systems mainly focus on (a) oculomotoric data (eye blinking,
eyelid movement, and saccade eye movement) , (b) EEG data  and (c)
behavioral expression data (gross body movement, head movement, mannerism, and
facial expression)  in order to characterize the fatigue and sleepiness state. Apart
from these promising advances in analysing facial and gestural expressivity, there has
been recently renewed interest in vocal expression and speech analysis. This fact is
mainly promoted by the progress in speech science and the gaining presence of
speech in voice guided HCI. Using voice communication as an indicator of sleepiness
e.g. within assistive technologies for elderly would have the following advantages:
obtaining speech data is non obtrusive, free from sensor application and calibration
efforts, “hands- and eyes-free”, and most important, speech data is often available in
In this paper we describe a speech adapted pattern recognition framework in order to
measure fatigue and sleepiness states. Our attention is focused particularly on the
processing step of feature computation. The rest of this paper is organized as follows:
In Section 2 computing high level contour descriptor features is explained. The
general speech adapted pattern recognition framework is provided in Section 3, a brief
summary of sleepiness detection results is given in Section 4.
2 Acoustic Features
Acoustic features can be divided according to auditive-perceptual concepts into
prosody (pitch, intensity, rhythm, pause pattern, speech rate), articulation (slurred
speech, reduction and elision phenomena), and speech quality (breathy, tense, sharp,
hoarse, modal voice) related features . Another distinction can be drawn from
using signal processing categories as time, frequency or phase space domain features.
Our approach prefers the fusion of perceptual features and purely signal processing
and speech recognition based features without any known auditive-perceptual
correlates. Typical frame level based acoustic features (Low-Level Descriptors, LLD;
see [12,13]) used in emotion speech recognition and audio processing [14,15] are
fundamental frequency (acoustic correlate to pitch; maximum of the autocorrelation
function), intensity, duration of voiced/unvoiced segments, harmonics-to-noise ratio,
position and bandwidth of 6 formants (resonance frequencies of the vocal tract
depending strongly on its actual shape), 16 linear predictive coding coefficients, 12
mel frequency cepstrum coefficients (“spectrum of the spectrum”), and 12 linear
frequency cepstrum coefficients (without the perceptually oriented transformation
into the mel frequency scale), see Tab 1.
Tab 1. Basic acoustic feature contours (low level descriptors, LLD, based on frame-level)
Frame level based feature Number of contours
Linear predictive coding (1-16)
Formants (F1-F6) position
Formants (F1-F6) bandwidth
Voiced segments duration
Unvoiced segments duration
Mel frequency cepstrum coefficients (MFCC, 1-12)
Linear frequency cepstrum coefficients (LFCC, 1-12)
After splitting the speech signal into frames and computing the above mentioned
frame level features, the values of each frame level feature are connected to contours.
This procedure results in 57 speech feature contours (e.g. the fundamental frequency
contour, the bandwidth of formant 4 contour etc.), which are joined by their first and
second derivates (velocity (Δ) and acceleration (ΔΔ) contours). Furthermore these 171
speech feature contours are described by elementary statistics (linear moments, values
and positions of extrema, quartiles, ranges, length of time periods beyond threshold
values, regression coefficients, etc.), and spectral descriptors (spectral energy of low
frequency bands vs. high frequency bands, etc.) resulting in 8,550 high-level speech
features (171 speech contours x 50 functionals), see Fig 1.
57 LLD features per
Fig. 1. Processing flow of acoustic feature computation including the computation of frame
level based features and contour descriptors (functionals) to capture sufficient temporal
features per sample
50 functionals per
171 LLD contours:
(raw, Δ, ΔΔ)
3 Speech Adapted Pattern Recognition Framework
The acoustic measurement process follows the speech adapted steps of pattern
recognition: (a) recording speech, (b) preprocessing, (c) feature computation, (d)
dimensionality reduction, (e) classification, and (f) evaluation. The following listing
gives a brief overview about possible variations in the measurement process.
Recording speech: Source of verbal material [human-to-human, human-to-machine
communication; monologue vs. dialogue situations; speech databases (e.g. AEC ,
Sympafly , EMO-DB )]; Speaking format [vowel phonation, isolated words,
connected speech, read speech, spontaneous speech]; Speaking style [intensity and
articulation related speaking style (e.g. hyperarticulation, whispering, shouting)];
Speech segment [vowels, consonant types (fricative, stop, glide), consonant clusters,
syllables, words, chunks phrases]; Recording situation [noisy vs. noise subdued
environment (e.g. driving with open window vs. laboratory recording); rough vs.
clean speech signal quality (e.g. telephone call, radio communication vs. clean
recording in 22.05 kHz, 16 bit)].
Preprocessing: Segmentation [manual, (semi-)automatic segmentation (e.g. MAUS
system ) of the speech signal in phonetic units of interest (e.g. specific vowels,
types of consonants or consonant cluster, stressed syllables, beginning or end of
phrases)]; Noise reduction [outlier detection, moving average filter, low bandpass
filter]; Framing and Windowing [size of frames (10-20 ms), degree of overlapping,
window function (hamming, hanning)].
Feature computation. Low level descriptors (LLD) [Fundamental frequency,
intensity, harmonics-to-noise ratio, formant position and bandwidth (F1-F6), LPC,
MFCC, LFCC, partitioning into voiced and unvoiced speech segments]; Functionals
[elementary statistics (e.g. linear moments, extrema values and positions, quartiles,
ranges, length of time periods beyond threshold values, regression coefficients),
spectral descriptors (e.g. spectral energy of low frequency bands vs. high frequency
bands) and state space features (e.g. largest lyapunov coefficient)]; Automatic feature
generation (genetic algorithms); Normalization [individual speaker specific baseline
correction, age/ gender specific normalization].
Dimensionality reduction. Subset selection [supervised filter-based (e.g. correlation,
information gain ratio), unsupervised (e.g. density, entropy, salience) or wrapper-
based subset selection (forward selection, backward elimination, sequential forward
floating search, genetic algorithm selection)]; Feature Transformation [unsupervised
(e.g. Principle Component Analysis, PCA Network, Nonlinear Autoassociative
Network, Multidimensional Scaling, Kernel PCA, Independent Component Analysis,
Sammon Map, Enhanced Lipschitz Embedding, SOM, Spectral Transformation,
Wavelet Transformation); supervised (e.g. Linear Discriminant Analysis)].
Classification. Classification granularity (binary or multiple class prediction);
Classifier choice [e.g. 1-nearest neighbour, multi-layer perceptron, support vector
machine, linear discriminant analysis, hidden markov model, decision tree, gaussian
mixture model]; Metaclassifier [bagging, boosting, voting, stacking]; Parameter
Validation. Evaluation strategy [2-, 5-, 10-fold cross validation; leave-one-sample-
out]; Reliability strategy [recordings on different days for retest reliability (e.g. leave-
4 Fatigue Related Speech Changes
The following fatigue and sleepiness related physiological changes can influence
voice characteristics: (a) decreased muscle tension (reduced facial expression and
smiling, unconstricted pharynx, softening of vocal tract walls, vocal fold elasticity
and tension), (b) decreased body temperature (reduced heat conduction, changed
viscoelasticity of vocal folds, changed friction between vocal tract walls and air as
well as impaired laminar flows), (c) reduced cognitive processing speed (impaired
speech planning) and (d) flat and slow respiration (low subglottal pressure). The
corresponding acoustical effects are lower fundamental frequency, intensity,
articulatory precision, and rate of articulation, as well as shift in the spectral energy
distribution due to changed filter characteristics. These filter characteristics can be
described in terms of formant frequencies and bandwidths. Formant bandwidth is
among others determined by the amount of energy loss in the vocal tract due to
softening of vocal tract walls, raising of viscosity, and increasing heat conduction
[20,21]. It is expected that especially the softening of vocal tract walls produces a
moderate increase in formant frequencies and broadening of formant bandwidths,
especially in lower formants. In an antagonistic way, the friction between air and the
vocal tract walls (due to a lowered body temperature and lowered heat conduction)
may cause a lowering of the formant frequencies.
Furthermore a general decreased facial expression and thus decreased lip spreading
[22,23] may result in a shortening of the vocal tract and therefore in lower F1 and F2
frequencies. A supplemental factor for causing this effect can be seen in articulatory
effects. The reduction of articulatorical effort leads to a smaller opening degree during
slackened articulation and a decreasing of the first formant. Another sleepiness related
voice phenomenon, which might influence the formant values, is the shift of speech
quality to a breathy, wide, lax, and non-tensed voice. The tensing of the vocal tract
raises the larynx, which could result in an increased formant frequency. On the
contrary, a lax voice is characterised by low F1 and wide F1 bandwidth. Similarly, the
breathier a vowel is spoken, the wider is the first formant bandwidth. However, little
empirical research has been done to examine the effect of fatigue and sleepiness on
acoustic voice characteristics. The aim of the following study is to introduce a fatigue
detection method based on the speech adapted pattern recognition approach.
5 Empirical Validation Results
We conducted a within-subject sleep deprivation design (N = 21; 8.00 p.m to 4.00
a.m). During the night of sleep deprivation a well established, standardised self-report
sleepiness measure, the Karolinska Sleepiness Scale (KSS) was used every hour just
before the speech recordings. The verbal material consisted of a 2 second sustained
phonation of the German vowel [a:]. The participants recorded other verbal material
at the same session, but in this article we focus on sustained phonation only. For
training and classification purposes, the records were further divided in two classes:
sleepy (SS) and non sleepy (NSS) with the boundary value KSS ≥ 6. (4 samples per
subject; 2 samples recorded at 8.30 and 9.00 p.m and 2 samples recorded at 3.00, 3.30
a.m.; total number of speech samples: 84 samples; 61 samples NSS, 23 samples SS).
During the night, the subjects were confined to the laboratory and supervised
throughout the whole period. Between sessions, they remained in a room, watched
DVD, and talked. Non caffeinated beverages and snacks were available ad libitum.
Fig. 2. Average harmonics-to-noise ratio contours of the sustained german vowel [a:]
phonation for sleepy vs. alert speakers.
The recognition rate (number of cases classified correctly divided by all cases; RR) of
the 1-nearest neighbour classifier was 83.3%, the class-wised averaged classification
rate (mean of recognition rates for each class; CL) was 76.5%. The five best single
features have the following correlations to self-reported sleepiness: HNR_mean = .29
(see Fig 2), formant4_max = -.29, formant2_bandwidth_1.quartil = .29,
intensity_1.quartil = .28, and formant3_max = -.28.
6 Concluding Remarks
Due to the hypothesized sleepiness related physiological changes in cognitive speech
planning, respiration, phonation, articulation, and radiation, the results for the
reported classification performance above chance level were largely as could be
expected. This is consistent with previous sleepiness related findings that suggest an
association of acoustic features [24, 25] with sleepiness. Nevertheless our results are
limited by the facts that we did not consider real life speaking situations including (a)
variations in speaking format (e.g. read speech, spontaneous speech), (b) variation in
speakers´ states (e.g. having a cold, after drinking milk, being nervous, aggressive or
in a depressive mood), (c) variations in speakers´ trait (e.g. strong dialect, older age),
and (d) variations in situational context factors (e.g. noisy environments, room
microphone). These confounders might influence the detection rate and the false
alarm error rate of the sleepiness measurement. Thus the present results are
preliminary and need to be replicated using natural speech environment. Moreover, it
would seem advisable that future studies address the following topics:
different validation designs: following the circadian sleepiness cycle,
pharmacological studies, randomized controlled trials (between-subject
temporal segmentation: finding sleepiness sensitive phonetic units (vowels
or consonant cluster in different positions within words and phrasal units).
feature extraction: computing state space domain based features (e.g. average
angle or length of embedded space vectors, lyapunov exponents, correlation
dimension, automutual information, time resolved density, fractal
dimensions, multiscale entropies, and recurrence quantification analysis);
using evolutionary feature generation.
classification: utilizing maximum-likelihood bayes classifiers, fuzzy
membership indexing, HMMs, gaussian mixture density models.
1. MacLean, A.W.: Sleepiness and Driving, Sleep Medicine Reviews 7, (2003) 507-521.
2. Melamed, S.: Excessive Daytime Sleepiness and Risk of Occupational Injuries in Non-Shift
Daytime Workers”, Sleep 25(3), (2002) 315-322.
3. Wright, N., McGown, A.: Vigilance on the Civil Flight Deck: Incidence of Sleepiness and
Sleep during Long-Haul Flights and Associated Changes in Physiological Parameters,
Ergonomics 44, (2001) 82-106.
4. Durmer, J.S., Dinges, D.F.: Neurocognitive Consequences of Sleep Deprivation. Seminars in
Neurology, 25, 2005. 117-129.
5. Nilsson, J.P., Soderstrom, M., Karlsson, A.U., Lekander, M., Akerstedt, T., Lindroth, N.E.,
Axelsson, J.: Less Effective Executive Functioning after one Night´s Sleep Deprivation.
Journal of Sleep Research, 14, 2005. 1–6.
6. Cañas, J.J.: Technology for special needs. An Interdisciplinary Journal on Humans
in ICT Environments, 2, 2006, 4–7.
7. Kollias, S., Amir, N., Kim, J., Grandjean, D.: Description of Potential Exemplars: Signals
and Signs of Emotion. HUMAINE Human-Machine Interaction Network on Emotions.
8. Caffier, P.P.: The Spontaneous Eye-Blink as Sleepiness Indicator in Patients with
Obstructive Sleep Apnoea Syndrome-a Pilot Study, Sleep Medicine, 2, (2002). 155-162.
9. Sommer, D., Chen, M., Golz, M., Trunschel, U., Mandic, D.: Fusion of State Space and
Frequency Domain Features for Improved Microsleep Detection. In: W. Dutch et al. (Eds.):
Int Conf Artifical Neural Networks (ICANN 2005), Springer: Berlin (2005) 753-759.
10. Vöhringer-Kuhnt, T., Baumgarten, T. Karrer, K., Briest, S.: Wierwille’s Method of Driver
Drowsiness Evaluation Revisited. Proceeding of International Conference on Traffic &
Transport Psychology. (2004).
11. Schuller, B.: Automatische Emotionserkennung aus sprachlicher und manueller Interaktion.
[Automatic Emotion Recognition from verbal and manual Interaction]. Dissertation,
Technische Universität München. (2006).
12. Schuller, B., Batliner, A., Seppi, D., Steidl, S., Vogt, T., Wagner, J., Devillers, L., Vidrascu,
L., Amir, N., Kessous, L., Aharonson, V.: The Relevance of Feature Type for the Automatic
Classification of Emotional User States: Low Level Descriptors and Functionals.
Proceedings of Interspeech, (2007) 2253-2256.
13. Vlasenko, B., Schuller, B., Wendemuth, A., Rigoll, G.: Combining Frame and Turn-Level
Information for Robust Recognition of Emotions within Speech. Proceedings of Interspeech,
14. Batliner, A., Steidl, S., Schuller, B, Seppi, D., Laskowski, K, Vogt, T., Devillers, L.,
Vidrascu, L., Amir, N., Kessous, L., Aharonson, V.: Combining Efforts for Improving
Automatic Classification of Emotional User States. In: Erjavec, T. & Gros, J.Z. (Eds.):
Language Technologies, IS-LTC 2006, Ljubljana, Slovenia: (2006) 240-245
15. Mierswa, I., Morik, K.: Automatic Feature Extraction for Classifying Audio Data. Kluwe,
16. Batliner, A., Hacker, C., Steidl, S., Noeth, E., D’Arcy, S, Rusell, M., Wong, M.: “You
stupid tin box” – Children interacting with the AIBO robot: A crosslinguisitc emotional
speech corpus, Proceedings of the 4th International Conference of Language Resources and
Evaluation LREC 2004 (LREC Lisbon 2004) (2004) 171-174.
17. Steidl, S., Hacker, C., Ruff, C., Batliner, A., Noeth, E., Haas, J.: Looking at the Last Two
Turns, I’d Say This Dialogue is Doomed – Measuring Dialogue Success ,Proceedings TSD
(Text, Speech and Dialog) (2004) 629-636.
18. Burkhardt, F., Paeschke, A., Rolfes, M., Sendlmeier, W., Weiss, B.: A Database of German
Emotional Speech, Proceedings of Interspeech 2005 (Lisboa, Portugal) (2005) 1517-1520.
19. Schiel, F.: MAUS Goes Iterative. Proc. of the IV. International Conference on Language
Resources and Evaluation, Lisbon, Portugal, (2004) 1015-1018.
20. Rabiner, L., Schafer, R.W.: Digital Processing of Speech Signals (Prentice-Hall, Upper
Saddle River, New Jersey, USA (1978).
21. Scherer, K.R.: Vocal affect expression: A review and a model for future research,
Psychological Bulletin, 99 (1986) 143-165.
22. Kienast, M., Sendlmeier, W.F.: Acoustical analysis of spectral and temporal changes in
emotional speech, Speech Emotion (2000) 92-97.
23. Tartter, V.C.: Happy talk - Perceptual and acoustic effects of smiling on speech, Perception
and Psychophysics, 27(1) (1980) 24-27.
24. Nwe, T.L., Li, H., Dong, M.: Analysis and Detection of Speech under Sleep Deprivation.
Proceeding of Interspeech, (2006) 17-21.
25. Krajewski, J., Kröger, B.: Using prosodic and spectral characteristics for sleepiness
detection. Interspeech Proceedings, (2007) 1841-1844.