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A PHONETIC STUDY OF SUBGLOTTAL PRESSURE EFFECTS ONSTRESS AND FUNDAMENTAL FREQUENCY
Shi Yu1, Sergio Hassid2 and Didier Demolin1
Laboratoire de Phonétique et Phonologie, UMR 7018, Université Sorbonne Nouvelle1Hôpital Erasme, Université Libre de Bruxelles2
[email protected], [email protected], [email protected]
ABSTRACT
An important issue in speech production is to under-stand the relation between subglottal pressure (Ps),fundamental frequency (fo) and stress. Attemptshave been made to identify the contribution of Psand fo on stress. An important point is to determinewhether stress is realised by an increase in Ps (there-fore intensity) and its effect on fo. Thus it is impor-tant to know whether fo variations on stress are vol-untary and programmed or not. This paper addressesthis issue by studying fo and Ps in a set of Englishsentences produced by 1 native English speaker. Psand speechwere recorded simultaneously and Pswasmeasured by direct tracheal puncture. Results showthat in declarative sentences there is an increase of Pson stress accompanied by a fall of fo. Interrogativesentences have an increase of Ps on stressed syllableswith a lower and slightly falling fo.
Keywords: stress, intonation, subglottal pressure,fundamental frequency, physiology.
1. INTRODUCTION
In speech sentences one important global attributeis the tendency of fundamental frequency (fo) todecline gradually from the beginning to the end,this phenomenon is usually referred to as declina-tion. One important question related to declinationis to know if it is the result of voluntary fo changesor physiologically determined therefore involuntaryfluctuations. Ohala [8] mentions that the cause ofdeclination remains an unresolved issue. Hart et al.[4] mentioned the perceptual relevance of declina-tion and notably the fact that it might be linguisticallyrelevant. These authors emphasise that declination isnot necessarily the product of voluntary control sincethe outcome of involuntary mechanisms can also re-sult in linguistic information.This issue is central to the understanding of the re-
lation between subglottal (Ps) pressure and fo, par-ticularly in intonational phenomena. What is thecontribution of Ps to fo at sentences level and in
speech in general, and how do they interact? Al-though there is an abundant literature discussing thisproblem, we can reasonably assume that fo is largelycontrolled by intrinsic and extrinsic laryngeal mus-cular adjustments and that the main contribution ofPs to fo is to raise the baseline in function of the in-tensity of speech. The higher the Ps and intensity,which are directly correlated, the higher the baselinewill be [2]. During speech, Ps shows a relativelyconstant and slightly decreasing value between aninitial rising phase and a final falling phase. In ad-dition, there are modulations of Ps that are due tochanges in the glottal or oral resistance and due tostress phenomena. One question arising from this is:how is the relatively constant Ps achieved despite thecontinuously decreasing lung volume? During nor-mal exhalation, there is a mechanical elastic recoilforce at play. This originates from the rib cage andthe lungs. The deeper the inspiration, the greater theelastic recoil. Along with these mechanical factors,exhaling is often prolonged during speech by the ac-tion of abdominal and internal intercostal muscles.Therefore claims such as those made by Hart et al.[4] that a serious candidate for explaining the decli-nation of fo of an utterance is the slow decrease ofPs over the time course seem to be debatable. How-ever they admit that several additional mechanismseffecting declination are conceivable.
In regard to stress, an important question is: doesrespiration have an active role in the production ofstressed syllables? Ladefoged [7] showed the pos-itive role of respiration in the implementation ofstress. It was demonstrated that for every stressedsyllable there is an increase in Ps. This means thatthe increase of Ps occurring with stress is mainly dueto changes in activity of the respiratory muscles.
This paper tries to contribute to the identificationof the possible physiological mechanisms responsi-ble for declination and to the explanation of inter-play between Ps and fo in stress phenomena. We ex-amined the production of a set of English sentenceswhere both intonation and stress position were var-ied to bring out the relation between fo and Ps.
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2. MATERIAL AND METHOD
Data consisted in a set of 36 sentences that wererecorded with one native English subject who re-peated each of the selected sentences 3 times (Table1). Groups 1/4 and 2/3 consist of respectively 3 and4 target words by varying the carrier sentence ”say... again” / ”repeat ... twice” which result in respec-tively 6 and 8 sentences. Group 5 contains 4 pairs ofdeclarative / interrogative sentences which differ bythe position of stressed syllable (in bold).
Table 1: 5 groups of recorded sentences.
Group 11/5. (say / repeat) pit (again / twice).2/6. (say / repeat) pity (again / twice).3/7. (say / repeat) pitying (again / twice).4/8. (say / repeat) pityingly (again / twice).Group 29/12. (say / repeat) play (again / twice).10/13. (say / repeat) playful (again / twice).11/14. (say / repeat) playfully (again / twice).Group 315/18. (say / repeat) photo (again / twice).16/19. (say / repeat) photographer (again / twice).17/20. (say / repeat) photographic (again / twice).Group 421/25. (say / repeat) man (again / twice).22/26. (say / repeat) manage (again / twice).23/27. (say / repeat) managerial (again / twice).24/28. (say / repeat) jeer (again / twice).Group 529./33. Jenny’s pie is pretty.30./34. Jenny’s pie is pretty?31./35. Jenny’s pie is pretty.32./36. Jenny’s pie is pretty?
Subglottal pressure (Ps), intraoral pressure (Po),oral airflow (Oaf) and the acoustic signal wererecorded synchronously. Ps was measured by tra-cheal puncture using a needle inserted in under thecricoid cartilage. The recording procedure preservedthe rights and welfare of human research subjects, inrespect of the ethical committee’s rules at the ErasmeHospital, Université Libre de Bruxelles. The nee-dle placement was made after administering 2% Xy-locaine injection for local anaesthesia. A plastictube linked to a pressure transducer was connectedto the needle. Po measurements were obtained us-ing a small tube inserted into the oropharynx throughthe nasal cavity. Oaf data were collected with a rub-ber mask placed over speakers’ mouth. The acous-tic signal was recorded with an AKG C 419 direc-
tional microphone positioned at a stable distance of4 cm from the rubber mask. The Ps and Po tubes, theOaf mask, and the microphone were connected to thePhysiologia workstation; a multi-parameter data ac-quisition system allowing simultaneous recording ofthe speech signal and various aerodynamic parame-ters. The data acquisition and signal processing werehandled with Phonedit. fo values are computed us-ing Praat pitch detection algorithm.
3. RESULTS
All three repetitions of each sentence were superim-posed in the same graphic, as the following figuresuggests, both fo and Ps values and their variationare relatively consistant throughout repetitions (Fig-ure 1). Each fo segment corresponds to the voicedpart of the sentence, the falling of fo value withineach segment is remarkable, and all results confirmthat each stress syllable has a notable increase of Ps.The first derivative of each repetition was then com-puted by taking a time step of 10 ms (Figure 2)
Figure 1: Variation of fo and Ps of sentence 2.(group 1)
The first derivative shows the variation of fo andPs. Each positive-negative pic of the derivative offo corresponds to the limit of voiced segment of thespeech. A clear initial rise and final fall can also belocated using the first derivative, both the end of thefinal fall and the start of the initial rise correspond tothe first and the last zero-crossing of the derivativeof Ps.The rapid falling fo segment (between each posi-
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Figure 2: First derivative of fo and Ps of sentence2 (group 1)
tive and negative pic) corresponds to an increase ofPs, and it is prominent for stressed syllable.
Figure 3: Variation of fo and Ps of sentence 17(group 3)
3.1. Declarative vs. Interrogative
The comparison of figures 4 and 5 show the dif-ferences between declarative and interrogative sen-tences, whereas fo in declarative sentences have al-
ways a rapid falling pattern, in interrogative sen-tences it is shorter and more flat.
Figure 4: Variation of fo an Ps of sentence 30(group 5)
Figure 5: Variation of fo an Ps of sentence 34(group 5)
The comparison of the first derivative (figure 6and 7) shows that, although the fo pattern at the endof the sentence is completely different (falling fordeclarative and rising for interrogative), the varia-tions of Ps are nearly the same for these two modal-ity, suggesting that fo is controlled independently of
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Ps.Figure 6: First derivative of fo an Ps of sentence30 (group 5)
Figure 7: First derivative of fo an Ps of sentence34 (group 5)
4. DISCUSSION
The first point to mention is that our data confirmprevious works done by [3] and [7] about the con-tribution of Ps to stress. The data also show clearexamples of fo declination on declarative sentences
and fo rise in interrogative sentences.The literature generally assumes that the question
of the phonetic correlates of stress is an intricate one[6]. For example, the interpretation of the physio-logical and acoustic correlates of stress is difficult tointerpret because of the ambiguous role of intensityin the perception of stress [6]. Even if modulationsof fo are largely due to adjustments of the intrinsicand extrinsic laryngeal musculature, Ps is one of thefactors that influences the rate of vocal fold vibrationand therefore some aspects of stress are connectedwith frequency. An increased Ps automatically trig-gers an increased rate of fo affecting the baseline ofan utterance. How are these variables controlled re-mains largely unknown.Results of the experiments presented in this paper
show that in addition of Ps rise accompanying stressthere is complex pattern of fo. In a declarative sen-tence stresse on fo generally shows a falling patternas displayed in Figures 1, 3 and 4. The level fromwhich this fall starts is generally higher on stressedsyllables confirming that there is an effect of Ps on foon stressed syllables. However, an important pointis to note that the comparison between declarativeand interrogatives sentences displaying the same lex-ical material shows that fo on stressed syllables canbe very different. In the sentences compared in Fig-ure 4 and 5 the initial syllables are stressed as theyshow an increase in Ps. However fo is falling as ex-pected in the declarative sentence while it is flat andlow at the start of the interrogative sentence. Thusa stressed syllable can either show a falling fo or aflat fo depending on the type of sentence. This sug-gests that, even if there is a small influence of Ps onthe rate of vocal folds vibrations, fo has a differentcontrol from Ps.Claims such as those made by [1] that the fo falls
over the course of an utterance because Ps also de-clines are not confirmed in our data. Even if at timesthere are some parallel movements between fo andPs contours, the latter remains largely independentof fo. Figure 3 show a clear example of a flat Ps pro-longed over three syllables suggesting that speakerdeliberately maintains this level to realise a suiteof stressed syllables where fo displays the expectedfalling pattern on each syllable.The fact that Ps and fo have a different control
might be a consequence arising from spinal inner-vation of the respiratory muscles, starting from theregion of the cranial nerves 3, 4, 5 and going tothe region of lumbar nerve 1 [5] while muscles con-trolling the laryngeal musculature are only cranialnerves (mainly the cranial nerve 10 the vagus nerve).This has to be explored in future research with more
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subjects.
5. REFERENCES
[1] Cohen, A., Collier, R., others, 1982. Declination:construct or intrinsic feature of speech pitch? Pho-netica 39(4-5), 254–273.
[2] Demolin, D. 2007. Phonological universals and thecontrol and regulation of speech production. Exper-imental approaches to phonology 75–92.
[3] Draper, M., Ladefoged, P., Whitteridge, D. 1959.Respiratory muscles in speech. Journal of Speech& Hearing Research.
[4] Hart, J., Collier, R., Cohen, A. 2006. A percep-tual study of intonation: an experimental-phoneticapproach to speech melody. Cambridge UniversityPress.
[5] Huber, J., Strathopoulos, E. 2015. Speech breathingacross the life span and in disease. The Handbook ofSpeech Production 13.
[6] Kent, R. D., Dembowski, J., Lass, N. J. 1996. Theacoustic characteristics of american english. Princi-ples of experimental phonetics 185–225.
[7] Ladefoged, P. 1968. Linguistic aspects of respira-tory phenomena. Annals of the New York Academyof Sciences 155(1), 141–151.
[8] Ohala, J. J. 1990. Respiratory activity in speech. In:Speech production and speech modelling. Springer23–53.
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