PHONETIC CHARACTERISRICS OF AN UNEXPLODED STOP IN HENDO

Didier Demolin, Hubert Ngonga-Ke-Mbembe and Alain SoquetPhonology Laboratory, Universit� Libre de Bruxelles

ABSTRACT

This paper describes the phonetic characteristics of a particularallophone of the vowel [i] in Hendo, a Bantu language spoken inCongo. This sound is made by an overshoot of [i] and behaveslike an unexploded stop. Acoustic, aerodynamic and articulatorytechniques have permitted a precise description of this sound.

1. INTRODUCTION

lohendo is spoken in Western Kasa� within the Kole area of theDemocratic Republic of Congo. Several studies have quoted thislanguage under the name of s�ng�mEn� (Guthrie [1]) or nkutu(Hulstaert [2], Van Bulck [3], [4], Bongo [5]). The first is anickname and the second is generic term. Bongo [5] points outthat ohendo is a nkutSu dialect. The survey of Zairian languages(now Congolese) [6] opts for lohe:hendo. From now on we willadopt the convention for Bantu languages which sates thatlanguages should be quoted by the root of their name, in this caseHendo. Within the Bantu languages classification (Guthrie [1])this language is classified in area C 82 but in the revisedclassification made by Tervuren [7] lohendo belongs to the samearea but in a different group C 73.

One of the striking phonetic features of this language is aparticular allophone of the vowel /i/ at the beginning of wordsbelonging to classes 5 and 8. This allophone transcribed with thesymbol [i] behaves differently whether it precedes a voiceless ora voiced consonant, it is realized [i] before voiceless consonantsand contributes to the lengthening of a voiced initial consonant asit is illustrated by the following forms: [ikatSi] hand, [ipEka]shoulder, [ddemba] body, [bba la ] mariage, [ipele] wall,[bbenNga] wild pigeon.

The aim of this paper is to describe the articulatory,aerodynamic and acoustic characteristics of this sound whoserealization is difficult to explain. Several propositions have beenmade to describe this particular sound, for Stappers (personalcommunication) it is a voiced glottal stop, for Bongo [5] this isan indication of the consonant lengthening, as well as in the caseof voiced as in the case of voiceless consonants. None of theseexplanations seem satisfying. We propose to describe [i] as anallophone of /i/ because there are a several arguments to justifysuch a position. One of the main arguments being that in Hendoword games, when an initial [i] (which is syllabic) is permutatedfor another syllabic position within a word , [i] is realized eitheras [i] or as [dZi]. For example the bi-syllabic word ipfo ÔhairÕbecomes pfodZi when both syllables of the word are permutatedand the word . Other arguments based on the production of thissound will be proposed to justify this hypothesis.

2. MATERIAL AND METHOD

Data have been recorded with two Hendo speakers at thephonology laboratory of the Universit� Libre de Bruxelles.Acoustic and aerodynamic parameters have been recorded with

the Physiologia workstation (Teston and Galindo [8]) consistingin a computer linked with an acquisition system equipped withdifferent transducers and the signal editing program Phonedit.Oral airflow measurements have been made with a small flexiblesilicon mask set against the mouth. Nasal airflow has beenmeasured at the end of a nostril via a small olive linked to theacquisition system.

Articulatory data have been acquired during three differentrecording sessions. The first consisted in a simultaneous record offace and profile images with a high speed video camera. Profileimages were obtained by putting a mirror against the cheek of thespeaker, at a 45¡ angle from the sagittal plan. These data wereintended to study jaw, tongue and larynx movementssynchronization during the realization of the sound [i].

The second recording session of articulatory data consistedto acquire two sets of data by using magnetic resonance imaging(MRI) at the H�pital Erasme of the Universit� Libre deBruxelles. The aim was to observe tongue and larynx positionsduring the production of [i]. MR imaging was performed with theTSE Zoom sequence [9]. One sagittal T1-weighted section of 6mm thickness was continuously acquired during at least 20 sec,using a quadrature neck coil at 1.5T (Philips Gyroscan ACS NT,Best, The Netherlands). The acquisition was implemented withthe following parameters: TR=250 ms, TE=30 ms, q=60¡,ESP=7.8ms, ETL=19 and 60% Partial Fourier acquisition, Fieldof View=300 x 150mm with a 32 x 128 Matrix. The TSE Zoomsequence is designed such that the initial 60¡ and the subsequent180¡ refocusing pulses excite perpendicular slabs, resulting intoan intersecting slice, free of foldover artifacts and withoutcompromising the spatial resolution.

The first set of MRI data consisted in the acquisition of [i]sustained for 6s. The second set of MRI data has been recordedwith a dynamic acquisition, in order to observe the articulatorycoordinations made to produce this sound. Material used for theserecording sessions is given in table 1.

Table 1. Data studied in the MRI experiments.

[dd�ka] sorcerer [ikaci] hand[bbve] stone [itoi] ear[ipfumba] ant [ipfo] hair[ikfundu] stomac [ipeke] raphia[ipaka] moth [ipfumbo] braid[ddeko] season [ddaka] promise[ikfuku] something in decomposition

The third set of articulatory data was acquired with aMovetrack [11] in order to observe jaw movements and the skinraising and lowering between the lower jaw bone and the neck .The latter movement expressing the vocal tract expansion by thelowering and raising of the tongue body. The Movetrack system(Branderud [11]) allows to record information on the movementof different pellets placed in the mid-sagittal plane. Three pelletswere glued respectively on top of the nose, on the lower incisive,

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and on the chin. The recorded signals correspond to the distancesbetween the pellets and the two coils mounted on the helmet.Both signals have been subsequently transformed into X and Ycoordinates.

3. Results

3.1. Acoustic data

Audio signal and spectrogram examination (figures 1 and 2 showthat [i] is always voiced and that there is an identifiableresonance around 250 Hz when [i] precedes a voicelessconsonant as well as when it precedes a voiced consonant. When[i] precedes a voiced consonant, there is a lengthening of theconsonant which is explained because the contact of [i] with theconsonant lengthens the voicing without any observabletransition. Before a voiceless consonant [i] appears with one ortwo weak amplitude resonances before the voiceless part of theconsonant which is preceding the constriction or closure release.The length of [i] + stop, or fricative or affricate ([p, t, k, f, pf, tS,kf]) is of 504.6 ms. (n=20). Those sequences were [i] lengthens avoiced stop is of 390.6 ms. (n=20). The length of [i] alone whenit precedes a voiceless stop is 314.4 ms. (n=20).

In all cases, the transition towards the following consonantis made by a gradual damping of vocal folds vibrations and bythe absence of a burst.

These data do not allow to explain the nature of this sound.Therefore it is necessary to turn to detailed articulatory andaerodynamic descriptions in order to understand the nature of thephenomenon.

3.2. Aerodynamic data

Examination of aerodynamic data and particularly of pharyngealpressure allows to have a better understanding of the productionof [i]. To start with, note that no nasal airflow has been observedin the data. This allows to claim that this sound is not nasalcontrary to the auditory impression sometime given by thissound. Figures 1 and 2 illustrate the realization of a wordcontaining [i] followed by voiceless and voiced consonants.Figures 1a and 1b show the variation observed in the raising ofpharyngeal pressure for the two speakers in the same wordcontaining [i].

The following events can be deduced from the pharyngealpressure plots. First, there is an expansion of the pharyngealcavity starting when the larynx is lowered with a closed glottisand with a closure in the vocal tract, since there is no negativeoral airflow. The other factors contributing to the expansion ofthe pharyngeal cavity are tongue body lowering and tongue rootfronting. Larynx lowering is important in the initial phases of theproduction of [i] but tongue movements are more salient in theremaining phases. Larynx lowering is conveyed by the initialnegative pharyngeal pressure. Generally speaking, the reductionof pharyngeal pressure consecutive to larynx lowering, tonguebody lowering and tongue root fronting is more important andlonger when voiced consonants follow [i]. Second there is araising of pharyngeal pressure as soon the pharyngeal cavity sizeis reduced by the setting back of the different parts of the tongueand by the upward movement of the larynx. This phenomenonhappens until stop or constriction release which means that thereis a second phase in this pressure building. This second phasebeing accounted for by the glottal setting, i.e. wide for voicelessconsonants and adduced for voiced consonants. This is what isobserved in the data were pharyngeal pressure raising is made in

two phases for both subjects of the experiments. The first phasecorresponding to the voicing of [i] and the second to the glottalsetting of the following consonant.

Time

1 2 3 4

Figure 1a. Spectrogram, audio-signal and pharyngeal pressurefor the word ipaka ÔmothÕ pronounced by the first speaker. 1)indicates the starting point of [i] and the start of pharyngealcavity expansion, 2) shows when the voicing starts, 3)indicates the end of the voicing and the end of the first phaseof pharyngeal pressure raising, 4) shows the release of thebilabial stop closure made for [p].

Time

1 2 3 4

Figure 1b. Spectrogram, audio-signal and pharyngeal pressurefor the word ipaka ÔmothÕ pronounced by the second speaker.1) indicates the starting point of [i] and th start of pharyngealcavity expansion, 2) shows when the voicing starts, 3)indicates the end of the voicing and the end of the first phaseof pharyngeal pressure raising, 4) shows the release of thebilabial stop closure made for [p].

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Time

1 2 3 4

Figure 2. Spectrogram, audio-signal and pharyngeal pressurefor the word ddaka ÔpromiseÕ pronounced by the first speaker.1) indicates the starting point of [i] and the start of pharyngealcavity expansion, 2) shows when the voicing starts, 3) showsthe end of the first phase of pharyngeal pressure raising, 4)shows the release of the alveolar stop closure made for [d].

One important fact to note when aerodynamic data arecompared with acoustic data is that one can always observe adecrease of voicing amplitude at the end of [i], consecutive to thebalance between pharyngeal and subglottal pressure at thismoment. Pharyngeal pressure raising and zero oral airflow showthat there is a closure in the vocal tract. This raising is moreimportant compared to stops not preceded by [i]. Mean value ofpharyngeal pressure is 14.5 hPa before voiceless and 9.74 hPabefore voiced consonants (n=20). Mean values for stops notpreceded by [i] are 7.3 hPa for voiceless consonants and 4.1 hPafor voiced consonants (n=20). The reason why pharyngealpressure reaches higher values for [i] preceding other consonantsis difficult to understand at first sight. However it is possible tosuggest that the reduction of pharyngeal cavity during the settingback of tongue body and tongue root contributes to the first phaseof the pressure raising. The second phase and the high pressurevalue is made when the glottis is set for the following consonant.The whole articulation of [i] and the following consonant beingalways made with stress.

The characteristic voicing of [i] starts immediately afterpressure reaches its lower value. This suggests a release of thevocal folds tension when the larynx goes up.

3.3. Articulatory data

3.3.1 Video data. A high speed video camera has been used todescribe the sequence of articulatory events producing [i] and tomake parallels with data obtained in aerodynamic and acousticmeasurements. The video camera has been configured to capturesgray scale images of 240 x 192 pixels at a rate 125 images persecond. Audio signal and trigger pulses corresponding to imageacquisition have been recorded simultaneously on a DAT. Thepulses were used to post-synchronize the audio signal with thevideo data. This technique permits a fine decomposition ofmovements and an observation of jaw, larynx and tongue body

positions. The latter being deduced from the movement made bythe skin between the jaw bone and the neck. The most difficultmovement to observe is larynx lowering because it is variable inamplitude and because of subjects physiological characteristics.During the production of [i], the lower jaw position is notsensibly modified by comparison with the sharp movement of thetongue which is observed in all cases for both subjects. Thismovement of variable amplitude is easily identifiable, its durationis quite short and corresponds to the large expansion of thepharyngeal cavity. When the tongue sets back to a higher positionthis corresponds to the increase of pressure in the pharyngealcavity. The pharyngeal cavity expansion is necessary to maintainvoicing during [i]. Figure 3 describes the main events observedby using this video technique for the word [ipaka] ÔmothÕ. Theimages presented in figure 3 reveal the following sequence ofevents. First, between cursors 1 and 2, there is a loweringmovement of the tongue body corresponding to the observednegative pharyngeal pressure. This movement is continued untilthe end of the [i] voicing (cursor 3). At this point one can alsonote a passive expansion of the vocal tract made by inflating thecheeks. Then there is a raising movement of the tongue bodysimultaneous of a lowering of the jaw. This last movement beingmade to anticipate the jaw position for the following vowel [a].

1 2 3 4

1 2 3 4

Time

Figure 3. Front and profile images of a speaker pronouncingthe word ipaka ÔpromiseÕ. Signal and spectrogram aresynchronized with the video images. 1) is the beginning of thepharyngeal expansion, 2) is the point were voicing starts, 3) isthe end of [i] voicing and the end of tongue body lowering 4)is the image just before the bilabial stop release.

3.3.2. MRI data. From an articulatory point of view, one lastthing must be established, this is the closure location of [i]. Basedon mid-sagittal MRI slices, one can say that there is a pre-palatal,or sometimes palatal, closure corresponding to the closure of theconstriction normally made for [i]. Consequently this is anarticulatory ÔovershootÕ of the vowel [i] which has generated thisparticular allophone. Figure 4 shows a mid-sagittal slice of [i]where it is possible to observe a pre-palatal contact, a largepharyngeal volume the larynxÕs low position.

Dynamic MRI sequence of images show that the order ofevents deduced from aerodynamic and acoustic measurements iscorrect. First there is a lowering of the larynx accompanied by anexpansion of the pharyngeal cavity made by the fronting of thetongue root and by the lowering of the tongue body. Second there

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is a reduction of the pharyngeal cavity made by the setting backof the different articulators.

Figure 4. MRI Mid-sagittal slice of a sustained [i].

3.3.3. Movetrack data. The Movetrack plots of figure 5 can beinterpreted as follows. Jaw movement along the X axis indicatesthat during the realization of [i], the jaw moves slightly fowardand starts to back up just before closure release in order toanticipate the production of [a]; along the Y axis one can observethat the jaw is in high position during [i] and starts to lowertowards the end of the bilabial stop in order to prepare [a]. Themovement of the underneath part of the tongue body along the Xaxis indicates a progressive fronting, along the Y axis one canobserve the tongue lowering. This movement reaching themaximum at the end of the [i] voicing. Thus the Movetrack datacorroborates the sequence of events observed in acoustic,aerodynamic, video and MRI recordings, in addition to providedata on the timing and amplitude of these events.

4. CONCLUSION

Acoustic, aerodynamic and articulatory techniques havepermitted a precise description of a particular allophone of [i] inHendo. This phonetic description corroborates structural factsobserved and described by Ngonga [10] and show that anaccurate phonetic description permits to support phonologicalfacts whose nature are not easy to understand and to describe.

5. ACKNOWLEDGEMENT

This research has been supported by the Convention ARC"Dynamique des syst�mes phonologiques" 98Ð02, n¡226.

Jaw X

Jaw Y

Chin X

Chin Y

Time

1 2 3 4

Figure 5. X and Y plots of the chin and jaw movementsrecorded with the Movetrack, for the word ipaka ÔmothÕpronounced by the second speaker. Signal has been recordedsimultaneously, 1) indicates the starting point of [i] and thestart of tongue body lowering and fronting, 2) shows when thevoicing starts, 3) indicates the end of the voicing and the endof tongue body lowering, 4) shows the release of the bilabialstop closure made for [p] with a lowered jaw.

6. REFERENCES

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[2] Hulstaert, G. 1950. Carte linguistique du Congo belge. Bruxelles,IRCB.

[3] Van Bulck, V. 1948. Les recherches linguistiques au Congo belge,r�sultats acquis, nouvelles enqu�tes � entreprendre. Bruxelles,IRCB.

[4] Van Bulck, V. 1952. Les deux cartes linguistiques du Congo belge.Bruxelles, IRCB.

[5] Bongo, . 1968. Esquisse phonologique du nkutshu, dialecteOhendo. M�moire de lÕUniversit� de Lovanium.

[6] Atlas Linguistique de lÕAfrique Centrale. 1983. Za�re. Paris.ACCT-Cerdortola.

[7] Bastin, Y., A. Coupez and B. de Halleux. 1983. Classificationlexicostatistique des langues bantoues (214 relev�s). Bulletin delÕAcad�mie Royale des Sciences dÕOutre-Mer. 173-199.

[8] Teston, B. and B. Galindo. 1995. A diagnostic and rehabilitationaid workstation for speech and voice pathologies. Eurospeech 4.Eur. Speech Com. Ass. Madrid. 4.

[9] Van Vaals J., G. Van Yperen, A. Hoogenboom and M. Duivestijn.1994. Local Look (LOLO) : Zoom fluoroscopy of a moving target.Proc. of the First SMR Meeting. Dallas 38.

[10] Ngonga, Ke-Mbembe, H. In preparation. Langage initiatiqueohendo : phonologie nte metaphonologie des langues d�riv�es.Doctoral dissertation. Universit� Libre de Bruxelles.

[11] Branderud, P. 1985. Movetrack Ð a movement tracking system. InProceedings of the French Swedish Symposium on Speech. 113-122. GALF. Grenoble, France.

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