2.3.1 The tonotactic approach

The ‘tonotactic approach’ (Creissels Reference Creissels, Larry and Charles1998; term suggested to us by a reviewer) postulates an exclusively tonally determined context for the occurrence of downstep in Tswana. More precisely, a downstep occurs across word boundaries when a word-final high tone is followed by at least two successive high-toned syllables. ‘This downstep occurs irrespective of the precise syntactic nature of the boundary between the two words and of their morphological structure’ (Creissels Reference Creissels, Larry and Charles1998: 146). Examples are provided in (5) below, where, according to Creissels (Reference Creissels, Larry and Charles1998: 145), downstep between the subject and the verb takes place in (5a) and is blocked in the same syntactic environment in (5b) because the tonotactic condition of three adjacent high tones, of which two follow the word boundary, is not met. The final high tone of the first word can be either lexical, or derived by High Tone Spread; Creissels gives examples from the latter (as in (5a, b)).

(5)Tswana (Creissels Reference Creissels, Larry and Charles1998: 145)

2.3.2 The phrasal approach

In contrast, Khoali (Reference Khoali1991) postulates a ‘phrasal approach’ (our term) to downstep in Southern Sotho (see also Kunene Reference Kunene1972 for a descriptive pilot study on downstep in Southern Sotho). Within the framework of Prosodic Domains (Nespor & Vogel Reference Nespor and Irene1986), Khoali (Reference Khoali1991) states that in a sequence of two adjacent high tones across word boundaries, a word-initial high tone is downstepped if both words occur within the same phonological phrase. Khoali (Reference Khoali1991) adopts the indirect reference approach (e.g. Nespor & Vogel Reference Nespor and Irene1986; Selkirk Reference Selkirk1986, Reference Selkirk, John, Jason and Alan2011; Truckenbrodt Reference Truckenbrodt1995) which proposes that phonology is not directly conditioned by syntactic information. Rather, the interface is mediated by phrasal prosodic constituents, like phonological phrase (abbreviated as φ), which are defined with reference to syntactic constituents but need not match them. Khoali (Reference Khoali1991) provides a phrasing algorithm that derives the phonological phrasing from the syntax.Footnote 2

According to Khoali’s phrasing algorithm, the subject and a following verb constitute one phonological phrase in Southern Sotho. An example from Southern Sotho with the phrase structure made explicit is shown in (6).

(6)Southern Sotho (Khoali Reference Khoali1991: 65, 76)

Khoali’s (Reference Khoali1991) approach predicts downstep on the high tone of the subject concord in (6a) and also in (6b).

Khoali adduces language-internal and external evidence for the phrasing in (6): Nouns with an underlying lexical high tone on the stem-initial syllable (pó- in (6a) and (6b)) are realized with an HL-pattern (as opposed to an HH-pattern) when they occur final in a phonological phrase. Thus, the High Tone Spread to the final syllable of the subject in (6) marks it as not phrase-final (Khoali Reference Khoali1991: 70).

In (7), the noun and following possessive concord are separated by a phonological phrase boundary according to Khoali (Reference Khoali1991).

(7)Absence of downstep in Southern Sotho (Khoali Reference Khoali1991: 114, 117)

Again, such a phrasing is supported by language-internal evidence. In non-phrase-final position, High Tone Spread to the word-final syllable would have taken place. As shown in (7), the final syllables of the head nouns are low toned, which Khoali interprets as an indication of their phrase-final status.

Interestingly, both approaches predict downstep for the data in (6), yet for different reasons. While for Khoali (Reference Khoali1991) the prosodic structure of two words within the same phonological phrase is decisive for downstep, for Creissels (Reference Creissels, Larry and Charles1998) it is the phonological condition of three adjacent high tones. Thus, we have two competing accounts of downstep for the Sotho-Tswana languages by Creissels (Reference Creissels, Larry and Charles1998) and Khoali (Reference Khoali1991), and the present study investigates experimentally which of the two accounts holds for the variety of Tswana under investigation. The data of the previous studies are based on elicitation (Creissels Reference Creissels, Larry and Charles1998) and introspection (Khoali Reference Khoali1991), and they come from different, though closely related languages. It is not clear whether the reported differences are linked to the languages investigated or if they form part of dialectal variation. After all, there is evidence that Tswana shares the phrasing of subject and verb (as in (6) above) and noun and possessive (as in (7a) above) with Southern Sotho, as will be shown and discussed in Section 4.1. Furthermore, both Creissels and Khoali explicitly state that they expect dialectal variation across the different varieties of Sotho-Tswana, including the tone system. The data in Creissels (Reference Creissels, Larry and Charles1998) come from a single speaker who is a native of the town Kanye and identifies herself as a speaker of the Sengwaketse dialect. In Cole’s (Reference Cole1955: xvi) classification, Sengwaketse belongs to the Central division of the Setswana cluster of dialects. Khoali’s work investigates the variety of Southern Sotho spoken in Qacha’s Nek in Lesotho and the Maluti area of South Africa.

The current study tests both approaches empirically with speakers of Tswana spoken around Vryburg in the North-West province in South Africa. Vryburg is reported to host dialects of the Southern division of Tswana (Cole Reference Cole1955: xvi). Geographically the three varieties of Sotho-Tswana discussed in this article are thus distributed across an extended area in Southern Africa. By running a production study obtaining acoustic data from several speakers, the current study thus presents the first instrumental approach to the occurrence of downstep in Tswana (as spoken in Vryburg).

3.1.1 Target sentences

In order to test the predictions outlined in (8), the production experiment followed a 2 × 2 design, such that two surface tonal conditions appeared in two boundary conditions. The tonal conditions had two adjacent high tones across word boundaries and differed in the third tone, either (i) H)_{ω ω}(HH, or (ii) H)_{ω ω}(HL. The boundary conditions at the ω-boundary distinguished between the absence and presence of a φ-phrase boundary: H)_ω, and H)_ω)_φ, respectively.

For each of the contexts A, B, C, and D (corresponding to (8a–d), four different sentences were constructed. Examples are given in (9). A full list can be found in the appendix.Footnote 3 The total set of test sentences comprised three repetitions of those 16 sentences.

(9)Illustration of contexts A, B, C, D

Although considerable care was taken in the construction of the stimuli, a number of asymmetries emerged at the locus of interest (final syllable of the noun (H1) and first syllable of the following word (H2)). These are listed here: (i) The stimuli vary in the number of syllables that precede the locus of interest in contexts A, B and C between one and three. (ii) In contexts A and B, a voiced obstruent intervenes in the locus of interest, whereas it is a voiceless obstruent in contexts C and D. (iii) There are lexical high tones and derived high tones in the locus of interest (on the verb in context A, on the noun in context B). Remember that this should not make a difference; see example (6) above, for instance, for a case of a derived high tone that causes downstep. (iv) The number of low-toned syllables following H2 in context D varies between one and two, and shows even a syllabic nasal in one case. (v) In contexts B, C and D, there are obstruents before H1, whereas it is sonorants in context A. Despite these asymmetries, we believe that none of them affect the presence or absence of downstep. Some of these factors may affect the acoustic analysis of downstep since microprosodic influences due to different types of segments show up in f0. However, this was taken care of by applying different types of f0 analyses (see Section 3.3.1 below).

3.1.2 Recording procedure and participants

The 48 sentences (4 contexts × 4 sentences × 3 repetitions) were presented in pseudo-randomized order using presentation software, interspersed with a small number of fillers showing a similar syntactic structure. Stimuli were presented in Tswana only, using Tswana orthography which does not mark tone. Care was taken that the same contexts did not occur twice after each other. Participants were given the possibility to read through the sentences first to familiarize themselves with the sentences. They were then asked to read out the sentences from the screen one-by-one. There was a short break after one third of the full set of stimuli.

Recordings were done in people’s homes in Vryburg and Huhudi, North-West Province, South Africa. An M-audio microtrack II recording device was used (sampling rate 44.1 kHz) together with a head-mounted microphone. Care was taken to minimize background noise. Data were transferred to a computer hard drive for further analysis.

Twelve speakers (five male and seven female, aged between 15 years and about 50 years) took part in the study. Speakers whose speech was considered representative for the area by the local pastor were invited by him to take part in the research voluntarily. According to Cole (Reference Cole1955: xvi), the variety of Tswana spoken in this region belongs to the Southern division of Tswana. In total, 576 sentences were recorded (4 contexts × 4 sentences × 3 repetitions × 12 speakers).

3.3.1 Data analysis

The main acoustic correlate of tone is fundamental frequency (f0) (see Myers Reference Myers1998), which can only be realized on voiced segments. In order to guarantee comparability across all stimulus items, the vowels of the high-toned syllables were delineated and analysed for their f0, following segmentation criteria in Turk, Nakai & Sugahara (Reference Turk, Nakai and Sugahara2006), except for double vowels which we divided by half of their total duration. For every sentence, the vowels carrying the noun-final high tone and the following word-initial high tone were the target vowels used for analysis.

Studies on the acoustic realization of a single high tone in the closely related Bantu language Northern Sotho have shown that fundamental frequency starts rising on the tone-bearing syllable but the f0 peak is only reached later (Zerbian & Barnard Reference Zerbian and Etienne2009). Its exact location varies according to a number of factors: It is reached earlier in syllables containing a voiceless onset than in syllables with a sonorant onset, it occurs earlier in utterance-initial onsetless syllables, and its exact location is speaker-dependent. Frequently, however, the f0 peak is only found between two or three syllables away, counting from the tone-bearing syllable. This behaviour has been described as High Tone Spread in the literature, and occurs when no low tone target follows the high-toned syllable.

Peak spread into the adjacent syllable as in High Tone Spread is not expected to occur in the present data as the second syllable is associated with a lexical high tone and has thus its own tonal target. We thus expect the tonal target to be realized locally.

It remains an open question where to measure the acoustic correlate of the tone of a syllable in such a case. In the following we motivate our choice. Myers (Reference Myers1998, Reference Myers1999) used local f0 maxima in his study on the alignment and scaling of high tones in Chichewa. Zerbian & Barnard (Reference Zerbian and Etienne2009, Reference Ziervogel, Lombard and Mokgokong2010) displayed time-normalized pitch contours across relevant syllables in their study on Northern Sotho and used the average f0 of a vowel for the statistical analysis. For the exposition of our acoustic results, we compared the three measures in (10), based on the respective vowels’ f0 average.

(10)Measures for comparing the tone of the final vowel of the noun (H1) to the initial vowel of the following word (H2)

Both the local f0 maximum (Myers Reference Myers1998) and the mean f0 over a vowel (measure (10a), Zerbian & Barnard Reference Zerbian and Etienne2009, Reference Zerbian and Etienne2010) are susceptible to microprosodic disturbances induced by segments and/or tones (Lehiste & Peterson Reference Lehiste and Peterson1961, Hombert, Ohala & Ewan Reference Hombert, Ohala and Ewan1979). While the latter is controlled for in the study and at least kept constant within contexts, the former could not be fully controlled due to restrictions posed by the language itself. For instance, there are few subject concords in Tswana which start with a sonorant. It was therefore unavoidable that we use subject concords starting with a voiced plosive which is known to lower the f0 slightly (Hombert et al. Reference Hombert, Ohala and Ewan1979). Similarly, in the possessive construction some concords start with voiceless plosives which raise the f0 slightly (Lehiste & Peterson Reference Lehiste and Peterson1961).

In measure (10b), the microprosodic influences by preceding and/or following segments and tones are cut off by using the mean f0 of only the central 50% of the vowel. However, it is known since Myers’ (Reference Myers1999) study that the pitch peak of a high-toned syllable in Bantu can be reached late in the syllable or even only in the following syllable. Zerbian & Barnard (Reference Zerbian and Etienne2009) showed that this holds if the syllable carrying the high tone has a sonorant onset. Thus, using only the central 50% for analysis might leave out the actual pitch target from analysis. This would speak in favour of measure (10c), which considers the f0 only in the last third of the vowel, thereby cutting off microprosodic influences of a preceding segment but retaining a possible pitch target towards the end of the syllable. However, the segment boundary and thus duration of the vowels in context B (bÁ-a- …) could not be determined reliably. Additionally, the long vowel was often accompanied with creaky voice (and hence undefined f0 values) halfway through the vowel. These facts would not affect measure (10b), which looks at the mean f0 of the central 50% around the mid of the vowel.

For the quantitative analysis items were excluded which contained hesitations, mispronunciations, or pauses affecting the syllables under consideration (n = 59), and/or a considerable amount of creaky voice (n = 13) which does not allow f0 measures. Hence, the quantitative analysis was based on 517 cases. A comparison of the means and standard deviations of the three measures in (10) shows that the differences between the measures are not substantial (see Table 3).

Table 3

Comparison of three phonetic measures for phonological tone in Hz.

Given that the differences between the measures are not substantial, the mean of the mid-50% of the tone-bearing vowel is used as acoustic reference because it cuts off microprosodic influences. The further analysis will be based on this measure.

As a reviewer suggested, an alternative method of measuring f0 contours is to fit them to polynomial curves based on f0 values at landmarks in the contour, such as onset of rise, peak, plateau, or offset of fall (e.g. Andruski & Castello Reference Andruski and Costello2004). Given that Tswana is a tone language with only two level tones and little tonal density, the mentioned landmarks are not necessarily local or salient. Figures 1 and 3 below illustrate the problem of defining suitable landmarks for tones in the current study. Whereas f0 maximum seems an appropriate landmark of the tonal target for H1 in context A, in context C, the f0 maximum of H1 clearly shows undesirable but unavoidable microprosodic influences of the affricate. We thus decided against such a modelling approach also because the main focus of the current study is the occurrence of phonological downstep, not the detailed phonetic modelling of the contour. Moreover, our acoustic measures are to be seen in conjunction with the auditory transcription reported in the previous section.

The presence or absence of downstep is established through a comparison between the f0 of H1 and H2 (see Snider Reference Snider1998, Reference Snider2007 for downstep in Bimoba and Chumburung, and Dolphyne Reference Dolphyne1994, Genzel & Kügler Reference Genzel and Frank2011 for downstep in Akan). Downstep occurs when H2 is systematically lower than H1.

3.3.2 Inter-speaker variability

In context A, both the tonotactic and the phrasal approach predict downstep between the final high tone of the first word (H1) and the first high tone of the second word (H2). We thus expected to find the comparison between H1 and H2 to be significantly different across all speakers, which means that downstep is realized. Surprisingly, we found considerable inter-speaker variability in this context (only), which we report on in this section. A series of paired samples t-tests were run examining for each speaker whether H1 and H2 were significantly downstepped in context A. Seven out of 12 speakers realized a significantly downstepped H2 (see Table 4). One speaker (speaker 06) very closely approached the significance level of p < .05. The results from auditory impression mirror the statistical results: There is an equal number of perceived downsteps and non-downsteps in context A, furthermore there was a high number of disagreements in this context. For two speakers (speakers 04 and 09) the difference between H1 and H2 in context A is not significant, which we conclude to be an indication that downstep was not realized. This is confirmed by the auditory impression as these speakers realize more contours that were perceived as non-downstepped than as downstepped. Two other speakers (speakers 05 and 07) show a significant effect, though in the other direction (indicated by negative t-values). Again, this result is mirrored in the results from auditory impression because these two speakers are not perceived as realizing downstep at all.

Table 4

Estimation of downstep realization between H1 and H2 for each speaker in context A.

Significance levels: ^*.05, ^**.01, ^***.001

The speakers who did not produce downstep in context A pattern with the other speakers in contexts B, C, and D though as will be evident later. There is no comparable inter-speaker variation in contexts B, C, and D. This means that the speakers do produce downstep in context B consistently, as well as they did produce no downstep in contexts C and D. The reason for the high inter-speaker variability in context A remains unclear, neither age nor gender seems to be decisive.