In Luganda, a Bantu language spoken in Uganda, there is a contrast between high (H) and low (L) tone (Tucker, 1962; Cole, 1967; Stevick, 1969; Hyman, 1982; Hyman, Katamba, & Walusimbi, 1987; Hyman & Katamba, 1993; Hyman & Katamba, 2010). In (1), for example, all the syllables have low tone except the boldfaced one. Here and henceforth, the high-tone span is boldfaced in the transcription, and the underlined syllables are the ones with underlying high tone. Morpheme gloss abbreviations are defined in the Appendix.

In (1), high tone is associated with a single syllable, which is a short high-tone span. Such a span in Luganda has an f₀ rise that begins before the onset of the high-toned syllable and ends in an f₀ maximum around the end of the vowel, followed immediately by an f₀ fall over the course of the next syllable (Myers, Namyalo, & Kiriggwajjo, 2018). The timing of the f₀ rise offset depends on the duration of the onset, vowel, and coda of the high-toned syllable, with longer segments associated with a later rise offset (Myers et al., 2018). The f₀ rise and fall is coordinated quite precisely with the segments, as in other languages (Silverman & Pierrehumbert, 1990; Prieto, van Santen, & Hirschberg, 1995; Arvaniti, Ladd, & Mennen, 1998; Xu, 1998; Myers, 2003).

However, there are also high-tone spans in Luganda that are longer than one syllable, which we will refer to as long high-tone spans. There are two processes of unbounded leftward spread of high tones (to be described in more detail below in Section 1.3), each of which yields long high-tone spans (Tucker, 1962; Cole, 1967; Stevick, 1969; Hyman, 1982; Hyman et al., 1987; Hyman & Katamba, 1993; Hyman & Katamba, 2010), as in (2).

In (2), the high-tone span extends over four syllables from the second syllable of the verb yalera to the antepenult of the object NP nnawolovu. There is no upper limit on the length of a high-tone span in Luganda.

There are various reasons that we might expect long high-tone spans to differ from short high-tone spans in f₀ level or f₀ timing. Xu and Wang (2001) and Xu and Sun (2002) have shown that even at normal speaking rates it takes most of a syllable’s duration for an f₀ target to be attained. In a short high-tone span, the speaker must then immediately begin the transition to the next target, while in a long high-tone span that next transition is in some later syllable. Time pressure is likely to be a greater factor in the realization of a short high-tone span than in the case of a long high-tone span.

One frequent effect of such time pressure is undershoot of the target (Lindblom, 1963; Moon & Lindblom, 1994). F₀ excursions were reduced in faster speech compared to normal-rate speech for one English speaker in Ladd, Faulkner, Faulkner, and Schepman (1999), and two French speakers in Fougeron and Jun (2000). In Thai, Gandour, Tumtavitikul, and Satthamnuwong (1999) found the pitch range was reduced at faster speaking rates in unstressed syllables for tones with greater f₀ excursions (high, falling, rising). Grabe (1998) and Grabe, Post, Nolan, and Farrar (2000) found that the f₀ range was truncated when the duration of the voiced vowel was shortened through addition of voiceless consonants, in German, and in English spoken in Leeds and Belfast.

Undershoot is also conditioned by time pressure in segmental categories. Long vowels tend to be more peripheral and so more spread out in F1 * F2 space than corresponding short vowels, e.g., in Finnish (Lehtonen, 1970: 86; Wiik, 1965), Chickasaw (Gordon, Munro, & Ladefoged, 2000), and Japanese (Hirata & Tsukada, 2009). Likewise, there are more complete closures with long stop consonants than corresponding short ones, e.g., in Japanese and Swedish (Löfqvist, 2005), Tashliyt Berber (Ridouane, 2007: 130), and Persian (Hansen & Myers, 2017). In general, shorter segments are more reduced and more subject to the coarticulatory influence of neighboring sounds than longer segments (Lindblom, 1963; Moon & Lindblom, 1994).

Another response to time pressure is to increase the velocity of speech movements. For example, for the majority of English speakers in Ladd et al. (1999) and Grabe (1998), faster speaking rate did not affect f₀ excursion, but was associated with faster f₀ change. For these speakers, the height of high-tone targets was maintained under time pressures, but the transition to peak f₀ was shortened.

Similarly, articulatory gestures in the production of short segments are significantly faster than in corresponding long segments. In German, Hertrich and Ackermann (1997) found less gestural stiffness (the slope of peak velocity against displacement) in the opening gesture for long vowels than for short vowels. Gestural velocity was found to be lower in the production of long consonants in Japanese compared to short ones (Smith, 1995; Löfqvist, 2007), and gestural stiffness was less for long labial consonants than for short ones (Smith, 1995; Löfqvist, 2005). Peak tongue position in medial lingual consonants in Japanese is attained later in long consonants than in short ones (Fujimoto, Funatsu, & Hoole, 2015). In vowel-vowel sequences in Finnish, the formant transition to the second vowel is longer if that vowel is long than if it is short, and this difference in formant transition duration has been shown to be a perceptual cue for vowel length in the language (Myers & Hansen, 2005). Löfqvist (2005) suggests that long segments are produced not by stopping at the target position and holding there, but by making the transitions to and from that target more gradual.

If the response to time pressure in Luganda tone is undershoot, then the f₀ rise excursion and the subsequent f₀ fall excursion will be smaller in short high-tone spans than in long ones. If the response is temporal compression, then the f₀ rise will be earlier in the span-initial syllable in short high-tone spans than in long high-tone spans, and the f₀ fall will be later in the span-final syllable than in the short high-tone spans. If time pressure is not a significant factor in tone realization, there will be no difference between short and long high-tone spans beyond the duration of the high f₀ plateau.

In addition to the length of the span, another important consideration in the phonetic realization of high-tone spans in Luganda is the source of the high tone. The high tones in (1) and (2) are lexical high tones, belonging to morphemes within the constituent words. There are also intonational tones. In (3), for example, there is no lexical high tone, but the sentence is realized with a high tone span extending from the second syllable of the verb phrase to the end of the sentence.

This high-tone span always includes the last syllable of the phrase, and it doesn’t belong to any segmental word. Hyman and Katamba (1993) thus identify it as a boundary tone H%. Its meaning and distribution are unclear: Hyman (1982) describes it as characteristic of list intonation, while Hyman and Katamba (2010) describe it as “indicating ‘finality.’” The citation-form transcriptions in Snoxall (1967), Cole (1967), and Stevick (1969) all include this intonational high tone for all items ending in two or more syllables without a lexical high tone. We have found such a final high-tone span to be typical in any Luganda statement ending in such a sequence.

The intonational high tone has always been transcribed the same as the lexical high tone in the literature on Luganda tone, and no distinction between the two was described in the detailed phonetic descriptions of Luganda tones in Tucker (1962, 1967). However, Fainleib and Selkirk (2014), in a small-scale instrumental study with one Luganda speaker, found that for their participant H% differed phonetically from lexical H. They found that the f₀ rise at the onset of an intonational H% span as in (3) was smaller than for a lexical high tone, and that there was a fall in f₀ over the course of the high-tone plateau in the intonational high tone but not in the lexical high-tone span. In other words, the raised f₀ interval was close to even in f₀ value in the case of a lexical high-tone span, but had a drop of about 9 Hz over the course of an intonational high-tone span.

H% can differ from other H tones in phonetic implementation. Pierrehumbert (1980: 26), in initially defining boundary tones for English, notes that “H* and H% are equally high tones,” differing primarily in where they occur in the phrase (on a stressed syllable, and on a phrase-final syllable, respectively). However, Pierrehumbert (1980: 90) argues that H% is subject to Upstep, while pitch accent H* is not. In Japanese, Pierrehumbert and Beckman (1988: 81) propose that the downtrend called catathesis is triggered only by accentual H, but can be undergone by either an accent H or boundary H%. Myers (1996) found that H% in Chichewa, unlike lexical high tones in that language, was not subject to downdrift, did not condition it, and was transparent to downdrift effects between lexical high tones on either side of it.

The findings of Fainleib and Selkirk (2014) suggest that lexical high tones in Luganda have greater f₀ excursions and less declination than the intonational high tones. However, only a single speaker participated in the study, so it does not provide a firm basis for generalization. In the following study, their claims about the phonetic differences between lexical and boundary high tone are tested with a larger sample of speakers.

There is still not much information available about the phonetic implementation of boundary tones, despite an important cross-linguistic body of work focused on the distribution and interpretation of such tones, e.g., the studies in Jun (2005) and Downing and Rialland (2017). The current study is intended to add to the still small literature on how boundary tones differ in phonetic realization from other intonational and lexical tones.

Since the following experimental study will deal with the difference between long and short high-tone spans, it is necessary to review what the literature has established about the conditions under which a high tone spreads or does not spread in Luganda.

One of the two sources of unbounded high tone spans is the process of High Tone Plateauing (Hyman & Katamba, 2010), in which the two separate lexical high tones are joined into a single extended high-tone span (Hyman et al., 1987; Hyman & Katamba, 1993; Pak, 2008; Hyman & Katamba, 2010). This process is given in Figure 1 in the formulation of Hyman and Katamba (2010), and it is exemplified in (4).

The verb yalera in (4b) and the object nnawolovu in (4c) each have one high-toned syllable when they occur in isolation. But when the two words occur in succession, as in (4d), there is a high-tone span extending from the high-toned syllable of the verb to that of the complement.

There are some syntactic conditions for this process (Hyman et al., 1987; Pak, 2008; Hyman & Katamba, 2010). According to Hyman and Katamba (2010), a sequence of words that meets the phonological structural description in Figure 1 is only subject to the rule if the words belong to the same tone group (TG). A tone group corresponds to a head-initial maximal projection XP, in which the head is not an ‘inherently-focused’ verb type, e.g., negative, imperative, or infinitive. It is further required that the complement in XP not include an ‘initial-vowel’ augment, a morpheme involved in the marking of focus.

The second source of unbounded high tone spans in Luganda is High Tone Anticipation. If a word with a high tone follows a word within the same phrase which ends in a low-toned syllable, the high tone in the later word is spread leftward up to the second syllable of the phrase or to the second syllable after a high tone (Hyman et al., 1987; Hyman & Katamba, 1993; Pak, 2008; Hyman & Katamba, 2010). This process is formalized by Hyman and Katamba (2010) as in Figure 2, where PW is a phonological word, and TP is a tone phrase. A tone phrase is a constituent superordinate to the tone group, and it consists of the VP or any preverbal maximal projection. An example of High Tone Anticipation is given in (5).

In the isolation forms in (5b) and (5c), the verb alera has no high tone and the complement omuwere has a high tone on the penult. But when they are juxtaposed in the sentence (5e), the high tone of the complement extends leftward to the second syllable of the verb. It is stopped from spreading onto the first syllable of the verb, in this analysis, by an initial L% boundary tone on the first syllable of the tone phrase.

If a high tone does not meet the conditions for High Tone Plateauing or High Tone Anticipation, it will not be spread and so it will retain a short high tone span on just one syllable. The high tone on the second syllable of the verb in (1), for example, is at the beginning of a tone phase (as defined above) and so does not meet the conditions for either tone spread process.

In this paper, we will assume for purposes of discussion that the tone-bearing unit in Luganda is the syllable, contrary to previous work that has identified the mora as the tone-bearing unit in the language (Tucker, 1962; Hyman, 1992; Hyman & Katamba, 1993; Hyman & Katamba, 2010). Myers et al. (2018) found that the timing of the f₀ peak and the subsequent fall depended on the duration of the onset, the vowel of the syllable, as well as the duration of the postvocalic consonantal interval. They did not find that this dependency was sensitive to whether or not the segment was moraic, and so concluded that the mora was not relevant to the timing of f₀ events in Luganda. However, in the following experiment, this controversy is moot, since the test syllables are all simple open syllables with short vowels, so that the syllable consists of just one mora.

Ten native speakers of Luganda participated in the study: three females and seven males. All had grown up in the Central region of Uganda and were living in the Kampala area at the time of the study. The speakers are described in Table 1.

The speakers ranged in age at the time of the experiment from 24 to 49. They came from all over the Central region of Uganda, the traditional homeland of the Baganda, from the Rakai district in the Southwestern corner of the region (S5) to Mukono in the Southeast (S2), Mubende in the Northwest (S3, S4, S10), and Kayunga in the Northeast (S7).

The test sentences in the study were all affirmative statements with a lexical subject, a verb in the present or past tense, and a nominal complement (either an object or a locative). Each had a medial high-tone span, within which all consonants were sonorant, and outside of which all syllables lacked lexical high tone. The initial and final syllable of the high tone span was always an open syllable with a short vowel.

There were four sentence classes: HH, LH, HL, and LL. These four classes differ in whether they have a long high tone span or a short one, and whether the high tone span consists of a boundary tone or a lexical tone. Twenty sentences of each of the four types were produced by each speaker, for a total of 80 sentences per speaker, and 800 sentences for the study. The test sentences are listed in the Appendix.

The HH sentences (listed in [1] in the Appendix) have a lexical high tone in the verb and also in the following complement. The verb does not belong to an ‘inherently focused’ inflection class, and the complement does not have an augment, so the verb and the complement belong to the same tone group, as defined in Hyman and Katamba (2010). High Tone Plateauing, as in Figure 1, thus applies, yielding a high tone span extending from the underlying verb-stem-initial high-toned syllable to the underlying high-toned syllable in the complement. The HH sentences have a long high-tone span with a lexical high tone.

The LH sentences (listed in [2] in the Appendix) have a verb with no lexical high tones, and a complement with one lexical high tone on a non-final syllable. The verb and the complement belong to the same tone phrase, as defined above, so they are subject to High Tone Anticipation, as in Figure 2. This yields a high-tone span extending from the second syllable of the verb (i.e., the first syllable of the verb stem) to the underlying position of the high tone in the complement. The LH sentences have a long high-tone span with a lexical high tone, like the HH sentences.

The HL sentences (listed in [3] in the Appendix) have a lexical high tone on the verb stem and none in the complement. A high tone in that position cannot spread leftward, since it is at the beginning of the tone phrase as defined above. As a result, the high tone in these sentences is realized on a single syllable. The HL sentences have a short high-tone span with a lexical high tone.

The LL sentences (listed in [4] in the Appendix) have no lexical high tones either in the verb or the complement. In such a case, an intonational H% is inserted and spread leftward under the conditions of High Tone Anticipation. The LL sentences have a long high-tone span with an intonational high tone.

Recordings were made in a quiet room on the campus of Makerere University in Kampala, Uganda, using a Shure SM10A head-mounted microphone and a Marantz PMD670 solid-state recorder, with a sampling rate of 44.1 kHz and 16-bit amplitude resolution.

The sentences were presented to the participants in a PowerPoint slideshow on a laptop computer, with each sentence on a separate slide. The slides were shuffled into a quasi-random order. Speakers were instructed to read each sentence to themselves first, and then to produce it without internal pauses, as a statement and a separate utterance (rather than as a member of a list). They were told that if they were not satisfied with their initial production, they could keep saying the sentence until they felt they had it right. They proceeded at their own pace through the sentences, but were instructed to finish saying a sentence before pressing the key to switch to the next one. Where the speaker had produced a sentence more than once, the last one was selected for analysis, unless it had a clear internal pause or slip.

The LL sentences were elicited in a separate block, since it was assumed on the basis of the previous literature that the final H% was optional, and that extra instructions would be required to elicit it. Speakers were presented with an example LL sentence, and were asked if they could say the sentence with more than one pitch pattern, but with the same meaning. The intention was to elicit the two pronunciations that have been described in the literature: one with all low-toned syllables, and one with the optional H% plateau. Speakers would then be instructed to produce each sentence first with one pronunciation, and then with the other.

However, it turned out that most speakers were unaware of the two options. Only three of the 10 speakers felt they had more than one pronunciation with the same meaning, and they were asked to produce each sentence both ways. The speakers who had only one pronunciation were asked to produce the LL sentences with that pronunciation.

The speakers who had just one pronunciation consistently produced the LL forms with a high-tone span extending from the second syllable of the verb to the final syllable of the sentence. This was also the pronunciation provided by all speakers for two instances of a sentence of the LL class that was mistakenly included in the main recording block without any separate instructions. For two of the speakers who felt they had two pronunciations of the LL sentences, both pronunciations had the final H% plateau, and the two pronunciations differed just in that one was produced with a greater pitch range than the other. Only one speaker produced the expected pair of pronunciations: one entirely low-toned, and the other with a high-toned plateau extending from the second syllable of the verb to the end of the sentence. It is probably worth noting that this speaker has been trained in Luganda phonetics, and was aware of how Luganda tone has been described.

For the speakers whose pronunciations differed just in pitch range, the production with the more reduced pitch range was selected for analysis, since it was produced more fluently and with less hesitation than the more emphatic, expanded-range productions. For the speaker with two tonally distinct pronunciations, the one with H% was selected for the analysis, since these are the only ones with a high-tone span to measure. For this speaker there were two tokens of LL sentences for which no H%-final pronunciation was offered. These two tokens were excluded from the dataset, leaving 798 utterances for the analysis.

Measurements were made in Praat (Boersma & Weenink, 2013). The onset and offset of the following segmental intervals were marked in the annotation tier: the consonant at the beginning of the first high-toned syllable in the high tone span (C₁), the vowel of that syllable (V₁), the consonant following V₁ (C_1a), the consonant at the beginning of the last high-toned syllable in the span (C₂), and the vowel of that syllable (V₂). The duration of each of these intervals was obtained, and the onset timepoints for C₁ and C₂ were selected as reference points for the timing of f₀ events. The onset of each consonant interval was set at the endpoint of the decline in waveform amplitude from the preceding vowel, while the offset of each consonant interval was set at the point at which amplitude began to rise at the end of the constriction interval. The vowel intervals were the intervals between consonant intervals. In the long-span conditions, the first syllable of the span was distinct from the final syllable, but in the short-span condition, the first and last syllable of the span were the same syllable, so C₁ was the same interval as C₂, and V₁ was the same as V₂.

A PitchTier representation of the soundfile was constructed using the autocorrelation method, with 5 Hz smoothing. The f₀ rise was defined as the interval of pitch points in the PitchTier which overlapped with the C₁-V₁ syllable and in which f₀ was monotonically rising. The rise onset was the first pitch point that was followed by a point with a higher numerical f₀ value, and the rise offset was the first point after that which was followed by a point with an equal or lower f₀ value. The f₀ fall was defined as the monotonically falling interval after that which overlapped with the span-final C₂-V₂ syllable. In the (short-span) HL condition, the rise offset was quite often at the same point as the fall onset, in which case both represented a single point that was the f₀ maximum of the rise-fall pattern. The f₀ plateau was the interval of raised f₀ points extending from the rise offset to the fall onset.

The following f₀ measurements were obtained:

F₀ timing measurements were made relative to the test syllable onset: the onset of C₁ for the f₀ rise, and the onset of C₂ for the f₀ fall. The following f₀ timing measurements were obtained:

Generally, there was a final fall in f₀ at the end of LL sentences, but in 32 cases, f₀ remained at the same high level to the end of the utterance, or actually rose at the end. Of these cases, 29 were produced by two speakers, S4 and S5, who thus had such productions in the majority of their LL sentences. The cases without a final fall were evenly distributed across the 5 LL sentences. These 32 cases are excluded from analyses concerned with the final f₀ fall, leaving 766 utterances for these analyses. Sample annotated displays are given below in Figures 3–6.

With regard to the f₀ rise and fall excursion in (6), the findings of Fainleib and Selkirk (2014) would lead us to expect a greater rise excursion and a greater fall excursion in the lexical tone spans (HH, LH, HL) than in the intonational tone span (LL). In addition, if long tone spans are hyperarticulated compared to short ones, we would expect the long spans (HH, LH, LL) to have a greater rise excursion and a greater fall excursion than the short spans (HL). On the basis of Fainleib and Selkirk (2014), it is also expected that LL will have a greater negative plateau excursion than in the lexical tone conditions (HH, LH, HL), reflecting declination during the plateau.

If long high-tone spans have slower f₀ transitions than short high-tone spans, we would expect the long spans to have greater peak delay than the short ones. At the other end of the span, slower f₀ transitions would be reflected in a lower mean fall delay (i.e., an earlier start to the f₀ fall) for the long tone spans compared to the short ones.

Mixed-model analyses were conducted, using the packages lme4 (Bates, Maechler, Bolker, & Walker, 2014), lmerTest (Kuznetsova, Brockhoff, & Bojesen, 2014) in R (R Core Team, 2017). The fixed effects were tone (LH, HH, HL, LL), and, for the analysis of peak and fall delay, the segmental durations C₁ and V₁, or C₂ and V₂. For tone, the default value was HH, and the other three categories were marked. The participant and the sentence were included as random intercepts, and the interaction of subject with tone was included as a random slope. To explore further pairwise comparisons among the tone conditions, the resulting mixed model was analyzed using lsmeans (Lenth & Love, 2018). The alpha level was .05.