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Acoustic correlates of stress in Afrikaans

The phonetic correlates of Afrikaans stress are, as in other languages, pitch, duration, intensity and vowel quality. Consequently, the acoustic signals of stressed vowels often have a higher fundamental frequency (F0), have greater duration (they are longer), they may be characterised by higher amplitudes (more intensity) and, in terms of their formant frequencies (F1, F2, F3), they are more peripheral in terms of the traditional vowel-space than unstressed vowels which are frequently reduced to a certain extent (i.e. closer to the mid-central position). For a general overview and reference list, see the topic on acoustic correlates of stress in Dutch, and Wissing (2012).


In this section, we demonstrate the acoustic features of a stressed vowel in comparison to its unstressed counterpart, and then report on some research findings concerning the acoustic correlates of stress in Afrikaans.

The figure in Table 1 depicts the stressed vowel /a/ in the first syllable of the word aanhou /'an.ɦœu/ ['an.ɦəu] continue (left) compared to the phonetically shortened and unstressed vowel [ɑ] in the first syllable of the derived form aanhoudend /an.'ɦœu.dənd/ [ɑn.'ɦəu.dənt] continuing (right).

The four acoustic correlates of stress are provided in the spectrogram (bottom window). Note the relatively higher pitch and intensity of the stressed vowel aa in aanhou, as well as its greater duration and lower F2, indicative of a perceivably more rounded and backed [ɒ]. The difference in stress is clearly perceivable in the soundfiles, waveforms and spectrograms of the first vowels of aanhou and aanhoudend in Table 1 below.

Table 1
Sound Sound waves and spectrogram
Figure 1
[click image to enlarge]

In the table below the respective acoustic measurements of these vowels are presented.

Acoustic measurements of stressed and unstressed vowels in Figure 1, viz. aanhou and aanhoudend
Table 2: Acoustic measurements of stressed and unstressed vowels in Figure 1, viz. aanhou and aanhoudend
Parameter Stressed /a/ áá(nhou) Unstressed /a/ aa(nhóúdend)
Duration (sec.) 240 ms 108 ms
Pitch (F0) 265 Hz 208 Hz
Intensity 77.5 dB 70 dB
Vowel quality (F1) 675 Hz 776 Hz
Vowel quality (F2) 970 Hz 1168 Hz

These values are all indicative of the difference between the same vowel (phonemically long, low /a/) in comparable phonetic contexts. It is well known, however, that especially duration, pitch and intensity do not always differ as dramatically across contexts as observed in the current example. Sluijter and Van Heuven (1996), for one, rightly points out that stress is never realised by a single acoustic property. Generally, duration and pitch are taken to be more important than intensity and vowel quality. In this specific example, stressed /a/ is notably more rounded (and thus acoustically backed) than its unstressed counterpart, the difference clearly discernable in the much higher F2 value of the later. Relatedly, Sluijter and Van Heuven (1996) found that intensity in specific frequency bands is a more important correlate of stress than total intensity. They show that, in the case of stressed syllables, only frequency components above 500 Hz exhibit increased intensity, and are thus of importance with regard to stress. Wissing (2007), in an experiment involving the production of the short /ɑ/, reported similar results when comparing vowels in accented position in a sentence to those in unaccented positions in a sentence. Similar results to those of Sluijter and Van Heuven (1996) were found when investigating intensity in the region 0.5 kHz – 2.0 kHz and in the region 2.0 kHz – 5.0 kHz. Furthermore, vowel quality was found to be a powerful parameter too. Less well-known parameters were investigated as well. Of those, the gradient of F0 for stressed vowels in accented positions in a sentence – but not in unaccented positiosn – turned out to be a strong indicator of stress as well.

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