2.2 The acoustic method

Using the acoustic method instead of the auditory method means to feed high-quality recordings into a speech analysis software. One example of such a software is Praat.  With the help of this software, acoustic differences between stressed and unstressed syllables can be measured. Stressed and unstressed syllables differ in vowel duration and quality, pitch height and intensity. These differences can all be investigated with a software like Praat. A varying formant structure shows differences in vowel quality (Gut 2013: 257). Since formants are a feature of the segmental level, this will not be analysed in this context – only the features of vowel duration, pitch height and intensity will be investigated in order to distinguish between stressed and unstressed syllables. It is impossible to investigate the differences between syllables across different speakers since a researcher first has to analyse the differences between all syllables of a word of one single speaker. This is because every person speaks with a specific loudness or pitch height, which has to be considered in the analysis. As an illustration, there is a considerable difference in pitch height between female and male speakers.  If these individual differences were not considered, the results would be skewed.

Vowel duration is measured in milliseconds (ms). In general, vowels in unstressed syllables are much shorter than vowels in stressed syllables. On average, stressed syllables in British English are 300 ms long; unstressed ones are about 150 ms long (Gut 2013: 247). Vowel reduction and vowel deletion in unstressed syllables in English are the reasons for this difference in length. In order to compute the duration difference one has to divide the duration of syllable 2 by the duration of syllable 1. If the result is bigger than 1, syllable 2 is longer; if the result is below 1, syllable 2 is shorter.

Pitch height is measured in hertz (Hz). However, Praat measures pitch differences in semitones, which is more convenient in order to receive significant results. To measure the pitch difference one subtracts the pitch of syllable 1 from the pitch of syllable 2. If the result is positive, the second syllable is of higher pitch (F0).

Intensity is measured in decibels (dB). In general, vowels in stressed syllables have a higher intensity than vowels in unstressed syllables. But Gut argues that intensity might be the “least consistent and least salient property of accentuation in British and American English” (2013: 247). To measure the intensity difference one subtracts the intensity of syllable 1 from the intensity of syllable 2. A positive result means that the second syllable is of higher intensity. Decibel (intensity) and semitones (pitch) are logarithmic units of measurement. Therefore, they can be subtracted, while seconds, as a measure of syllable duration, should be divided in order to receive more reliable results.

The advantage of the acoustic method is that it is more valid and reliable compared to the auditory method. Gut also values the acoustic method as “much more appropriate to demonstrate sources of variation in word stress within and between languages and accents of a language” (2013: 248). On the other hand, it is a very time-consuming method and high quality recordings are required.

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