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4.1.1.Samples

For finding spectrogram distances between all IPA segments we need samples of one or more speakers for each of them. We found the samples on the tape The Sounds of the International Phonetic Alphabet on which all IPA sounds are pronounced by John Wells and Jill House. On the tape the vowels are pronounced in isolation. The consonants are sometimes preceded, and always followed by an [a]. We cut out the part preceding the [a], or the part between the [a]’s. We realize that the pronunciation of sounds depends on their context. Since we use samples of vowels pronounced in isolation and samples of consonants selected from a limited context, our approach is a simplification of reality. However, Stevens (1998, p. 557) observes that

“by limiting the context, it was possible to specify rather precisely the articulatory aspects of the utterances and to develop models for estimating the acoustic patterns from the articulation”.

The burst in a plosive of the IPA inventory is always preceded by a period of silence (voiceless plosives) or a period of murmur (voiced plosives). When a voiceless plosive is not preceded by an [a], it is not clear how long the period of silence which really belongs to the sounds lasts. Therefore we always cut out each plosive in such a way that the time span from the beginning to the middle of the burst is equal to 90 ms. Among the plosives which were preceded by an [a] or which are voiced (so that the real time of the start-up phase can be found) we found no sounds with a period of silence or murmur which was clearly shorter than 90 ms.

In voiceless plosives, the burst is followed by an [h]-like sound before the following vowel starts. A consequence of including this part in the samples is that bursts often do not match when comparing two voiceless plosives. However, since aspiration is a characteristic property of voiceless sounds, we retained aspiration in the samples. In general, when comparing two voiced plosives, the bursts match. When comparing a voiceless plosive and a voiced plosive, the bursts do not match.

To keep trills comparable to each other, we always cut three periods, even when the original samples contained more periods. When there were more periods, the most regular looking sequence of three periods was cut.

The Levenshtein algorithm also requires a definition of ‘silence’. To get a sample of ‘silence’ we cut a small silent part on the IPA tape. This assures that silence has approximately the same background noise as the other sounds.

To make the samples as comparable as possible, all vowel and extracted consonant samples are monotonized on the mean pitch of the 28 concatenated vowels. The mean pitch of John Wells was 128 Hertz; the mean pitch of Jill House was 192 Hertz. In order to monotonize the samples the pitch contours were changed to flat lines. The volume was not normalized because volume contains too much segment specific information. For example it is specific for the [v] that its volume is greater than that of the [f].