7.3.3 Factors Influencing Phonetic Variation

One important factor that influences phonetic variation is the rate of speech, generally measured in syllables per second. Rate of speech varies both across and within speakers. Many kinds of phonetic reduction processes are much more common in fast speech, including flapping, is vowel reduction, and final /t/ and /d/ deletion (Wolfram, 1969). Measuring syllables per second (or words per second) can be done with a transcription (by counting the number of words or syllables in the transcription of a region and dividing by the number of seconds), or by using signal-processing metrics (Morgan and Fosler-Lussier, 1989). Another factor affecting variation is word frequency or predictability. Final /t/ and /d/ deletion is particularly likely to happen in words which are very frequent like and and just (Labov, 1975; Neu, 1980). Deletion is also more likely when the two words surrounding the segment are a collocation (Bybee, 2000; Gregory et al., 1999; Zwicky, 1972). The phone [t] is more likely to be palatalized in frequent words and phrases. Words with higher conditional probability given the previous word are more likely to have reduced vowels, deleted consonants, and flapping (Bell et al., 2003; Gregory et al., 1999).

Other phonetic, phonological, and morphological factors affect variation as well. For example /t/ is much more likely to flap than /d/; and there are complicated interactions with syllable, foot, and word boundaries (Rhodes, 1992). As we will discuss in Ch. 8, speech is broken up into units called intonation phrases or breath groups. Words at the beginning or end of intonation phrases are longer and less likely to be reduced. As for morphology, it turns out that deletion is less likely if the word-final /t/ or /d/ is the English past tense ending (Guy, 1980). For example in Switchboard, deletion is more likely in the word around (73% /d/-deletion) than in the word turned (30% /d/-deletion) even though the two words have similar frequencies.

Variation is also affected by the speaker's state of mind. For example the word the can be pronounced with a full vowel [dh iy] or reduced vowel [dh ax]. It is more likely to be pronounced with the full vowel [iy] when the speaker is disfluent and having "planning problems"; in general speakers are more likely to use a full vowel than a reduced one if they don't know what they are going to say next (Fox Tree and Clark, 1997; Bell et al., 2003; Keating et al., 1994).

Sociolinguistic factors like gender, class, and dialect also affect pronunciation variation. North American English is often divided into eight dialect regions (Northern, Southern, New England, New York/Mid-Atlantic, North Midlands, South Midlands, Western, Canadian). Southern dialect speakers use a monophthong or nearmonophthong [aa] or [ae] instead of a diphthong in some words with the vowel [ay]. In these dialects rice is pronounced [r aa s]. African-American Vernacular English (AAVE) shares many vowels with Southern American English, and also has individual words with specific pronunciations such as [b ih d n ih s] for business and [ae k s] for ask. For older speakers or those not from the American West or Midwest, the words caught and cot have different vowels ([k ao t] and [ka a t] respectively). Young American speakers or those from the West pronounce the two words cot and caught the same; the vowels [ao] and [aa] are usually not distinguished in these dialects except before [r]. For speakers of most non-American and some American dialects of English (for example Australian English), the words Mary ([m ey r iy]), marry ([m ae r iy]) and merry ([m eh r iy]) are all pronounced differently. Most American speakers pronounce all three of these words identically as ([m eh r iy]).

Other sociolinguistic differences are due to register or style; a speaker might pronounce the same word differently depending on who they were talking to or what the social situation is. One of the most well-studied examples of style-variation is the suffix -ing (as in something), which can be pronounced [ih ng] or [ih n] (this is often written somethin'). Most speakers use both forms; as Labov (1966) shows, they use [ih ng] when they are being more formal, and [ih n] when more casual. Wald and Shopen (1981) found that men are more likely to use the non-standard form [ih n] than women, that both men and women are more likely to use more of the standard form [ih ng] when the addressee is a women, and that men (but not women) tend to switch to [ih n] when they are talking with friends.

Many of these results on predicting variation rely on logistic regression on phonetically-transcribed corpora, a technique with a long history in the analysis of phonetic variation (Cedergren and Sankoff, 1974), particularly using the VARBRUL and GOLDVARB software (Rand and Sankoff, 1990).

Finally, the detailed acoustic realization of a particular phone is very strongly in-

fluenced by coarticulation with its neighboring phones. We will return to these fine-grained phonetic details in the following chapters (Sec. ?? and Sec. ??) after we introduce acoustic phonetics.

7.4 ACOUSTIC PHONETICS AND SIGNALS

We will begin with a brief introduction to the acoustic waveform and how it is digitized, summarize the idea of frequency analysis and spectra. This will be an extremely brief overview; the interested reader is encouraged to consult the references at the end of the chapter.