Phonetics and Phonology
Think about the difference between aspirated and unaspirated allophones once more. The sounds [p,t,k] for example share a phonetic feature - they belong to the category "plosive" sounds.
Since all plosive sounds that occur at the beginning of a word or a stressed syllable are produced with such a puff of air, i.e. they are aspirated, we can formulate a general rule that learners of English could bear in mind, when they start to learn the English language:
Aspiration Rule in English: Aspiration occurs on all voiceless stops occurring as the first sound in a stressed syllable. Although aspirated stops and unaspirated stops are physically different , we consider both to be the same sound. For English, aspiration is not employed to create a meaning difference.
And here are some more rules for you to keep in mind:
Dissimilation Rule: This type of rule refers to processes whereby two neighboring sounds become less similar. An example is the rule of fricative dissimilation. Consider how hard it is for Germans to produce the th sound... In learning the ordinal numbers, the numbers fifth and sixth for example present a pronunciation challenge in this regard. It is difficult to pronounce two fricatives next to one another when one of them involves the th sound that does not exist in one's language. Interestingly though, such sound sequences appear to present challenges for many native speakers, too. Therefore they tend to make one of the two sounds more different from the other. As a result, fifth is pronounced as [fIft] and sixth as [sIkst]. The second fricative becomes a stop, which makes it more dissimilar and easier to pronounce.
Insertion Rule: In this kind of process a sound is added that is not present in slow pronunciation or spelling. For example, when we pronounce the word hamster at a regular speed, most of us will say and hear hampster with a p. This can be confusing when teaching spelling, especially to non-native speakers who don't have a history of reading and hearing English words and their spelling.
Deletion Rule: Finally, there are types of pronunciation processes where sounds are left off. For example, when pronouncing the word police, the word often sounds like pleace and may be confused with please if one is not used to hearing voiced s. This unstressed vowel deletion is fairly common in fast speech and can be confusing to non-native listeners in ESL contexts, who often hear words long before they need to spell them.
When we can predict the environment in which phonemes or allophones will occur, we can write a rule that represents their distribution.
The general form of a phonological rule is:
A -> B/C___D
“A becomes B following C and preceding D”.
Let's go through one last example: Consider the following words:
- write -- ride
- rope -- robe
- lock -- log
- cute -- cued
- pick -- pig
- tap -- tab
Is there a difference in the vowel sounds?
There is! The change-triggering consonants /p t k/ all differ in the same way from their counterparts /b d g/; they are unvoiced, whereas the counterparts are voiced.
A -> B/C___D “A becomes B following C and preceding D”.
V -> V:/___C (voiced) Vowels are lengthened preceding voiced consonants.