When I first studied phonetic transcription I learned about broad and narrow transcription, where narrow transcription contains much more detail, like the presence of aspiration on consonants and fine distinctions of tongue height. Of course it makes sense that you wouldn’t always want to go into such detail, but at the time I didn’t think about what detail was excluded from broad transcription and why.
In phonology we learned about phonemes, and how phoneme categories glossed over many of those same details that were excluded from broad transcription. For reasons I never quite grasped, though, we were told that phonemic transcription was a very different thing from broad transcription, and we were not to confuse them. Okay.
I got a better explanation from my first phonetics professor, Jacques Filliolet, who used three levels of analysis: niveau généralisant, niveau pertinent and niveau particularisant. We can translate them as general, specific and detailed levels.
When I started teaching phonology, I realized that the broad vs. narrow distinction did not reflect what I read in books and papers and saw at conferences. When people are actually using phonetic transcription there is no consistent set of features that they leave out or include.
What people do instead is include the relevant features and leave out the irrelevant ones. Which features are relevant depends on the topic of discussion. If it’s a paper about aspiration, or a paper about variation where aspiration may or may not be relevant, they will include aspiration. If it isn’t, they won’t.
I realized that sometimes linguists need to go into more detail than phonetic transcription can easily handle, so they use even finer-grained representations like formant frequencies, gestural scores and voice onset times.
Recently I realized that this just means phonetic transcription is a form of communication. In all forms of communication we adjust the level of detail we provide to convey the relevant information to our audience and leave out the irrelevant parts.
Phonemes are another, more organic way that we do this. This explains why phonemic transcription is not the same as broad transcription: we often want to talk about what sounds go into a phoneme without adding other details. For example, we may want to talk about how English /t/ typically includes both aspirated and unaspirated stops, without talking about fundamental frequency or lip closure.
Another possible translation of Filliolet’s niveau pertinent is “the appropriate level.” This is really what we’re all aiming for: the level of detail that is most appropriate for the circumstances.
Finding the right level of detail for phonetic transcription is actually not hard for students to learn; they do it all the time in regular language. The simplest way to teach it is to give the students assignments that require a particular level of detail.
Students are sometimes frustrated that there is not a single way to transcribe a given utterance. In addition to these differences of level of description, there are stylistic differences: do you write [r] instead of [ɹ] for an English bunched /r/?
Of course the International Phonetic Alphabet was sold as just such a consistent system: one symbol for one sound, in contrast with the messy reality of writing systems. To me this feels very Modernist and Utopian, and it is no accident that it was invented at the same time as other big modernist projects like Esperanto, Principia Mathematica, and International Style architecture.
The IPA falls short of the ideal consistent representation that was sold to people, but has largely succeeded in providing enough consistency, and keeping enough of the mess at bay, for specific purposes like documenting language variation and language acquisition.