Note: for some reason the usual devoicing diacritic refuses to display properly, so I have used an ad hoc replacement, a following minus sign. So, for example, [w-] represents a devoiced labial-velar approximant.
Relax. This has absolutely nothing to do with a new novel by the awful Dan Brown.
I don’t know about you, but on the odd occasion I utter the word whew! I use a sound for which there is no recognised symbol. It is a voiceless labial-palatal fricative. I suppose we could use [ɥ-], but that really means a devoiced labial-palatal approximant and I don’t think the two sounds are equivalent. There are at least two ways of making air turbulent. One is to speed up the air-stream. The other is to narrow the stricture through which the air passes. My feeling is that for devoiced approximants the first way is used alone. The fact that the vocal folds are not vibrating means that the air-stream is relatively unimpeded until it reaches the oral stricture and is therefore travelling faster. For voiceless fricatives, on the other hand, I think the stricture is narrower than that for the equivalent voiceless fricative, and this combined with the faster airflow means that the friction noise is more intense.
For some voiceless approximants we already have recognised voiceless fricative symbols to accompany them:
- [w-] ~ [ʍ]
- [ʋ-] ~ [f]
- [l-] ~ [ɬ]
- [j-] ~ [ç]
- [ɰ-] ~ [x]
- [ʁ-] ~ [χ]
However, there are at least two voiceless approximants where there is no symbol for the matching voiceless fricative: [ɥ-], as already mentioned, and [ɹ-].