Approximants Versus Fricatives
In addition to less turbulence, approximants also differ from fricatives in the precision required to produce them. When emphasized, approximants may be slightly fricated (that is, the airstream may become slightly turbulent), which is reminiscent of fricatives. For example, the Spanish word ayuda ('help') features a palatal approximant that is pronounced as a fricative in emphatic speech. However, such frication is generally slight and intermittent, unlike the strong turbulence of fricative consonants.
Because voicelessness has comparatively reduced resistance to air flow from the lungs, the increased air flow creates more turbulence, making acoustic distinctions between voiceless approximants (which are extremely rare cross-linguistically) and voiceless fricatives difficult. This is why, for example, the voiceless labialized velar approximant (also transcribed with the special letter ⟨ʍ⟩) has traditionally been labeled a fricative, and no language is known to contrast it with a voiceless labialized velar fricative . Similarly, Standard Tibetan has a voiceless lateral approximant, and Welsh has a voiceless lateral fricative, but the distinction is not always clear from descriptions of these languages. Again, no language is known to contrast the two. Iaai is reported to have an unusually large number of voiceless approximants, with /l̥ ɥ̊ w̥/.
For places of articulation further back in the mouth, languages do not contrast voiced fricatives and approximants. Therefore the IPA allows the symbols for the voiced fricatives to double for the approximants, with or without a lowering diacritic.
Occasionally, the glottal "fricatives" are called approximants, since typically has no more frication than voiceless approximants, but they are often phonations of the glottis without any accompanying manner or place of articulation.
Read more about this topic: Approximant Consonant