There are problems in distinguishing voiceless approximants from voiceless fricatives. Phonetic characteristics Fricative consonants are generally said to be the result of turbulent airflow at a place of articulation in the vocal tract. However, an audible voiceless sound may be made without this turbulent airflow: makes a distinction between "local friction" (as in or ) and "cavity friction" (as in voiceless vowels like and ). More recent research distinguishes between "turbulent" and "laminar" airflow in the vocal tract. It is not clear if it is possible to describe voiceless approximants categorically as having laminar airflow (or cavity friction in Pike's terms) as a way of distinguishing them from fricatives. write that "the airflow for voiced approximants remains laminar (smooth), and does not become turbulent. Voiceless approximants are rare in the languages of the world, but when they do occur the airflow is usually somewhat turbulent." Audible voiceless sounds may also be produced by means of turbulent airflow at the glottis, as in ; in such a case, it is possible to articulate an audible voiceless sound without the production of local friction at a supraglottal constriction. describes such sounds, but classes them as sonorants. Distinctiveness Voiceless approximants are rarely if ever distinguished phonemically from voiceless fricatives in the sound system of a language. discuss the issue and conclude "In practice, it is difficult to distinguish between a voiceless approximant and a voiceless fricative at the same place of articulation ... there is no evidence that any language in the world makes such a distinction crucial." Disagreement over use of the term Voiceless approximants are treated as a phonetic category by (among others) , , and . However, the term voiceless approximant is seen by some phoneticians as controversial. It has been pointed out that if approximant is defined as a speech sound that involves the articulators approaching each other but not narrowly enough to create turbulent airflow, then it is difficult to see how a voiceless approximant could be audible. As John C. Wells puts it in his blog, "voiceless approximants are by definition inaudible ... If there's no friction and no voicing, there's nothing to hear." A similar point is made in relation to frictionless continuants by : "There are no voiceless frictionless continuants because this would imply silence; the voiceless counterpart of the frictionless continuant is the voiceless fricative." argue that the increased airflow arising from voicelessness alone