Friction can be generated in two ways in the vocal tract. One way is to produce a constriction of close approximation. To achieve this, two articulators are far enough apart so that air can pass between them, but close enough together so that when it does, it becomes turbulent and produces friction noise just forward of the maximal constriction. This is how friction is produced for labiodental fricatives [f v] and dental fricatives [θ ð] (as in ‘think’ and ‘then’). The other way is to direct a channel of air at another surface, such as the back of the teeth or the alveolar ridge, and when the moving air hits this surface, it becomes turbulent. This is how friction is produced for alveolar fricatives [s z] and postalveolar fricatives  (as in ‘ship’ and ‘pleasure’).

How much friction is generated depends on a number of variables. First, the width of the channel between articulators affects the pressure of air through the constriction. With a narrower channel (e.g. for [s] and [ʃ]), the pressure increases, and so the turbulence increases too. With a wider channel (e.g. for [f ] and [θ]), there is less pressure and therefore also less friction.

Secondly, the volume of air affects the volume of friction generated. The more air forced through the constriction, the greater the pressure, and therefore the greater the turbulence and the amount of friction noise. This will depend on the amount of air being expelled from the lungs.

If this seems a bit abstract, imagine watering a garden with a hosepipe. If the pipe is not blocked and the water pressure is not too high, water flows through the hose and comes out in a steady stream. If you narrow the end of the pipe by putting your finger over it, the pressure within the pipe increases, generating turbulence in the flow of water, and producing a spray. You can also increase pressure in the hose by opening the tap, which increases the flow of water through the pipe and therefore also the pressure behind the stoppage. To get a really good spray (in other words, a lot of turbulent water), you can increase the pressure by both opening up the tap and making the constriction narrower. Air moving through the vocal tract is a little similar: the constriction in the vocal tract corresponds to your finger at the end of the hosepipe, and the water moving through the pipe corresponds to the air being pushed out of the vocal tract.

Fricatives are often classed as strident or non-strident. Strident fricatives, , have a lot of friction noise, especially at higher frequencies, caused by a comparatively narrow constriction. Non-strident fricatives are [f v θ ð]. If you compare the sounds [θ] and [s] in particular, you will hear that one of the main differences between them auditorily is that [θ] has a much ‘flatter’, quieter sound than [s], which sounds ‘sharper’ or ‘brighter’ and louder.

Fricatives in English all have airflow down the mid-line of the vocal tract. You can tell this by producing a fricative, holding the articulators in place and then sucking air in. The part of the mouth that goes cold and dry as you do this should be symmetrical around the middle of the vocal tract.

Some speakers use fricatives with lateral airflow: the voiceless lateral fricative  is a Welsh sound (spelt ) that occurs in many place-names, such as ‘Llandudno’, ‘Llangollen’ and ‘Llanfair’. Many English speakers replace this sound with a sequence like [tl kl θl]; but it is not difficult to produce . First, make a [l] sound, and hold it; secondly, remove the voicing (as in the  of ‘pay’); thirdly, raise and tense the sides of the tongue a little. This should produce a good .

English fricatives all have oral airflow: the velum is raised and forms a tight seal preventing air from flowing through the nose. This can be demonstrated by pinching the nose while producing a sustained fricative such as [s:::]. It makes no difference to the sound that comes out because [s] requires a good seal, preventing leakage of air through the nasal cavities and therefore weakening the friction.

Affricates have no place on the IPA chart, because they are sounds composed of two elements, plosive+fricative. In English, there are two such sounds at the systematic level, , usually represented by  and  respectively in the spelling. Affricates were also discussed, but their fricative components  are described here.

There are fricatives at five places of articulation in most standard varieties of English: labiodental, dental, alveolar, postalveolar and glottal; and two degrees of voicing: voiced and voiceless.

Other fricatives also occur in English, for a range of reasons.

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