Friction only accompanies some sounds, usually obstruents. It can be heard when the air flows through a narrowed opening in the speech tract. When we blow out a candle or a match, we make such a narrowing between our lips.
Many speech sounds have friction. Clear examples of sounds that only consist of friction are the voiceless fricatives [s], [f], [x] etc. When we narrow the glottis, whisper (which is glottal friction) is produced, as we have seen in the previous section. (Whisper a few vowels softly. When you whisper them louder, you may also generate friction in the mouth itself, in particular for a vowel like [i]. Try it!)
Of course, friction can also accompany voiced sounds: imitate the buzzing of a bee or a gnat to produce [z] as in Dutch ijzel or English easy.