Rarely do we consider how perception affects our speech. Most of us are able to pronouce our words effortless and, as a result, we don't take into consideration the processes which enable us to do so.
In chapter 13 we learn that a phoneme is "the shortest segment of speech that, if changed, would change the meaning of a word." The number of phonemes actually vary depending on a language as sounds are pronounced differently in various languages. For example, the American English language has 47 phonemes whereas the African language has up to 60 phonemes.
Another aspect of speech perception is the acoustic signal (or acoustic stimulus), which is produced by patterns of of pressure changes. The acoustic signal is created by "air that is pushed up from the lungs past the vocal cords and into the vocal tract." The examples offered in the text are interesting. I admit, I found myself attempting to pronounce the letters without allowing my mouth to make the expected/typical movements; it didn't work, the words were terribly distorted. For example, when we pronounce the sound /d/, we place our tongue against the ridge above our upper teeth and when we pronounce the sound /f/we place our bottom lip against our upper front teeth and then push air between the lips and teeth. (I know you're trying it now!!!).
Obviously, we don't think about the movements our mouths are making as we pronounce letters or words, we just do it automatically.