The sound of the letter S is [s] only if it follows an unvoiced consonant. Otherwise, it becomes a Z in disguise. When an S follows a vowel, a voiced consonant, or another S, it turns into a [z]. The following exercise will let you hear and practice S with its dual sound. There are many more Z sounds than S sounds.
Exercise 10-1: when S becomes Z
Under contrast, in the list that follows, notice how the voiced word is drawn out and then repeat the word after me. Both voiced and unvoiced diphthongs have the underlying structure of the tone shift, or the double stairstep, but the shift is much larger for the voiced ones.
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