Universitario

/z/ sound and /s/
the z sound is voiced (the vocal cords vibrate during its production), and is the counterpart to the unvoiced s sound. tocreate the z sound, the front of the tongue is placed close to the tooth ridge. the tip of the tongue should be close to the upper backside of the topfront teeth. the tongue is kept tense as air is pushed between a small groove along the center of the tip of the tongue and the front of the toothridge. the front sides of the tongue touch the side teeth toward the front of the mouth. the lips are held slightly tense during the sound.
the ssound:  make the s by lightly placing your tongue at the top of your mouth on the ridge behind your front teeth. the air should pass over the top ofthe tongue in an even way.  sometimes people develop a whistle or slight lisp on s because the tongue is not even. if you struggle with making thes sound, get more practice with s sound pronunciation.
there is a very confusing aspect of the s sound in american english. s has more of a lightz sound in some common words when the s follows a vowel.  this rule is not constant.  as a good example: is has the z sound, butthis doesnot.  the phonetic rule is this: if s follows a vowel or a voiced consonant, it sounds like z.  however, because there are exceptions and this rule isalready a subtle one, i recommend that you simply be aware of it and listen for it.  also, begin to use it in some of the common words listed below.