As with other varieties of Arabic, Najdi Arabic (NA), spoken in central Saudi Arabia, contains a set of emphatic consonants, /? – ? – ð?/, which have an uncontrollable phonological effect on neighboring segments whereby the presence of an emphatic in a word entails “emphasis spread” to the adjacent non-emphatic segments. Acoustically, emphasis is manifested by lowering the frequency of the second formant (F2) of vowels in the vicinity of an emphatic. The purpose of this study is investigate the domain and directionality of emphasis spread in NA and whether it is blocked by any opaque phonemes by measuring and comparing the F2 frequencies of vowels in minimal pairs contrasting on the basis of the presence/absence of emphatics. The results show that the domain of emphasis spread in NA is the entire phonological word consisting of the stem plus any inflectional/derivational affixes. In addition, emphasis spreads rightward and leftward throughout the phonological word in a gradient manner in an inverse relationship, where emphasis peaks in the syllable containing the emphatic and gradually decreases as it moves into the following/preceding syllables. Finally, the high front phonemes /y – ? – ?/ act as opaque segments as they block rightward spread but not leftward spread.