The objective of the present paper is to understand whether the "density" of the vowel system of a language, in other words the large or small number of vowels in the language, plays a significant role in the production and perception of vowels. Three languages (French, English and Spanish) are analysed. This work rests upon the comparison between speech production and the cognitive processing of linguistic units. Two different methodologies were employed in this aim : observation of phonetic productions and analyses of behavioural measures. First results lead us to hypothese that perception units can be larger than production units. In other words, the density of a vowel system seems to influence vowel perception but not vowel production.