定比特音又称为基本元音、正则元音（英语：cardinal vowel），是一组用以参考的元音，让语音学者描述语言的元音之标准位置，例如普通话的“笔”字（bǐ），其元音可以描述为定比特音1[i]，也就是最接近它的定比特音。

当舌头位于极端的位置，发出的元音就是定比特音，不论位置的前后或高低。现时的系统由丹尼尔·琼斯（Daniel Jones）在20世纪初设计，然而这一概念早已由其他语音学者提出（一种说法是于19世纪晚期，由语言学家埃利斯首先提出），而“cardinal”这一术语由贝尔选定，后为斯威夫特所采用。

Cardinal vowels are a set of reference vowels used by phoneticians in describing the sounds of languages. For instance, the vowel of the English word "feet" can be described with reference to cardinal vowel 1, [i], which is the cardinal vowel closest to it. It is often stated that to be able to use the cardinal vowel system effectively one must undergo training with an expert phonetician, working both on the recognition and the production of the vowels. Daniel Jones wrote "The values of cardinal vowels cannot be learnt from written descriptions; they should be learnt by oral instruction from a teacher who knows them".