There is an acknowledgement that a principle of economy affects phonetic and phonological procedure. In this paper, I focused on the articulation of vowels in the light of economy of articulatory labor and established the articulatory energy level for English cardinal vowels. With the help of this level I could prove schwa /○/ is the weakest vowel in English maintaining 0 level and propose the basis for vowel weakening from /i/, /e/, /o/, and /a/ to schwa /○/. The articulatory places of /r/ and /○/ are so closely related that I could decide that this gives the phonetic background for intrusive /r/.