Half-Open Vowels (Third Degree of Aperture)
You can hear each sound by clicking on his IPA transcription.
Half-Open Front Vowels
This symbol designates an unrounded front vowel that is more open than the two previous examples. It falls between the third and fourth degrees of aperture.
Half-Open Central Vowels
Central vowel (half-open). In reality, this vowel falls between the third and fourth degrees of aperture. This articulation is considered neutral from the point of view of lip-rounding as well (neither rounded nor unrounded).