Vowels keep the vocal tract open and vary little by context, unlike consonants

Evidence

A vowel is a speech sound where the vocal tract is relatively open without major constrictions. Vowels are produced relatively independently of their context. Consonants vary more in the manner of production. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 572

Categorization

• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 572