What characterizes a vowel in phonetics?

In phonetics, a vowel is characterized by being produced without any significant restriction or blockage of airflow in the vocal tract. This means that during the articulation of a vowel, the vocal cords vibrate freely, and the air can flow unobstructed, allowing for a clear and resonant sound. Vowels serve as the core of syllables in languages and possess specific qualities such as openness, backness, and rounding, which contribute to their distinct auditory characteristics.

Other choices suggest characteristics that do not apply to vowels. For instance, airflow blockage would describe consonants, which typically involve partial or complete closure of the vocal tract. Being always voiceless applies to certain sounds in language but does not define vowels, which can be voiced. Lastly, the notion that vowels require a consonant to be formed is incorrect, as vowels can exist independently as standalone sounds or syllables in various languages. Thus, the defining feature of vowels is their unobstructed airflow during production.