What term refers to the acoustic properties that determine the quality of vowel sounds?

The term that refers to the acoustic properties determining the quality of vowel sounds is known as formants. Formants are specific resonance frequencies of the vocal tract that characterize different vowel sounds. Each vowel has a unique pattern of formants, typically identified as F1, F2, and sometimes F3, which correspond to the first three resonance frequencies. These formants are critical in distinguishing one vowel from another in spoken language.

Phonemes refer to the smallest units of sound in language, which can include both consonants and vowels, but do not specifically describe the acoustic properties of vowel sounds themselves. Consonants are sounds that are produced with some degree of constriction in the vocal tract and do not embody the acoustic properties associated with vowels. The vocal tract is the physical structure through which sound passes, and while it contributes to the formation of formants, it is not the term that specifically identifies the quality of vowel sounds. Thus, formants are the most accurate answer in this context.