Which class of sounds is characterized by a narrowed vocal tract where the airstream is forced through a narrow passage?

The correct answer is fricatives because this class of sounds is produced when the airstream passes through a narrow constriction in the vocal tract, creating turbulence and resulting in a continuous sound. During the production of fricatives, the articulators come close together, but do not completely block the airflow. This constriction can occur at various points in the vocal tract, such as the teeth, the alveolar ridge, or the glottis, producing characteristic sounds like /f/, /v/, /s/, and /z/.

The nature of fricatives also differentiates them from other classes of sounds. For example, liquids involve a less constricted vocal tract, allowing the air to flow freely around the tongue without producing significant turbulence. Affricates, on the other hand, begin as stops (complete closure of the vocal tract) and release into a fricative quality, but they do not solely represent fricative sounds. Stops completely block airflow before releasing it suddenly, which contrasts with the continuous airflow seen in fricatives.