Final answer:
The presence of an anatomical obstacle such as teeth in front of a constriction during speech production increases the frequency of the fricative noise by causing a more turbulent airflow, which increases the higher frequency components.
Step-by-step explanation:
The acoustic effect of an anatomic obstacle, such as teeth, in front of a constriction when forming fricatives in human speech is to increase the frequency of the fricative noise. This phenomenon is related to the way in which sound waves resonate within the vocal tract. When air is forced through a narrow constriction, it causes turbulence, resulting in the production of fricative noise. The presence of a physical barrier like teeth alters the airflow and can serve to increase the frequency due to more rapid fluctuations in air pressure and flow patterns around the obstacle, enhancing higher frequency components. Additionally, the way sound is shaped within the vocal tract can be likened to an air column closed at one end, creating a pattern of overtones that are unique to each individual's speech due to variations in the shape and positioning of the tongue and mouth.