Which aspect of communication situation can directly affect a person's lipreading effectiveness?

The viewing angle of the speaker is crucial for effective lipreading because it directly impacts the visual access the listener has to the speaker's mouth and facial movements. When the listener is positioned at an optimal viewing angle, they can better see the articulation of the words, which includes lip movements, facial expressions, and even some non-verbal cues. If the viewer is off to the side or at a poor angle, it can significantly obscure their ability to accurately interpret what is being said.

Effective lipreading requires not only the visibility of the lips but also the surrounding facial features that can convey additional contextual information and nuances in communication. This visual access is essential since many sounds are not clearly distinguishable through lip movements alone, and visual cues play a key role in enhancing understanding, especially in noisy environments.

While the other aspects mentioned—such as prior knowledge of the topic, clarity of the speaker's voice, and the listener's state of relaxation—are important for communication overall, they do not impact the visual effectiveness of lipreading as directly as the viewing angle does.