Which acoustic property tends to change the least when stressing a syllable or word according to research?

The correct choice highlights that amplitude tends to change the least when stressing a syllable or word. When a syllable or word is stressed, speakers usually increase the loudness and also alter other characteristics like pitch (fundamental frequency) and duration. However, while amplitude can change, it is typically the least variable compared to the other properties during stress placement.

In research surrounding speech prosody, it is found that fundamental frequency, or the pitch of the voice, significantly increases during stressed syllables due to heightened vocal fold tension and airflow. Similarly, duration often extends for stressed syllables, as speakers slow down slightly to emphasize the syllable. Speech tempo can also be affected by stress, as speakers might adjust their rate of speech to articulate the stressed syllable clearly.

In contrast, while amplitude does increase with stress, the extent of its variability is not as pronounced as the changes seen in fundamental frequency and duration, making it a more stable variable in the context of stress in speech. This stability contributes to the understanding of how stress is perceived and processed in spoken language.