Perceptually, the two categories are hard to distinguish. In both cases, the overall F0 excursion is high and flat. However, the acoustic analysis shows a slight difference. Under narrow focus, the final word is more prominent, while under broad focus, the pitch range for the sentence initial word is slightly greater. It is suggested that, under broad focus, stress falls on two places.
Primary stress falls on the last word, whereas the secondary stress falls on the first one.
An Acoustic Study of Sentence Stress in Mandarin Chinese. Ph.D. dissertation, Ohio State University. All rights reserved.