This paper concerns the relationships amongst acoustic phonetic features of speech signals, perceived vocal effort, and speech clarity. It is presented from a statistical analysis of a good number of subjective testing on an anechoic speech corpus with 5 different vocal efforts, namely hushed, normal, raised, loud, and shouted, with an aim to map objective acoustic phonetic features onto subjective ratings. Results show that listeners can differentiate vocal effort from subtle acoustic phonetic variations. There is also a correlation between clarity and vocal efforts. A regression model is further established to predict vocal effort from acoustic phonetic analysis.