Many different instrumental techniques for deriving an energy property of the speech wave have been employed in communication studies. There is as yet, however, no satisfactory method of calculating the loudness of the various types of complex quasi‐periodic sounds that appear as time functions in speech. This paper does not present a method of making such loudness calculations, but suggests that any such method would be inadequate for deriving physical correlates of accent or stress judgments about speech. The study reports volume indicator and instantaneous amplitude observations on sustained vowels produced under various conditions of speech effort, and on vowels produced in CNC syllables in a carrier phrase with stress held constant. The untested theory is proposed that the perception of linguistic stress is based upon judgments of the physiological effort involved in producing vowels. The relative amplitude and spectrum of the vowel provide the chief basis for judgment of the physiological effort.