It's normal enough for people to say that we can attribute the percept of 'stress accent' to certain acoustic correlates – usually higher relative fundamental frequency, intensity, and duration of syllables. It's easy enough to measure duration and compare that across 'stressed' and 'unstressed' syllables. But what about intensity?
There seem to be two things about intensity that I can't wrap my head around.
If intensity is measured in RMS, that means it's an average over time. So an intensity trace on Praat, for example, is working out some kind of average as it goes along from left to right. So what exactly should you measure then - the average RMS for the whole vowel? the highest RMS value in the intensity trace? is that intensity peak already somewhat flattened out if it's measured in RMS?
Once you have a measure, how do you compare them across syllables? Do you need to do a log transform of the RMS dB in order to make it a linear scale when you do a comparison, or would that not make sense because your ears hear logarithmically anyway?
I should point out that my research relates to Australian languages, not English, so the cues are considerably more subtle, and vowel quality does not change much between stressed and unstressed syllables.