The central aim of our experiments is to find a phonetic explanation for the phonological irrelevance of the onset (i.e. prevocalic consonants) for syllable weight. From a set of pilot experiments (cf. [1]) we deduced that the cause of this irrelevance might be found in speech perception rather than speech production. Subjects proved to be less sensitive to duration changes in the onset than to such changes in either nucleus or coda. Assuming that perceived duration is the primary phonetic correlate of syllable weight, we may have found a clue as to why the onset does not count in the determination of syllable weight. This answer immediately yields another question. We do not know why subjects are unable to reliably perceive onset durations. So, we must go one step deeper into the matter, and find out what it is that prevents us from correctly determining onset duration. This study focuses on two experiments. The first is an experiment that is designed to check whether listeners remain insensitive to onset duration under different circumstances. In the other experiment we explore one line of reasoning that may provide an answer to the follow-up question.