Languages with binary stress systems frequently tolerate a stress lapse over the final two syllables, but almost none tolerate a word-initial stress lapse. Lunden (to appear) argues that this lapse asymmetry can be explained by the presence of word-level final lengthening, which can then create the perception of prominence alternation in languages that use duration as stress correlate. The results of a production and a perception study with English speakers are presented which compare /ɑ/s that occur under stress lapse to /ɑ/s in non-stress-lapse positions. While word-final unstressed /ɑ/ is always longer than non-final unstressed /ɑ/, it is significantly longer when immediately following an unstressed syllable. Similarly, unstressed word-final /ɑ/ has a higher F1 and lower F2 than non-final unstressed /ɑ/, but word-finally this less-reduced vowel is closer to a full vowel when the final syllable is part of a stress lapse. The perception study finds that these differences have perceptual consequences that can lead to a perceived continued rhythm in stress lapse. The phonetic differences explain why a word-final unstressed vowel can be perceived as relatively strong when following an unstressed syllable but as relatively weak when following a stressed syllable.