The lower formant frequencies of the whispery vowel are known to be slightly higher than those of the modal vowel. This paper attempts to interpret this phenomenon acoustically, based on an electrical circuit model of the vocal tract, taking into account acoustic coupling with the subglottal system. A three-dimensional vocal tract shape was measured from a magnetic resonance image (MRI). It was found that the narrowing of the tract in the false vocal fold regions and weak acoustic coupling with the subglottal system are primary causes of the rise of the lower formant frequencies.