A model of acoustic coupling between the oral and subglottal cavities is developed and predicts attenuation of and discontinuities in vowel formant prominence near resonances of the subglottal system. One discontinuity occurs near the second subglottal resonance (SubF2), at 1300-1600 Hz, suggesting the hypothesis that this is a quantal effect [K. N. Stevens, J. Phonetics 17, 3-46 (1989)] dividing speakers' front and back vowels. Recordings of English vowels (in /hVd/ environments) for three male and three female speakers were made, while an accelerometer attached to the neck area was used to capture the subglottal waveform. Average speaker SubF2 values range from 1280 to 1620 Hz, in agreement with prior work. Attenuation of 5-12 dB of second formant prominence near SubF2 is found to occur in all back-front diphthongs analyzed, while discontinuities in the range of 50-300 Hz often occur, in good agreement with the resonator model. These coupling effects are found to be generally stronger for open-phase than for closed-phase measurements. The implications for a quantal relation between coupling effects near SubF2 and [back] are discussed.