1. A computer implemented method for determining whether alcohol is consumed by a person in a vehicle, the computer implemented method comprising: receiving a voice signal from said person and converting the received voice signal into a plurality of voice frames; extracting a voice frame corresponding to a voiced sound among the plurality of voice frames; extracting a plurality of formant frequencies of the voice frame corresponding to the voiced sound; calculating a formant slope between two formant frequencies among the plurality of formant frequencies; determining whether alcohol is consumed by said person according to the calculated formant slope, wherein the determining comprises: counting the number of voice frames each having a formant slope less than a predetermined threshold, computing a ratio of the counted number of voice frames to the total number of voice frames, and determining whether alcohol is consumed by said person when the computed ratio is greater than a predetermined value; and enabling or disabling the vehicle based on the determination whether alcohol is consumed by said person.
8. The computer implemented method of claim 1 , wherein the calculating a formant slope between two formant frequencies among the plurality of formant frequencies comprises calculating a formant slope between the first formant frequency and the fourth formant frequency.