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Listening to a lecture in a classroom is a common process for studying in Thailand. Ability of learning is affected by the ability of hearing the instructors' speech. Acoustical environments of the classroom, hence, can influence speech intelligibility. In this research, acoustical parameters, reverberation time, listeners' locations in classrooms and their effects were studied. By using an assumption of linear system of the classrooms, the room reverberation, background noise, and other classroom acoustical factors can be implicated as an impulse response of the system. Maximum length sequence was used to identify the impulse responses at listeners' locations, which are at front, middle, and rear section of the classrooms. A clean speech, recorded in a semi-anechoic room, was convoluted with a series of the measured classrooms' impulse responses to yield a set of simulated reverberant speeches that the listener at each location in the classes would have heard. A number of volunteers were invited to test an ability of understanding the speech. The results showed that the reverberation and the listeners' location severely affected the speech intelligibility. Students sitting at front of the classes would gain better speech clarity than those sitting at middle or rear parts of the classes. Location of listening at back section, closed to the rear solid walls in the classrooms with bad reverberation times was the worse location for gaining speech intelligibility.