Speech transmission index variation due to ventilating and air-conditioning system in university classrooms

A major factor to be considered in the study of teaching-learning process based on spoken communication is sound field. Excessive background noise levels or reverberation time values inside a classroom interfere with oral communication, creating an acoustic barrier to the teachinglearning process. In previous work, reverberation time was measured in a sample of university classrooms in the City of Córdoba, observing that such parameter has tripled the value recommended by international literature. Furthermore, background noise level was measured in two different conditions: heating, ventilation and air-conditioning system turned on and off. Then, these levels were evaluated according to NC and RC criteria established by IRAM 4070, where more than 50 % of the population under study complied with the NC criteria, and none with the RC criteria. Moreover, spectrum of heating, ventilation and air-conditioning system noise was observed to significantly interfere with the frequency range with greater contribution to speech intelligibility. In this paper, results of speech transmission index (STI) measurements for both conditions of heating, ventilation and air-conditioning system are presented. Such measurements were performed by the impulse response method according to IEC 60268-16, with head and torso simulator as source. Results of STI and signal-to-noise ratio obtained in both conditions are compared. Variations of up to twelve times the just-noticeable difference of STI were found.