This work presents an approach for measuring material volumes in a closed cylindrical silo by using acoustic waves and resonance frequency analysis of silo’s acoustic systems. With an assumption that the acoustical systems were linear and time-invariant, frequency responses of the systems were identified via measurement. A sine sweep was generated, amplified and fed to a loudspeaker inside the silo. Acoustic waves were picked up by a microphone and processed to yield the silo's frequency response. Resonance frequencies and wave mode numbers of standing waves in the frequency range below 900 Hz were analyzed and used for calculation of air-cavity lengths. With known silo's dimension, the material volume estimations were achieved. Sets of experiments for estimating volumes of sand, cement, water, rice grain, and stone flakes in a closed silo, were done. It was found that the approach could successfully estimate the volumes of sand, cement, and water with a satisfactory accuracy. Percent errors of the estimations were less than 3% from the actual volumes. However, the approach could not estimate the volume of rice grain and stone flakes, since their sound refractions were neither resulted in standing waves nor acoustical modes in the silo.
Attending a class and listening to a lecture given by an instructor is a common process in Thailand education. 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 and listeners’ locations in classrooms and their effects on the speech intelligibility were studied. By using an assumption of linear systems of the classrooms, the room reverberation, background noise, and other classroom acoustical factors can be implicated as impulse responses of the system. Maximum length sequence was used to identify the impulse responses at listeners’ locations in 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 experimental results showed that the reverberation and background noise at listeners’ locations severely affected the speech intelligibility. A classroom, that seemed to have a good averaged reverberation time, did not always yielded good speech clarity for all the locations in the class. In fact, for the classroom used in the study, the rear section of the class was poor for intelligibility and the back corner closed to a noise source was the worse location for speech hearing.
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