Subjective indices related to speech transmission in classrooms – an appraisal

Speech intelligibility is a subjective performance index defined as the percentage of a message understood correctly. Often the results of speech intelligibility tests would suggest that conditions are acceptable, with Intelligibility Score (IS) of the order of 90% or more, however even if the intelligibility is so good, speech transmission performance may not be satisfactory. Subjective ratings of the Listening Easiness Score (LES) provide an alternative approach. A total of 2176 intelligibility tests were administered in a primary school to 272 pupils, yielding an useful data set of 1912 values. The Speech Transmission Index (STI) was also measured for each test setting in seven different positions in the laboratory classroom used for the test. Both IS and LES are inherently bounded, and their data distributions exhibit a significant accumulation of scores in the upper part, mainly for IS. The resulting truncation problem have been addressed in order to allow meaningful comparison between indices.