Some effects of reflections and delayed sound arrivals on the perception of speech and corresponding measurements of the speech transmission index

Abstract

Although the effects of reflections and later arriving sound repetitions (echoes) have been well researched and published over the past 60 years—ranging from Haas, Wallach, and more recently to Bradley & Sato and Toole, their effect on Speech Transmission Index measurements and assessments has only be cursorily studied. Over the past 20 years, the speech Transmission Index (STI) has become the most widely employed measure of potential speech intelligibility for both natural speech and more importantly of Public Address and emergency sound systems and Voice Alarms. There is a common perception that STI can fully account for echoes and late, discrete sound arrivals and reflections. The paper shows this not to be the case but that sound systems achieving high STI ratings can exhibit poor and unacceptable speech intelligibility due to the presence of late sound arrivals and echoes. The finding is based on the results of a series of listening tests and extensive sound system modeling, simulations and measurements. The results of the word score experiments were found to be highly dependent upon the nature of the test material and presentation.