Speech transmission index or rapid speech transmission index for classrooms? A designer's point of view

Abstract

Classrooms, especially those related to the early development of individuals, are very important nowadays as they play a crucial role in the development of human characters and the attitude towards learning, both of which eventually define whether or not an individual will have a successful life. It is therefore very important that a high level of speech intelligibility inside classrooms can be achieved so that the communications between teachers and students and the passing of instructions from teachers to students can be effective. This must be done in the design stage as it is usually more difficult and very costly to correct for acoustic defect after the construction is completed. Research suggests that speech intelligibility can be described by several indices which take into account the room reverberation and the background noise level. Examples of which include the articulation index [1], the speech transmission index (STI) [2], the percentage articulation loss of consonants (%ALcon) [3] and the useful-to-detrimental sound ratio (U50) [4], among which the STI and the %ALcon appear to be the more common ones adapted in the acoustic profession. The results of Bradley [5] indicate that there are substantial correlation between STI, %ALcon and U50; implying that these indices are equivalent in the assessment of speech intelligibility. The rapid speech transmission index (RASTI) was developed in the 1980s by Houtgast and Steeneken [6] as a quick mean for the estimation of the speech intelligibility. The principles of STI and RASTI are very similar except that the calculation of the latter requires information from the 500 Hz and 2 kHz octave frequency bands only such that the measurement time is substantially shortened. RASTI has been used in many studies related to speech intelligibility in indoor spaces 4(for instance, Refs. [7,8]). The advances in measurement technology and the computer nowadays make it possible to estimate both the STI and RASTI from a captured pulse decay in an enclosure within seconds [9] and thus, the idea of ‘rapid’ for RASTI is no longer meaningful. However, the speech intelligibility in a room cannot be predicted in the design stage without information of the octave band background noise levels and the room reverberation, but the former may not be precisely known ARTICLE IN PRESS