ODEON APPLICATION NOTE – Calculation of Speech Transmission Index in rooms

Scope Sufficient acoustic quality of speech communication is very important in many different situations and places; classrooms in schools, auditoria, PA systems in public buildings, alert and warning systems in noisy environments etc. Speech transmission index (STI) is an objective method for prediction and measurement of speech intelligibility. This is a guide how to use ODEON for calculating the speech transmission index in a room in accordance with the international standards IEC 60268-16 [1] and ISO 9921 [2]. This application note is not applicable for open-plan offices, which are dealt with in another ODEON application note 1. This application note refers to ODEON version 12, all editions. Method Although IEC 60268-16 is a measurement method, the measurement procedure can be simulated in ODEON, thus providing a tool for the acoustical design of performance spaces and other rooms where speech communication is important, or for the design of a loudspeaker system in a room. Two different types of cases will be discussed: Direct communication between two people in the same environment and public address (PA) systems where an electro-acoustic system is used to address a group of people. The original method [1] is based on a special modulated speech-shaped signal and the measurement of modulation transfer functions in seven octave bands with 14 different modulation frequencies. However, an alternative and equally good method uses the impulse response as suggested by Schroeder [3]. This so-called indirect method is the one used in ODEON. Ambient noise Noise due to all other sound sources than the communication speech. Background noise level Sound pressure level in octave bands due to ambient noise, spatially averaged over the relevant listener area. Application note – Calculation of STI in rooms 2 Public address system PA Electro-acoustic system used to address a group of people. Reference speech level Speech level equivalent to 60 dB(A) at 1 m distance in front of the talker's mouth. Signal-to-noise ratio SNR Difference between the sound pressure level of the speech or test signal and the sound pressure level of the background noise. Speech intelligibility Rating of the proportion of speech that is understood. Speech transmission index STI Objective method for prediction and measurement of speech intelligibility. Rapid speech transmission index RASTI A simplified and approximate measurement method for rating speech transmission in auditoria, with or without sound systems. NOTE: STIPA is only applied as a direct method, not relevant for the indirect method …