Effect of Acoustic Environment on the Sensitivity of Speech Transmission Index to Source Directivity

Source directivity has the potential to affect speech transmission index (STI) measurements. When the source is a model of a human talker, there is a question of how accurate that model needs to be. However, in many acoustic environments source directivity has little effect on STI because other factors dominate. One instance is soundfields in which the direct sound is dominant (e.g., in the quiet outdoors, in the nearfield, or in an anechoic room): directivity will have no effect on STI so long as the direct sound is strong enough. Another instance is soundfields in which the reverberant soundfield is dominant (e.g., in the far field in a room with moderate or more reverberation). This paper examines theoretical situations where source directivity has a substantial influence on STI because of the balance between direct (or early) and reverberant soundfields, as well as the role that background noise can have in increasing the importance of source directivity.