Balloons of Directivity of Real and Artificial Mouth Used in Determining Speech Transmission Index

In room acoustics, one of the most used parameters for evaluating the speech intelligibility is the Speech Transmission Index (STI). The experimental evaluation of this STI generally employ an artificial speaker (binaural head) and listener (artificial mouth). In many cases ( i.e. big rooms or system of telecommunications) the precision of directivity of artificial mouth doesn’t influence too much the result; on the contrary inside cars, but also in other cases, the shape of the whole balloon of directivity is important for determining correct and comparable values and different mouths give really different results in the same situation. For these reasons we have measured, in an anechoic room, the 3D directivities of a statistical population of speakers. The post-processing of the results enabled us to determine the average and the standard deviation of human speech directivity. These results constitute a valuable source of information for assessing the compliances of artificial mouth to reality.