Influence of artificial mouth’s directivity 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 employs an artificial speaker (binaural head) and listener (artificial mouth). In this study, the influence on the measurements of the emission directivity of the artificial mouth was investigated for different acoustic environments and we have found that, in many cases (i.e. big rooms or systems of telecommunications) the results is not sensitive to modifications of the directivity; on the contrary, inside cars, the shape of the whole balloon of directivity is important for determining correct and comparable values and the different mouth studied gives really different results in the same situation.