Bozzoli F., Farina A.

The optimal listening conditions are very important in assessing the 'comfort' of a space: intelligibility is certainly the most important when verbal communication becomes central. For example in a classroom, listening correctly to the teacher is the basic acoustical requirement. One of the intelligibility’s parameters that has been reveled to be more sensitive is the Speech Transmission Index The STI, as exposed in IEC standard n.60268-16, is based on the reduction of the modulation index of a test signal simulating the speech characteristic of a realtalker, due to the environment. The test signal is emitted by a sound source situated at thetalker’s position and is recorded by a microphone at any listener’s position. This signal consists of a noise carrier with a speech-spread frequency spectrum and a sinusoidal intensity modulation at frequency F. Even if STI uses a binaural head , it is a monaural parameter because of it doesn't consider correlation between the two ears' signals. For showing this aspect we have compared situations with the identical STI (same modulation indexes, same signal to noise ratio) and the same spatiality of the signal (it was always emitted from a loudspeaker in front of the listener) but changing noise's spatiality: in one case noise was emitted by the same loudspeakers used for the signal, in the other case we have used two loudspeaker placed around the listener. We have done many subjective tests based on words' recognition and we have found that noise's spatiality influences intelligibility. We have tried to find a correlation between intelligibility and spatiality for defining a corrective factor for STI that considers also this aspect.