The relation between spatial impression and the law of the first wavefront

In the field of room acoustics, it is popular belief that the early and late reflections contribute to auditory source width (ASW) and listener envelopment (LEV), respectively. However, some papers have demonstrated results not necessarily in agreement with the belief. In this paper, a hypothesis is proposed to clarify the essentials of ASW and LEV from point of view of the auditory phenomenon. The hypothesis is that the components of reflections under and beyond the upper limit of validity for the law of the first wavefront contribute to ASW and LEV, respectively. Two experiments were performed to evaluate the hypothesis. In the first experiment, the results showed directly that the components of reflections under the upper limit of validity for the law contribute to ASW. In the second experiment, four kinds of threshold were measured to evaluate the relation between the effect and LEV: image-splitting which corresponds to the upper limit of validity for the law, LEV, reverberation perception, and reverberation disturbance. The results showed that the threshold of image-splitting coincides with the that of LEV. This suggests that the components of reflections beyond the upper limit of validity for the law contribute to LEV. In conclusion, it seems that the results of experiments shown in this paper favor the hypothesis.