Perceptual detection thresholds for alterations of the azimuth of early room reflections

Abstract

Spatial audio, that is, the reproduction of an acoustic event including all spatial information, has become a major research topic in recent years. The computer games industry, for example, is looking for ways to reproduce the created spaces as realistically as possible, not only visually but also acoustically. A particularly large area of interest in spatial audio is the authentic reproduction of a room. The question arises how far the reproduced angles of a reflection can deviate from the actual angle without the room being perceived differently. This has already been investigated for single reflections, but how and when several shifted reflections together cause a perception change is still largely unknown. Therefore, in this work it is investigated how far the azimuth angles of the first five reflections of a room impulse response have to be shifted, so that the original room impulse response can just be distinguished from the manipulated one. For this purpose, 20 subjects performed a headphone-based adaptive ABX threshold test in which the selected reflections, initially all coming from one side, were shifted closer to or further away from their original positions at each adaptation step until the individual point of just noticeable difference was found. For this, the number of steps for moving each reflection from the adaptation start to the original position was the same, but the step size differed due to the individual angle differences. The recordings of a drum and a male speaker were used in the test. The results show that the five selected reflections for both test signals can be shifted between 4.33 and 17.13 , depending on the step size of the reflection, until a difference is audible. The perceived differences between the rooms were described as changes of the center of gravity in the room or the localization of the sound source. No major difference between the median values of the drum and the speech signal could be found. The inexperienced listening test participants were mainly responsible for all outliers found in the threshold tests while the median thresholds were slightly higher, suggesting that the reproduced angles of the reflections could potentially deviate more for the broad society. An additional ABX test comparing the original room with a room where the first reflections were mirrored showed a clear perceptual difference.