EFFECTS OF EARLY REFLECTIONS ON LISTENER ENVELOPMENT

It is well known that later reverberant sounds contribute to listener envelopment (LEV). On the other hand, the effect of early reflections on LEV has not sufficiently been clarified. In this paper, listening tests were carried out in order to examine the effects of early reflections on LEV. As the result, it was confirmed that early reflections affected not only the auditory source width (ASW) but also LEV. In addition, there were some cases in which early reflections suppressed LEV, specifically when the early reflections increased ASW. INTRODUCTION It is well known that later reverberant sounds contribute to listener envelopment (LEV)[1]. On the other hand, the effects of early reflections on LEV have not sufficiently been clarified. According to LG[2] which Bradley et al. proposed as an evaluation index of LEV, there is absolutely no effect of early reflections. Early reflections do not completely affect LEV? Moromoto et al. has clarified that not only the front to back ratio(FBR) of the later reverberant sounds but also FBR of early reflections also contribute to LEV[3]. Authors proposed SBTs[4] as an evaluation index of LEV. SBTs indicates that early reflections also affect LEV since SBTs is based on center time(Ts). The LF[5] is a measure of early lateral reflections. LF is used well when the acoustic quality of a concert hall is evaluated. Supplying early lateral reflections is an important point of view in the acoustical design of the concert hall. It is understood today that LF is the evaluation index of the auditory source width (ASW)[6]. LF can be controlled in the arrival direction of the early reflections and the level of the early reflections. In this paper, listening tests were carried out in order to examine how the arrival direction and the level of the early reflections, respectively, influence LEV. METHOD Two experiments were conducted using a simulated sound field in an anechoic chamber. We installed 16 loudspeakers, which were 1.5m apart from the listeners at equal intervals of 22.5 degrees in a horizontal plane including the two ears of a listener. When carrying out the experiments, we selected the required number of loudspeakers from among the sixteen and reproduced direct sound or reverberant sounds. We used the virtual sound source distribution that produced the basic sound field shown in Fig. 1 in our experiments. The reverberation time of the basic sound field was about two seconds and C80 was 0.5dB, while Lf was 0.2 and LG was 2.1dB. First, the virtual sound source distribution was divided into the early reflections which is up to 80ms and the late reflections which is after 80ms. Next, the early reflections up to 80ms were divided into two parts, the left and right. The late reflections after 80ms were horizontally divided into eight parts. Third, the directional responses required for reproduction were synthesized from the virtual sound source of each part. As a result, two early responses and eight late responses were prepared. As a stimulus, we used a sound composed of the directional response waveform convolved with a portion of the first movement which was a Mozart divertimento about ten seconds long. The experiments were conducted according to Scheffe’s paired comparison method[7]. Each pair of sound fields was randomly presented. Nine subjects rated the LEV and ASW of the second sound field of each pair on a seven-point scale in comparison with the fi rst one. Previous to the experiments, we explained to the subjects the differences between LEV and ASW, and made sure they understood the possibility of change in a variety of auditory senses (such as reverberance and loudness). Fig. 1.Virtual sound source distribution and impulse response of the basic sound field DEFINITION OF SBTs AND LTs SBTs(Spatially Balanced Ts) SBTs is based on center time (Ts). First, we newly defined Ts i as indicated in Formula 1 below. The difference between Ts and Ts i is the numerator in their formula. The numerator in the Ts i formula is a directional response in each direction. Ts i means the contribution of individual directional responses in direction i to Ts. Therefore, the total of Ts i becomes Ts. Where, p(t): omnidirectional impulse response, pi(t): reflections arriving from direction i or directional responses in direction i. Next, we corrected Ts i, as indicated in Formula 2, taking the contribution of the direction of arrival to LEV into account. Where, i a : level, time and direction factors of reflections arriving from direction i, èLi: angle from binaural axis. In addition, the value of (1+cosèLi)/2 in Formula 2 reaches a maximum in the binaural axis direction and changes so that it does not become zero in the front/back direction. This is because the contribution of frontal reflection is not zero in our previous experiment[4]. dt t p dt t p t Ts i i ∫ ∫ ∞ ∞ ⋅ = 0 2 0 2 ) ( ) ( 2 ) cos 1 ( Li θ + ⋅ = i i Ts a (1)