Perceptual Aspects of Dynamic Binaural Synthesis based on Measured Omnidirectional Room Impulse Responses

The auralization of acoustic environments applying dynamic binaural synthesis can be used for multiple applications. Circular sets of binaural room impulse responses (BRIRs) are often acquired by performing measurements with rotated dummy heads. This procedure is rather costly and therefore not always feasible in practice. Recently, an approach to synthesize circular BRIR sets based on a single omni-directional room impulse response (RIR) was presented [1]. Direct sound, early reflections and diffuse reverberation are extracted from the latter and treated separately. Spatial information is added according to generic geometric assumptions and by exploiting certain psychoacoustic knowledge about diffuse reverberation, like for example the perceptual mixing time. While it is the highest goal in auralization to archive authentic reproduction of an auditory environment, i.e. the recipient cannot distinguish between a real environment and an equivalent virtual one, a sufficiently plausible representation is actually adequate for many practical applications. This is the basic assumption of the presented approach. Naturally, there are certain physical and perceptual differences between actual measured BRIRs and synthesized ones which are based on a single omnidirectional RIR. In order to examine these perceptual differences, a listening experiment based on a SAQI test paradigm [2] for four rooms with different room acoustic properties was conducted. The results show that the proposed solution is promising and has the potential to create circular sets of BRIRs from one single one-channel measurement. Introduction Binaural synthesis can be used for a headphone based presentation of auditory virtual environments. This approach can be applied in various areas like audio engineering, telecommunication, or architectural acoustics to create a natural and plausible room impression. By considering the listener’s head movements in the auralization, localization accuracy increases and externalization of virtual sound sources improves. Several commercial or scientific rendering engines are available which allow adapting the sound field presented via the headphones according to the head movements in real time (e.g. [3]). However, applications using this technology in everyday life are still rare. One reason is the complex procedure for the acquisition of spatial sound fields which cannot be conducted with standard equipment and techniques. Two different approaches to collect room data are commonly applied: Either simulated sound fields based on a geometric model are used. This allows for a free positioning of the listener in the virtual room and an adaptation of the sound field according to the listener’s movements. Alternatively, measurements of binaural room impulse responses (BRIRs) can be performed. If the reproduction aims at being authentic, i.e. the listener cannot distinguish a presented virtual environment from an equivalent real one, head rotations of the listener need to be considered. For this a circular (or even a full spherical) set of BRIRs needs to be captured. However, such a measurement implies the use of complex devices (e.g. a rotatable artificial head) and an enormous amount of time. Thus for many state-of-the-art applications in the field of spatial audio, the effort is so high that circular sets of BRIRs are not used. The algorithm introduced in this paper aims at a sufficiently plausible auralization of a room, based on a simple measurement procedure. For many of the applications mentioned above, a plausible presentation is quite sufficient and no authentic reproduction of the sound field is pursued. The approach described here requires only one measured omnidirectional room impulse response to synthesize a complete set of BRIRs. The presented study investigates to what extent one measured omnidirectional room impulse response can be enhanced in order to deliver a plausible auralization when using dynamic binaural synthesis. The paper is organized as follows: Section 1 describes the proposed algorithm performing this spatialization. Section 2 illustrates the performance and the results of a psychoacoustic evaluation of the algorithm. Measurements of circular sets of BRIRs and of omnidirectional impulse responses of four different rooms were used [4]. Within two psychoacoustic experiments, perceptual differences between the measured sets of binaural impulse responses and the synthesized sets were evaluated. In Experiment 1 the general difference was investigated while in Experiment 2 applying a SAQI test paradigm [2] a large number of perceptual attributes was tested. The results show to what extent a plausible auralization can be obtained and which of the perceptual cues are responsible for the remaining differences. Basic idea of the algorithm The aim of the algorithm described here is the synthesis of a circular set of binaural room impulse responses (BRIRs) from a single omnidirectionally measured room impulse response (RIR). Therefore, predictable information from geometrical acoustics as well as knowledge regarding the perception of diffuse sound fields is applied. The RIR is split into different parts. In the early part the direct sound is identified. According to the direction of incidence of the direct sound, which has been determined by the user in advance, the appropriate head-related impulse response (HRIR) is picked. This HRIR is convolved with the time frame of the omnidirectional RIR which contains the direct sound. The early reflections reach the listener according to a room and position-specific reflection pattern from various directions. In the algorithm, arbitrary spatial reflection patterns are chosen, which are regarding the directions of incidence not at all adapted to the geometrical shape of the room. The temporal structure of the early reflections is detected from the omnidirectional RIR by analyzing peaks and dips. By convolving the HRIRs of each of the directions of sound incidence with the time section of the respective reflection a binaural synthesis of the early geometric reflected part is applied. The diffuse reverberation part is considered to reach the listener temporarily and spatially equally distributed. Several studies [5-7] have shown that the part of the measured impulse response after a perceptual mixing time can be simplified. In this investigation, the diffuse part is replaced by binaural noise, which is frequency-dependently shaped according to the decay of the omnidirectional impulse response. Even though an exact reconstruction of the temporal and spatial structure is ideally not required [6] small perceptual differences might remain [8]. Several studies have shown that the value of the mixing time is room dependent and can be chosen according to predictors which are calculated based on geometric room properties [7]. However, as the transition from the early reflection part to the diffuse reverberation part is quite soft, a smooth crossover has been chosen in the design of the algorithm. Since relevant information about the binaural sound field is absent in the omnidirectional RIR, the algorithm presented here incorporates several inaccuracies and errors. For example, a point source is assumed for all synthetic BRIR sets. Thus, it is not possible to rebuild source width and other properties of the source correctly. Furthermore, the directions of incidence of the synthesized early reflections are not in line with the original ones. Hence, differences in perceptual spatial properties (e.g. envelopment) between the original room and the synthesized room may occur. Finally, the diffusion of the early reflections is not rebuilt precisely. In the algorithm, only a simple model estimating the part of the geometrically reflected and the diffusely reflected energy is applied. In the listening experiments, which complete this study, it is evaluated which perceptual influences these approximations have. In the literature, several approaches to enhance an omnidirectional impulse response to a binaural one have been proposed [9, 10]. They rely on statistical considerations and perform an adequate reconstruction of the coherence. Implementation The algorithm was implemented in Matlab and includes the following steps: The algorithm requires the omnidirectional RIR as well as basic geometric information about room volume and the incidence direction of the direct sound as input data. Furthermore, the algorithm has access to a set of HRIRs and to a pre-processed sequence of binaural noise. Both the HRIRs and the binaural noise were acquired based on measurements with a Neumann KU 100 artificial head [11]. The algorithm only applies to frequency components above 200 Hz. For lower frequencies the coherence of a typical binaural impulse response is nearly one and the omnidirectional impulse response can be maintained. The first relevant maximum in the impulse response is identified as direct sound. A frame of 5 ms starting slightly before this maximum is convolved with the HRIR of the direct sound incidence direction. The time section from 5 ms to 150 ms, is assigned to the early reflections and the transition towards the diffuse reverberation. In order to determine the early reflections, the energy of a sliding window of 8 ms length is calculated and areas are marked which contain high energy. Peaks which are 6 dB above the RMS are determined and assigned to geometric reflections. Smoothly windowed sections around the peaks are convolved with the HRIRs according to the predefined reflection pattern. The omnidirectional RIR excluding the sections of the direct sound and the geometric reflections is split up into 1/6 octave bands by applying a near perfect reconstruction filterbank [12]. In each of the frequency bands, the binaural diffuse part is synthesized. Therefore, the envelope of the binaural noise is shaped according to the envelope of the omnidirectional impulse response. Thu