Real-time auralization of propagation paths with reflection, diffraction and the Doppler shift

The auralization of early reflections is often implemented using digital signal processing and steady-state filtering by assembling multiple propagation paths into a single filter. To render the Doppler shift in interactive auralization systems, the constraint of linear and time-invariant filtering must be dropped. Instead, resampling during real-time audio processing must be performed. Especially in urban outdoor scenarios, it is not only necessary to account for time variant movement of fast sound sources, but also for dynamics of the built environment. Consequently, if specular reflections and diffraction paths should be considered for real-time auralization, a combination of resampling and filtering must be deployed. A single-input multiple-output variable delay line (SIMO VDL) is suggested to effectively create the Doppler shift for early reflections and to prevent comb filter effects during filter exchange. It consists of multiple interpolating read cursors and subsequent short filtering modules that apply individual spectral attenuation effects caused by medium propagation and boundary interaction. Sound propagation simulation To auralize virtual acoustic environments (VAEs), algorithms and models can be employed that approximate the propagation effects of traveling sound waves from a source to a receiver along the wave front normal by means of Geometrical Acoustics (GA) [11], as shown in Figure 1. Especially in the context of interactive indoor and outdoor auralization applications, those GA methods can deterministically find propagation paths that include loworder specular reflections off walls [1] and, with some computational efforts, include diffracted energy as well [2, 9]. The resulting intermediate representation from path finding algorithms can be transformed into time-discrete impulse responses (or transfer functions in frequency domain) that include all steady-state effects such as directivity, medium attenuation, spreading loss, absorption, etc. This approach, however, cannot handle time-variant influences like source and listener movement. The frequency shifts that are hereby caused require resampling [10] using variable delay lines (VDLs) [3, 15]. Adapting to scene dynamics by exchanging filter coefficients or impulse responses and cross-fade the audio streams cause comb filter effects, which become severe when differences in distance increase [12]. For room acoustics, this problem is mostly undetectable [6] and can be suppressed by high filter exchange rates of early reflections and slow sound source movement [7]. Steady-state real-time auralization Applications that adapt dynamically for changes of the VAE (like moving sources and receivers) operate in real-time, if those adaptions are performed smooth and fast. An impression of immersion is achieved, when human perceptual thresholds are undergone, and the system can readily be used for Virtual Reality applications [11]. This low-latency constraint is demanding and requires fast propagation simulations algorithms, efficient filter coefficient assembly (e.g. FIR filter design) and rapid artifact-free exchange of digital signal processing (DSP) module parameters. Hybrid simulation approaches that handle direct sound, early reflections and diffuse reverberation separately have proven to work sufficiently well when combined with DSP units that implement lowlatency block convolvers [6, 12]. The steady-state propagation approach treats sources, receivers and the built environment as entirely static snapshots and, consequently, scene adaptions can only be performed by jumps from one static representation to the next. Time-variant real-time auralization For fast moving objects, switching filters that combine all transmission paths can be interpreted as jumps between scenes and result in audible artifacts. Additionally, they lack the possibility to integrate Doppler shifts. Instead, delay lines are commonly used to simulate the medium propagation delay [4, 8, 10]. Interpolating write cursors can effectively auralize the occurring Doppler shift when the source moves relatively against the medium. Furthermore, interpolating read cursors auralize the Doppler shift when Figure 1: Simplified example of a virtual outdoor environment with buildings, a fast-moving sound source (fire brigade) and a sound receiver (humans). Direct sound is occluded by the red building, hence reflected and diffracted paths dominate the acoustic transmission and individual Doppler shifts per indicated path must be considered. DAGA 2018 München