Interactive Acoustic Modeling of Complex Environments

  


Thomas A. Funkhouser, Ingrid Carlbom, Gary Elko, Gopal Pingali, Mohan Sondhi, and Jim West

Overview:

Computer-aided acoustic modeling tools are important for design and simulation of three dimensional environments. For instance, an architect might use such a tool to evaluate the acoustic properties of a proposed auditorium design. Or, a factory designer might be able to predict the sound levels of any machine at any position on a factory floor. Acoustic modeling can also be used to provide sound cues to aid understanding, navigation, and communication in interactive virtual environment applications, particularly if acoustical simulations can be updated at interactive rates. For example, the voices of users sharing a virtual environment may be spatialized according to each user's avatar location.

The primary challenge in acoustic modeling is computation of reverberation paths from a sound's source position to a listener's receiving position. As sound may travel from source to receiver via a multitude of reflection, transmission, and diffraction paths, accurate simulation is extremely compute intensive. Prior approaches to acoustic simulation have used the image source method, whose computational complexity grows with O(n^r) (for n surfaces and r reflections), or ray tracing methods, which are prone to sampling error and require lots of computation to trace many rays. Due to the computational complexity of these methods, interactive acoustic simulation has generally been considered impractical.

We have developed data structures and algorithms to enable interactive simulation of acoustic effects in large 3D virtual environments. Our approach is to precompute and store a spatial data structure that can be later used during an interactive session for evaluation of reverberation paths. The data structure is a ``beam tree'' that maps the convex pyramidal beam-shaped paths of significant transmission and specular reflection from a source point through 3D space. The beam tree is generated by: 1) partitioning 3D space into convex polyhedral regions, 2) computing the convex polygonal boundaries between regions, and 3) recursively splitting and tracing convex polyhedral beams from a source point through region boundaries (e.g., reflecting beams at opaque boundaries). The precomputed beam tree data structure can be used to compute specular reflection and transmission paths from a source position to any point in space at interactive rates. The lengths and directions of computed reveration paths may be used to spatialize audio source signals to a receiver moving under interactive control by a user.

These data structures and algorithms have been integrated into a system for interactive acoustic modeling. The system takes as input: 1) a set of polygons describing the geometry and acoustic surface properties of the environment, and 2) a set of anechoic audio source signals at fixed locations. It outputs an audio signal auralized according to the computed delays, directions, and attenuations of the specular reverberation paths from each source to the receiver point. The receiver point can be moved interactively by the user, allowing real-time exploration of the acoustic environment.

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