Geometric Sound Transmission


Demo movie

Status 2007-12-12

Transmission through single walls now works. Correct transmission through walls with empty space is not currently supported. Since I have not found attenuation factors that describe sound inside of the space between walls, it is unlikely that this can be implemented in the near future. Thin wall attenuation is as described in many papers from the Journal of the Acoustical Society of America.

In the current implementation, sound can pass through wall with either a refractive property, or direct transmission. Attenuate is handled through the use of sound materials. The attenuation factor demonstarted in the movie is similar to two sheets of 1/2 inch plywood with 3 inches of air between the sheets.

The result is a realtime simulation of realistically sounding sound transmission. The simulation runs at 0.5 hertz on a common dual core PC.

Status 2007-11-20

I currently have transmission frusta working correctly. After a primary frusta intersects a surface, a transmission frusta is generated. The transmission then proceeds to propagate through the scene. The transmission attenuation factor is currently the same as the diffraction/reflection value; this will be changed in the future. Two screenshots follow, one with transmission and one without.


For my class project, I am going to propose an extension of sound-frustum tracing that allows the transmission of sound through objects. This phenomenom is often seen throughout everyday life and is an important aspect to creating realistic interactive soundscapes. Sound transmission is the the ability of sound waves to transmit through an object and continue propagating on the other side. This is a complex interaction, influenced by many factors, such as the resonate frequency of the material, the material's absorption coefficient, and the contents of the object.

Current state-of-the-art

Current geometric implementations of sound propagation use techniques similar to beam tracing or path tracing. Each has advantages and disadvantages, such as a speed, simplicity, or accuracy tradeoff. None of the methods described support sound transmission, however, with modifications, simple geometric propagation could be generated, since it is a simple extension of raytracing refraction.

Beam Tracing
This method involves constructing pyrimidal beams and tracing them through the scene. The beams may specularly reflect off surfaces or diffract and propagate further through the scene. This method relies on a precomputation stage that makes dynamic scenes impossible. Also, due to the reliance on beam tracing, it has severe limitations when used in scenes with highly complex geometry.
A Beam Tracing Approach to Acoustic Modeling for Interactive Virtual Environments.

Frustum Tracing
Frustum tracing is similar to beam tracing. Instead of shaped beams, frusta are traced through the scene. These frusta may also be further propagated throught specular reflection or diffraction. This method relies on advanced raytracing techniques, and has been shown to perform real-time on complex dynamic scenes. However, do the the fact that the frusta may partially intersect objects, it does not guarantee accurate geometric propagation.
Interactive Sound Propagation in Dynamic Scenes Using Frustum Tracing

Other methods
Other methods include techniques such as phonon tracing and path tracing. These techniques are similar to point sampling the scene to trace the sound propagation. Due to the intense discritezation of the sampling, these methods may fail to generate a frequency response in certain areas.
Phonon Tracing for Auralization and Visualization of Sound

Proposed tasks

I plan on exploring this topic using the frustum tracing technique developed at UNC as the basic structure. The engine already has support for many advanced ray tracing techniques and sound transmission would enable a more perceptually pleasing simulation.

Alternative ideas

The method outlined above describes a basic geometric method to transmit sound through materials. Another approach would be to accumulate incoming sound beams on the object before recomputing a response audio clip and transmitting it on the other side of the material. This would involve accumulation techniques similar to what is used in photon mapping and would reduce the number of transmitted beams while maintaining a realistic audio simulation.


November 20, 2007
Have transmitting frusta working in the raytracing engine

December 3, 2007
Allow refractive index to alter transmition path
Damp the refracting beams through the use of STC values

December 12, 2007
Correctly compensate the damped value for cases when refracted beam passes through negative space