Ray tracing
This page is dedicated to the first 10 years of the development history in ODEON and how some of the main algorithms were constructed based on ray tracing and image source modelling. In 1984 the Ray tracing algorithm was selected; in 1990 an improved hybrid model combining ray tracing and image source method was developed, and two years later the Secondary source method was introduced as a substantial improvement. Later, in 1995 Vector based scattering was implemented as a more efficient method in ray tracing.
Relevant papers describing the development:
From the very beginning of the development of ODEON ray tracing models known from optics and visual inspection of acoustics were selected and investigated as basis for room acoustic simulation. The Ray tracing was selected in 1984 instead of similar methods like e.g. cone tracing or beam tracing. The ray tracing method later turned out to be a very convenient choice because rays can be treated as both explorers of a geometry in order to find relevant reflective surfaces and as transporters of energy information to be collected for calculation of the room impulse response.
Vector based scattering
In 1995, the Vector based scattering method was implemented in ODEON as a way of combining the scattered and specular directions of reflections during the ray tracing. With a typical moderate scattering coefficient on the surfaces, this leads to stable and reliable results with relatively few rays compared to the traditional Monte Carlo method for scattering, which needs a huge amount of rays.
Particle tracing
A special application of ray tracing is the Particle tracing, which means that the rays are considered carriers of patches of acoustic energy travelling around the room along the rays with the speed of sound, and after each reflection the energy is reduced according to the absorption properties of the surfaces. The total energy in all the particles can be displayed as a function of time and this is the Global Decay function of the room, from which a quite accurate estimate of the reverberation time can be calculated. This method takes into account the source position, the location of absorption material in the room, and the degree of scattering, i.e. the lack of diffusion in the room. However, there is no receiver point; the decay of energy in the entire volume is calculated. In 1995 this method was introduced in ODEON together with the vector based scattering.
Image source method
Specular reflections can be constructed geometrically using image sources. Modelling with image sources is beneficial for describing the early directional energy in a room with little scattering on the surfaces. But for higher order reflections the method is too precise geometrically, i.e. some reflections can be calculated althought they are without any significance when the wave length of sound is taken into account. In addition this method is very time consuming for higher order reflections.
Hybrid Methods
In 1989 hybrid models were introduced in ODEON to accomodate both the Ray tracing and the Image source method, combining the best features in both methods. For early reflections Raytracing is used to detect what image sources are relevant for a certain source position, and thus a large number of potential, but not relevant image sources are left out; in this way the combination of the two methods saves calculation time. In ODEON version 1 the late reflections were also calculated by a special kind of image source method.
Secondary source method
In 1992 the Secondary source method was developed as a hybrid model to benefit from the combined ray tracing and a new secondary source method for more accurate calculation of the late reflections. The rays are considered carriers of patches of acoustic energy, which is reduced after each reflection of the ray according to the absorption coefficients of the surfaces; the secondary sources are located in each reflection point during the ray tracing. Each receiver point in the room then collects information about the energy from all visible secondary sources in the room. In combination with the Vector based scattering this method has proven to be very efficient, especially in complicated room geometries
