## 📄️ Solver concept

Covering the entire audible spectrum with the wave-based solver requires substantial compute resources and it is in most cases considered impractical, but also to some extend, unnecessary to do. Therefore Treble's state-of-the-art Geometrical Acoustics (GA) solver is used to compute the high frequency part of the acoustic response, which is later combined with the wave-based results via hybridization.

## 📄️ The Image-Source Method

The image-source method (ISM) is a widely used method to model the propagation and reflection of acoustic waves[1,2]. The basic concept is to mirror the original source across each of the planes in the model, thereby creating an image source. The geometrical model of the space generally consists of many planes, that each will create an image source. Once the original source point is mirrored across all planes, we can check the reflection path of the first reflection by tracing a ray back from the receiver point. First towards the image source, and then from the intersection point of the ray and the plane to the original source

## 📄️ The Ray-Radiosity Method

The traditional method of acoustic ray tracing is to trace rays around the space while looking for intersections between the rays and a small sphere. This method is straightforward and intuitive, but it can require a fairly high amount of rays to generate enough hits to produce a meaningful energy histogram. To limit the amount of rays needed, Treble GA solver uses a Ray-Radiosity (RR) approach to raytracing the scattered and the late part of the response. This method generally assumes that the sound field is diffuse, which is considered fair for the scattered and the late part of the response. It is also known as "Diffuse rain" or "Secondary Source", and is related to radiosity methods.

## 📄️ Scattering

The definition of the scattering coefficient is that it denotes the ratio of sound energy that is reflected in other directions than the specular direction. Therefore the scattering coefficient is always a value between 0 and 1, where 0 means that the reflected wave is fully specular and 1 means that it is fully diffuse. The scattering coefficient is to a large extend a concept that needs to be included into GA due to the lack of diffraction and other wave-effects in the basic underlying approximations and assumptions.

## 📄️ Directivity

The treble GA solver is capable of modelling the directive characteristics of sources. Currently, the directivity is modelled with magnitude only, pr octave band for both the pressure-based ISM and the ray-radiosity(RR) solver.