III.2 Image source technique

This chapter presents the image-source technique and how to use it to find all the specular reflection paths in a room geometry. We start with a single reflection from a surface, and then gradually move to higher order reflections, and more complex geometries such that in the end we have a general algorithm that can be used in any room geometry composed of planar surfaces. The examples here are all in 2D, but they generalize directly to more realistic 3-D geometries as well.

III.2.1 An ideal specular reflection
An ideal specular reflection from a rigid surface can be represented by a mirror image source that is obtained by reflecting the sound source against the reflecting surface. If the mirror image source emits an identical wavefront as the original source, it is seen as a reflected wave-front inside the room.

- Try moving the 'Time' slider to see how a circular wavefront propagates from a point-source.
- Check the 'Show outside' to see the reflected image-source and corresponding wavefront as a complete circle.
Note that in all the illustrations the surfaces are single-sided meaning that only their front-sides are reflective. Each surface has its normal-vector pointing in this direction. It is seen as a little red marker at the center of each surface.

Time: Show outside

Actually, this reflection model is physically accurate if the reflecting surface is of infinite size and ideally rigid. However, in practice the surfaces are of limited extent and the edges of a surface typically cause the sound field to be diffracted. Modeling of diffraction is a separate topic and will be discussed later in this book. Also the assumption of ideal rigid surfaces does not usually hold. In this section the main emphasis is on computation of the specular reflection paths and handling of acoustic properties of the reflecting material will be discussed in conjunction with reconstruction of impulse response from reflection paths.