An echo is simply caused by sound bouncing off a flat surface. Sometimes it can be heard bouncing between parallel flat surfaces many times. Echoes can cause all sorts of effects that are undesirable. They can interfere with the positioning of effects in a home theatre, the stereo image from a pair of speakers or even make it hard for musicians to play in a room. Most echoes can be fixed by the use of absorption of diffusion. Absorption just reduces the size of the reflected sound by making it travel through a suitable medium and losing some of it's energy. Diffusion solves the problem by breaking the reflection into many smaller ones and scattering them in different directions. Absorption is the usual way to remove echoes but if too much is used the room will be too ‘dead' and it will feel unpleasant. This is when diffusion is used, to fix the remaining echoes without making the room too dead sounding.