Early Reflections and Home Cinema Design - November 14th, 2008
Performance of home cinema installations is influenced by the presence of boundaries, and so it is important to understand how sound propagates in a home cinema room. Let’s imagine a home cinema with a starting speaker and a listener located somewhere at the home cinema seating area. Let’s now assume that at some time (t=0) a signal is played through the speaker. There are three main aspects to how the sound of a speaker behaves in the cinema room, which are as follows:
1. After a short delay the listener in the cinema room will hear the sound of the starting speaker, which will have travelled the shortest distance between it and the listener. The delay will be a function of the distance, as sound travels 344 meters per second. The shortest path between the starting speaker and the listener is the direct path and therefore this is the first thing the listener hears. This component of the sound is called the direct sound. The direct component is important because it carries the information in the signal in an uncontaminated form. Therefore a high level of direct sound is required in home cinema installations, for a clear sound and good dialogue intelligibility. We need to point out that the intensity of the direct sound reduces as the square of the distance from the source, in the same way as a sound in free space.
2. A little time later the listener will then hear sounds which have been reflected off one or more surfaces (walls, floor, etc.). These sounds are called early reflections and they are separated in both time and direction from the direct sound. These sounds will vary as the home cinema speaker or the listener moves within the cinema room. We use these changes to give us information about both the size of the home cinema room and the position of the home cinema speakers in the space. If any of these reflections are much delayed, total path length difference longer than about 30 milliseconds, they will then be perceived as echoes. Early reflections can cause interference effects, and these can both reduce the intelligibility of dialogues of home cinema installations, and cause unwanted timbre change in music in the home cinema room. The intensity levels of the early reflections are affected by both the distance and the surface from which they are reflected. In general most surfaces in dedicated home cinemas absorb some of the sound energy and so the reflection is weakened by the absorption.
3. The absorption coefficient of acoustic wall treatments in home cinema installations defines the amount of energy, or power that is removed from the sound when it strikes them. In general the absorption coefficient of real acoustic treatments in a home cinema room will vary with frequency. The amount of energy, or power, removed by a given area of an acoustic wall panel will depend on the energy, or power, per unit area striking it. As the sound intensity is a measure of the power per unit area this means that the intensity of the sound reflected is reduced in proportion to the absorption coefficient.
High end home cinema designs will take early reflection analysis into consideration, in order to produce an acoustically correct home cinema environment.