First reflections and home cinema design

Performance home theater facilities is influenced by the presence of borders, so it is important to understand how sound spreads in a home theater room. Imagine a home theater with a loudspeaker and a listener’s departure located somewhere in the home theater. Now suppose that, at a time (t = 0), a signal is reproduced by the loudspeaker. There are three main aspects of how the sound of a loudspeaker in the theater, which behaves as follows:

1. After a short delay, the listener in the theater will hear the sound of the speaker, who will have traveled the shortest distance between him and the listener. The delay will depend on the distance, as the sound travels 344 meters per second. The shortest path between the speaker and the listener’s departure is the direct path and therefore the first thing the listener hears. This sound component is called direct sound. The continuous component is important because it carries the signal information in a non-contaminated. Therefore, a high level of direct sound is required in the home theater installations, for a clear sound and a good intelligibility of the dialogue. It should be noted that the intensity of the direct sound decreases as the square of the distance from the source, in the same way as a sound in free space.

2. A little later, the listener will hear sounds that have been reflected in one or more surfaces (walls, floors, etc.). These sounds are called the first reflections and separated in time and direction from the direct sound. These sounds vary the home theater speaker or the listener to move inside the theater. We use these changes to give us information about the size of the home theater room and the position of home theater speaker systems. If one of these reflections is much delayed, the total length difference more than about 30 milliseconds, then perceived as echoes. Early reflections can cause interference effects, and these can reduce the intelligibility of the dialogues of the home theater installations and bring an unwanted music sound change to the home theater. The intensity levels of the first reflections are affected by the distance and the surface from which to reflect. In general, most home theater dedicated surfaces absorb some of the sound energy and therefore reflection is weakened by absorption.

3. The absorption coefficient of acoustic wall treatments in home theater installations defines the amount of energy or power is suppressed from the sound when hit. In general, the absorption coefficient of actual acoustic treatments in a home theater room varies with frequency. The amount of energy or power removed by a given acoustic panel wall depends on the energy or power per unit area that the affected area. Since sound intensity is a measure of power per unit area, this means that the intensity of the reflected sound is reduced proportionally to the absorption coefficient.