Perception of Sound Reflections in Home Cinemas - October 30th, 2008
In a normal sounding environment, the sound from a given source, such as a home cinema speaker, reaches our ears via a number of different paths. Some of the sound arrives by a direct path, but a good deal of it only reaches our ears after one or more reflections from the surfaces of the home cinema room. In spite of this, we are not normally aware of these reflections, or echoes, and they appear to have little influence on our judgements of the direction of the sound source. Thus, we are still able to locate a speaker in a reverberant cinema room where the total energy in the reflected sound may be greater than that reaching our ears by a direct path.
Home cinema installations are not a different case. Multiple speakers are located around the cinema room and of course, reflections from the room’s boundaries are going to occur. The home cinema designer needs to address these issues when designing the room. An important point to mention is the so called “precedence effect.” If two successive sounds are heard as fused, the location of the total sound is determined largely by the location of the first sound. It has also been called the “Haas effect.”
The precedence effect plays an important role in our perception of sound in a home cinema. It enables us to locate, interpret, and identify sounds in spite of wide variations in the acoustical conditions in which we hear them. Without it, listening in a reverberant room would be an extremely confusing experience.
Sometimes, on the other hand, the effect can be an inconvenience. An example of this is found in the reproduction of music from the left and right home cinema speakers. If the sound is equally intense in both front channels, then the sound is located in the centre, between the two channels, provided the speakers are equidistant from the listener. If, however, the home cinema system is not properly calibrated and the sound from one loudspeaker leads in time, and if the time disparity exceeds 1ms, the precedence effect operates; the sound appears to originate entirely from that loudspeaker.
In a normal home cinema installation this gives the listener latitude of about 60cm on either side of the central position. Deviations greater than this produce significant changes in the “stereo image.” Almost all of the sound appears to come from the closer loudspeaker. Thus, the notion of the “money seat” in a home cinema room is quite close to the truth.