Over the years many articles have been written and classes taught about proper seating position in a home theater or media room. Most talk about mystical rules of thirds or some precise fraction of the rooms length and width for proper placement. Others focus on the proper position of the listener as a point in an equilateral triangle. I’m not writing to debunk any such rules but rather to explain the science behind them and show where they tell only part of the story.
There are two prime factors, of many, for the proper placement of listeners. First, the ideal locations with respect to the speakers and second, the best location for smooth bass response. Both of these factors depend on the type of system and the size of the seating area. For large rooms, like commercial cinemas the directivity of the speaker, the speaker locations, and speaker pointing are critical. In small rooms, like a home theater, we face additional issues that complicate our designs. In this article, I’ll discuss how front left, center, and right (LCR) speaker placement can affect our seating layout beyond simply using the correct angular separation. Look for more articles in the future to fully cover this subject.
The concept of separating speakers to create an angle of 45º to 60º from the perspective of the prime listener is well known. What isn’t so well know is that the speaker separation also dictates the size and shape of the “sweet spot”. Plus, the addition of a center channel goes well beyond simply adding a dialogue speaker; in fact, this is a minor role for the center channel.
The True Function of the Center Speaker: The center channel has inherited the term dialogue speaker not because that is its sole purpose but rather because the movie industry jumped on the logic and convenience of porting most if not all dialogue to it. The original purpose of the center channel was to create a wider area of good focus (good imaging) for stereophonic sound. Diagram 1 shows the typical area (in green) in front of two speakers where balanced and focused stereo imaging is heard. The layout shows the left and right speakers at an approximately 60º angle. The darker green represents the space where the imaging is best centered. Depending on the directivity of the speaker (directivity being the polar dispersion) the sweet spot or sweet area is a narrow aisle of focused sound running lengthwise in the room. Speakers with a narrower directivity tend to have a narrower sweet area.
Diagram 2 shows how the center channel creates a sweet triangle instead of an aisle. The advantage of this design is obvious; a much wider area of balanced focused imaging. This is ideal for home theater where the audience is often more than a single listener. You can see that this setup, while widening the good listening area, is still not sufficient to bring all the listeners into the sweet triangle. Allowance must be made for the desired seating layout with reference to the speaker separation.
Diagram 3 shows how decreasing the separation between the left and right speakers pulls the sweet triangle forward. The advantage here is full coverage of the forward seating area with balanced focused sound. The goal of the acoustical designer is to maintain the desired angular separation of the left and right speakers for accurate stereo imaging without leaving any seats out of the sweet area. The best designs preserve the correct placement of stereo images across the soundstage while keeping the extreme side seats in the good focus zone. It is possible to have side placed listeners retain a balanced soundstage but the design is always pushing the envelope of sweet triangle width versus proper left/right separation. Part of the science of the acoustical designer and the art of the acoustical calibrator is creating a solid layout that accomplishes this goal. All too often, without an understanding of this phenomenon, systems are designed strictly based upon the 45º to 60º rule compromising forward seating with an unbalanced soundstage.