by perry sun
For surround sound professionals, the issue of where to place speakers for small-room multichannel audio production and monitoring is one that is highly likely to be inevitable, and also not without some controversy. The underlying problem seems to be that, for music production at least, the world of positioning speakers for surround sound is not yet well-established. There really isn’t an existing professional environment to follow from as a model, such as the dubbing stage, which is considered the reference sonic environment for smaller-room facilities that are devoted to working with film soundtracks, such as for DVD.
Professionals working with multichannel audio should perhaps refer to the home theater in seeking some guidance as to speaker placement. After all, home theater aficionados and gurus have been tackling this issue for years. And since the end product of what you’re doing will end up in a home theater of sorts, it seems to make sense that you would want to create a listening environment that duplicates as much as possible what the consumer will ultimately experience. The dubbing theater is commonly considered the reference for movie soundtracks, even for the home, but many home theater buffs will contend that a film soundtrack experienced in a home environment can sound dramatically more compelling than in a typical cinema, particularly in terms of the perception of spatial coherence (or sonic imaging).
Therefore, we believe that awareness of home theater approaches to speaker placement for the professional can be beneficial for monitoring of both movie soundtracks and music. To this end, I would like to explain in detail the common ways consumers set up their surround sound speaker systems. They seem to fall into what I would consider as the “movie theater” and the “performance” approaches.
The “Movie Theater” Approach
Home theater has traditionally been thought of as bringing the movie experience to the home. Therefore, it has made sense, particularly during the early years, to follow the movie theater as a model with respect to the placement of speakers and the expectations in regards to the multichannel soundfield experience. Many home theaters are designed to replicate the cinema, often at great lengths and expense, with the incorporation of movie theater-like d�cor and seating.
Setting up a speaker system for the express purpose of reproducing cinema-type sound is what I would refer to as the “movie theater” approach. An example of a such a speaker placement scheme is shown in fig. 1. The screen speakers are collinear, and placed behind a screen, just like for a typical cinema. In this example, the surrounds are mounted into the wall and are of dipolar design (which, of course, is part of the THX specification for home theater). If the theater is large enough, it may be justified to use two monopole speakers per surround channel.1The “movie theater” approach brings cinema sound to the home, and results in the reproduction of certain sonic characteristics of a movie theater. There are some potential advantages. Because the screen speakers are typically identical (as in a THX system), the problem of timbre mismatching is non-existent. Also, the use of dipoles or a small array of monopoles (in a large home theater) can reproduce the diffusive, enveloping nature of the surround soundfield, characteristic of a larger, auditorium-sized space.
The “movie theater” approach to speaker placement, though very effective in replicating cinema-type sound, has compromises as well. As we all know, home theater is expanding beyond movies and into surround sound music. To reproduce multichannel music to its fullest potential, a speaker system should be able perform at its best with imaging, in addition to fidelity. This is likely to be less than ideal for the “movie theater” approach, since the screen speakers, positioned to match the physical boundaries of the screen, are usually placed too closely to allow for the best possible imaging between them. Also, the sense of spatial coherence is inherently lost within the surround soundfield, which purposely compromises directivity in favor of diffusivity.
The “Performance” Approach
I have always interpreted home theater to be the encompassing of the general cinema speaker placement methodology in order to allow for reproduction of the directional attributes of film sound. But I have also always considered home theater to be of almost unlimited potential in terms of performance. Seasoned home theater experts believe that a sound system for home theater can perform significantly beyond what is capable in a movie theater system. To this end, I would argue that placement of speakers should be approached with the objective of not just delivering movie sound, but also to bring out the best with music as well – both 2-channel and multichannel sources.
In other words, realistic, accurate reproduction of a multichannel recording’s spatial attributes should be the primary objective, and can potentially be realized through the “performance” approach to speaker placement. A diagram showing this approach, under what I believe are optimal conditions, is shown in fig. 2. In this case, the same screen is used as for fig. 1, but the speakers are placed in front, and with particular focus toward optimizing their imaging potential. All of the speakers are identical monopoles. [Dipole surrounds can be used in music reproduction as well – a subject that will be discussed in a future article -Ed.]
One of the important aspects of such an approach to speaker setup is a defined, central listening position, or the “sweet spot” (shown in cyan). Often, home theaters based on the “movie theater” approach sacrifice this listening position because of the cinema-like seating arrangement (as shown in fig. 1), or placement of a projector right where the optimal seating position would be. Another important aspect is the positioning of the speakers along a circle, each maintaining equal distance from the listening position. Even if electronic time delay is used to compensate, the situation is still can be sub-optimal, because a speaker’s perceived spectral and spatial characteristics can vary with distance. (Professional multichannel facilities do not electronically impose time delays, so equidistant speaker positioning is especially important). By eliminating the variables associated with seating and speaker positioning as much as possible, an important step toward realizing optimal lateral and depth-of-field imaging can be achieved.
The angular positions of the speakers shown in fig. 2 roughly conform to the ITU-R recommendation. The included angle of around 60 degrees between the front left and right speakers, along with the listening position, should, in my opinion, allow for palpable imaging with 2-channel music (or 5.1 mixes without a center channel). Also, with identical speakers, phantom imaging can be realized between any of the front channels as well. However, some may prefer that the surrounds be placed further behind the sweet spot than shown in the diagram. Still others might have the desire to decrease the above included angle to better match the frontal image with the screen. The important point to be made is that the “performance” approach to home theater allows for such flexibility with speaker placement.
Which Approach To Adopt?
With multichannel audio, as is the case for stereo, it’s not just about the ability to deliver audio from each of the principle speaker locations, but is also about how these locations work with each other to obtain a cohesive, spacious, palpable listening experience. I’m likely to be preaching the obvious to many of you, particularly if you work exclusively with multichannel music. But I also believe that using this approach is appropriate for the purpose of movie soundtracks for home theater. However, many home theaters do utilize dipolar surrounds, so perhaps the best compromise for your monitoring purposes would be to have both monopole and dipole surrounds installed, or to use M&K Sound’s Tripole speakers. The “performance” approach is actually what the majority of home theater systems are based upon, more or less, since the “movie theater” method typically requires specialized installation and a dedicated room, and therefore is usually more expensive to achieve.
Perry Sun is the Movie Sound Editor for Widescreen Review. Perry can be contacted at [email protected].