How Many Speakers, and Where Do We Put Them?
A current project here at Russ Berger Design Group underscores a situation faced by anyone planning new or upgraded rooms for surround audio production. This project includes fifty technical spaces with 5.1 surround monitoring in seven distinctly different configurations. The surround mix must be transportable between rooms, although they vary greatly in size and function. It is critical to select monitoring methods that meet this transportability requirement while providing the best possible end product.
When the decision is made to create an audio production room for surround use, it is necessary to identify the surround monitoring philosophy that is to be implemented. Factors that must be identified include the format(s) to be accommodated and the primary target for the end product (consumer video, cinema, television/radio). However, if the room is to be successful at meeting its intended purpose, the single most important factor to be considered is “who’s listening”?
By “who’s listening” I primarily refer to those persons in the room who must be able to hear the audio elements and the mix with a high enough degree of confidence to make technical and artistic decisions. This single consideration, more than any other, drives design decisions regarding room size and shape, monitor type and placement, ergonomics, and acoustical treatment. Also to be considered, of course, is the final target media and venue for the surround product. Figures 1 through 3 represent three current surround monitor placement philosophies in audio control room settings. Subwoofer placement issues are deliberately omitted here, and will be the topic of a later column.
In Figure 1, five identical monitors are implemented, all aligned to converge at the mix position. The left and right front monitors and the left and right rear monitors (usually) form identical, but opposing, triangles in a sort of “Quadraphonic Mark II” (Quintaphonic?) arrangement. This configuration is gaining considerable favor in 5.1 channel music production. It has the advantage of providing the best articulation and definition for elements placed fully in the left or right surround channel. Note that this advantage is not as definitive when trying to pan between the left rear and right rear. Many of the auditory mechanisms that support the illusion of spatial localization when the source is in front of the listener are not effective when the source is from the rear.
The major disadvantage of this configuration is that the only position in which critical monitoring is possible in this room is at the console. Anyone seated at the producer’s desk will obviously hear the rear monitors at a considerably higher relative level than the engineer. This level differential problem can be somewhat reduced, but not eliminated, if the room is very large, but, for reasons of cost and ergonomics, this is not normally a practical alternative. Also, depending upon the monitors chosen, finding room to properly place and align the surround monitors can be difficult both for physical and acoustic reasons. When producing sound for picture, or in any other situation where the goal is to use the surround channels to convey a sense of space or ambience (classical or other acoustic music, for example), discrete monitors are not as effective as the diffuse methods that follow.
Figure 2 shows an implementation of dipolar surround monitors. For those unfamiliar with this type of monitor, they are imports from consumer home theater (specifically the Lucasfilm THX home theater specification) that have crossed over into the professional community. They utilize two sets of drivers in each monitor, one set firing forward and the other firing rearward. The forward and rearward drivers are wired in opposing polarity to create a lateral null perpendicular to the wall on which they are placed. The purpose is to create a diffuse surround field at the listening position by assuring that only reflected sound energy from differing paths makes up the soundfield. If the monitors selected have a large enough null and the distance between the mix position and the producer’s desk is small enough, the surround monitor may be placed so that both locations are in the null, thereby achieving similar surround channel levels for each.
There are several drawbacks to dipolar surround monitors when used in a typical production environment. For proper and predictable operation they rely on substantial reflective areas – common in home theater and listening rooms, but generally avoided in professional production rooms – to direct the surround information back into the listening position(s). This means that if they are to be used effectively, reflective areas must be strategically placed within the room geometry in such a way that the surround field is adequately supported without adding unwanted early reflections from the front monitors. This is a difficult, but not impossible, balancing act.
Dipolar surround monitors also typically have relatively low output capability since (a) they provide little diaphragm area when compared with normal left, center and right main monitors, and (b) much of the potential output of that area is “thrown away” in the creation of the direct path null. They have very little output below 80 Hz – 100 Hz, so the low frequency content of the surround channels must be routed to the mains or a subwoofer via a bass management system. There are few professional monitor vendors making dipolar surround monitors as of this writing, and most consumer versions are not sturdy enough when used in a professional environment, where an inadvertent mispatch, digital artifact, high-level slate tone or tape scrub “accident” can quickly damage them.
Figure 3 shows surround monitoring using a point source array. This method is analogous to that used in film mixing for theaters and cinemas. It uses a number of small monitors arranged to create an even, diffuse surround field. If the surround monitors are properly chosen and placed, it provides the broadest area of monitoring of the methods presented. Six to ten surround monitors, ideally chosen so that the combined output of them (in acoustic watts) equals or exceeds that of one forward channel, are usually required for average to large control rooms. It is important that the closest surround monitors to listeners at any decision-making location (console, producer’s desk, etc.) is beside, or preferably behind, them to avoid having the surround field perceived as coming from the front.
The cost and complexity of obtaining and mounting so many loudspeakers can be problematic. This surround monitor array will have better low-frequency performance than dipolar monitors due to the quantity and proximity of the sources, but in most cases it will still be necessary to redirect some low-frequency content from the surround channels.
Which one of the above is the best choice in your situation based on who’s listening? In a small Avid or digital workstation room where one or two persons seated at the edit position make all the decisions, the five equal discrete monitors as shown in Figure 1 can provide an effective solution. Widening the rear monitor triangle to between 120 and 150 degrees inclusive will allow the surround channels to be heard more accurately and the room to be more compliant for use with multiple surround formats.
Dipolar surround monitors are particularly appropriate for use in small (~400 sq. ft. or less) production and playback spaces where the listeners are in fairly close proximity fore and aft, and where the room design can accommodate the acoustic conditions necessary for their operation. Dipolar monitors should be carefully selected to assure that durability will be adequate and the sonic match to the front monitors acceptable.
The cinema-style point source array lends itself to use in mid-size to larger rooms (~300 sq. ft. and up) where listeners are located throughout the room. In the case of a room with multiple rows of listeners, such as a screening room, this monitoring method is the only effective choice.
Combinations of these monitoring philosophies are also possible. A room dedicated primarily to discrete 5.1 channel monitoring can include a point source array and switch between the two as production projects dictate. Conversely, rooms incorporating diffuse surround monitoring can (and arguably should) easily include a set of nearfield discrete monitors for use at the console.
The most important listener to consider in all of this is the one who is paying the bill. The end consumers’ listening environments are another world entirely; one that we have little to no control over. Fully understanding the chaotic nature of the consumers’ listening situations is critical to successfully navigating and translating surround product through your carefully crafted monitoring environment.