Sound Image Costs and Benefits, Part 2
Apr 4, 2013 1:38 PM, By Bob McCarthy
Practical sound system design for the best preservation of imaging.
Main speaker placement
In the most basic sense we can model the source and loudspeaker(s) arrangement as a speaker array. The most successful configuration would be the uncoupled point source in both the horizontal and vertical planes. In short, all of the sources reconcile back to a virtual source point behind the original stage source. For example, outward-facing left and right speakers relate as a point source to the stage (favorable) and inward-facing left and right speakers (unfavorable) create an inverted point source, known as a “point destination” array. If the left and right speakers move to a straight-ahead orientation, the array relationship is an uncoupled line source (neutral). Central listeners will be pulled outward away from the stage and those on the sides will find the speakers guiding them inward toward the stage. The center speaker has a greater challenge since we tend to place both performers and audience members on the floor and center speakers high in the air. In the horizontal plane the center speaker lines up very favorably with the source, but in the vertical plane, we are sure to have listeners for whom the center speaker will appear as an inverted point source with tendencies to pull the image skyward. The only way those listeners will have an uncoupled point source orientation to the stage source and center speaker is if they are in a high balcony area looking down on both the stage and the loudspeaker. This is certain to be a minority of the listeners. Fortunately, we are far less precise in our vertical localization than our horizontal, so these errors are less noticeable to the listeners.
So we now have a basic placement strategy for the main system design: left and right should be as close in to the stage as practical (within sightlines and gain before feedback limits), at the lowest practical height that gives us even level from front to back. Center should be as low as possible (almost always a scenic/sight line limit).
Fill speaker placement
The main system can get coverage help from the auxiliary fill systems such as frontfills, sidefills, infills, and delays. Let’s look at the best way to position these systems to aid our image placement goals. First up is the frontfill array. This is the easiest of all possible subsystems to place in an image enhancing location. By being spread across the stage front they are assured of bringing the focus onto the stage horizontally and only slightly below the stage source vertically. The worst-case scenario is we appear to have people singing out of their feet, which is far less an issue than our mains faced. The frontfills have the ideal imaging location but can only be used for a limited distance, typically about three rows.
Next in line is a sidefill speaker, which could horizontally extend the L/R main coverage to the near outside seating. Like the L/R mains, we want to place them as far inward as possible and aim them outward to continue our point source orientation to the stage (and the mains). Keeping these at the lowest possible level (relative to the mains and stage source) will help keep our focus inward.
Infill speakers can extend the horizontal range of the L/R mains to cover the central area beyond the reach of the frontfill. This is a dangerous pursuit as this area can become very crowded with arrivals from the center main, the frontfills, and the infill from the opposite side. The infills are an inverted point source to the stage source in the sensitive horizontal plane, so the image distortion risks are very high. The best strategy for infill arrays is a minimalist one: Keep the level as low as possible so that only a small number of seats are covered exclusively by the infills by giving way to frontfills and center mains as much as possible. In summary, infill speakers should be placed as far onstage as possible and run at the lowest level you can get away with to fill in the gaps.
Delayed speakers present the next challenge. Like the frontfill, under (or over) balcony delays have a very favorable horizontal orientation to the source, once again an uncoupled point source. But like the center main, they are vertically challenged because they are invariably an inverted point source to the stage with an upward orientation. On the positive side, however, is the fact that in many cases, the vertical angle between the stage source and under-balacony speaker is very small, leaving us with a low risk of substantial error. Therefore, we are working to overcome small angular distortions in our less-sensitive plane (the vertical). Contrast this to the far more difficult task faced by the center main, which has extremely high vertical angle distortion in the front parts of the room. The placement of under-balcony delays is described by the component spacing and the depth of the coverage. It is a given that they will be mounted on the ceiling, so we won’t have the option of going lower. In the horizontal plane we are better off not to attempt to use widely spaced, wide-coverage angle speakers. To do so would place many listeners in locations where they may have a substantial outward horizontal image pull toward the delay speaker. Instead, an appropriate quantity and spacing of speakers with 60 to 100 degrees of coverage reduces the horizontal image risks. The vertical image is kept in check by reducing the coverage depth to the minimum required, and therefore allowing us to keep the level low. If the delays have to throw too far they will inevitably be too loud at the rows closest to them, which is exactly where the vertical image distortion risks are the highest. It helps to aim the delays to the deepest area of coverage, thereby keeping the nearest rows underneath their coverage. Far too often we see under-balcony speakers hung out on the front edge of a long balcony and aimed downward when their best placement would be much deeper underneath and aimed at the rear row.
Overbalcony speakers are a variation on the same theme as underbalcony speakers, but with a few twists. First is the fact that we are not necessarily forced to mount our speakers directly onto the ceiling but may have the option of dropping them down until sightline limits apply. A favorably low position can greatly reduce the vertical image distortion risks. In the horizontal plane the same issues apply, but it is worth restating the disadvantages of using a small quantity of overly wide delays up there. It is not uncommon to see designs with a single point source cluster of two or three speakers creating a wide horizontal spread to cover the overbalcony. This is inadvisable since it opens up large angular distortions in the sensitive horizontal plane for those with seats in the sides. If two or three speakers are needed to provide sufficient coverage, it is better to spread them apart in an arc or line and minimize the angular distortion for the center and sides alike. That covers the mains and typical subsystem placements. Now let’s move on to how to set the timing sequence.
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