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Radial Coverage

Jul 13, 2012 12:39 PM, By Bob McCarthy

Designing sound for the fan-shaped room.

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Figure 5: (a) Coverage width for a 90-degree speaker placed on the floor at 6.25 meters, 12.5 meters, 25 meters, and 50 meters, (b) coverage width for same speaker from a height of 25 meters, (c) same from a height of 12.5 meters. See larger image.

A field example

Our example room is still 90 degrees at 50 meters. Our clusters are located 22 meters forward of the apex (at the stage edge) at a height of 8 meters. Our closest seats are 6 meters forward of the cluster(s), which is 28 meters from the apex. Can we do it with one? In the front row we need 40 meters of radial coverage length. The 90-degree speaker only gives us 14 meters of coverage width there. Bzzt! Thanks for playing. Could a wider speaker do the job? Sorry. We are losing the game by a 3:1 margin here, so it would take a 270-degree speaker. Any takers? Next option: We could raise the speaker up to 24 meters, but now it’s above the roof. Two clusters would expand our coverage to 28 meters and three would give us 42 meters. We needed 40 at minimum. We have a winner. Well, there you have it: It takes three 90-degree clusters to cover this 90-degree fan-shaped room. But wait. The position for the three-cluster scenario blocks the video screen. One option is to move the clusters up by one meter (about 10 percent) and use 80-degree speakers. The extra height allows us to use a narrower speaker in the front, and this will help us to reduce some of the extra coverage at the far end. How about four clusters? Four 90-degree clusters would be too much, but there are other options. The first is four 60-degree clusters and another would be a pair of 90s and a pair of 45-degree clusters. The latter is the one that was actually used on the design.

Let’s summarize things to make sure this all comes together. We have established how to characterize a fan-shaped room, its apex, its length, and angular width. We explored how to fill the shape from the apex or any point forward of that. We saw how the height of the cluster affects the width of the coverage on the floor, making the coverage shape change from front to back. Finally we put it all together to show how to make decisions on how many clusters of what coverage angle.

So that takes care of the fan-shaped room. What about the fantangle? Remember that hybrid of fan and rectangle. In the front it widens with distance, and then it stays the same width in the rear. No speaker is going to do this. Our approach is to first see which is the dominant shape. If there is more of the room in the fan, then go with the fan approach. If more of the room is the rectangle, then go with the rectangle design approach. What’s the rectangle design approach? Well, that’s another story.

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