Oct 19, 2012 1:42 PM, By Bob McCarthy
When are they needed?
As sound designers, we often have to decide whether we should go through the added expense and trouble of mounting, wiring, and buying under-balcony speakers—an extra array. There is no glory in an under-balcony speaker system design since their ultimate compliment is not knowing whether they are turned on, but there can be shame if we assure a client they would not be needed when they are. How can we determine when under-balcony delays are needed?
The decision is not as simple as you might think. For example, if an under-balcony area is 100ft. deep, we surely need some dedicated speakers, right? Not if the under-balcony area was also 100ft. high. How about ceiling height as the defining feature? Again this is too simple. If we consider the ratio of under-balcony depth and height, we are getting closer, but still not quite there. Just like a hall with balconies, we will have to study this challenge in layers.
The decisive factors are derived from three distances: the length from the main speakers to the first seats under the balcony, the last seats there, and the height of the balcony. Relationships between these parameters will give us most of what we need, with other factors such as the room acoustics and the speaker response also playing a role.
MAKING DELAY DECISIONS
We begin by laying out the basic configuration: We have a speaker system covering the room, which we will call the “main” system. This might be several different subsystems, but suffice to say, these speakers would cover the room if the balcony were not there. Second, we have (or don’t need) the under-balcony (U/B) system. Let’s reduce the decision-making logic to the simplest level and then proceed from there. Level one presents the following question: Do all under-balcony listeners have a line of sight to the main speakers? If the answer is “no,” then we have an automatic decision: you need a U/B system. This is a special (and also worst) case because the listeners do not hear the U/B system as an extension of the mains, but rather as the only sound source. Needless to say this is to be avoided if at all possible by optimizing placement of the mains to achieve an unobstructed line of sight and sound. If the answer to the question is, “Yes, I can see the mains,” then we may or may not need a U/B system.
The next level requires us to look at the ratio of under-balcony depth and height. Getting the depth is easy and is simply the distance from the first row under the overhang to the last listener. (Note: Use the last row of seats for this rather than the rear wall, unless you are expecting standing room only.) Computing the height is trickier since very often the floor and ceiling do not run in parallel. Often the floor rakes upward (with a flat ceiling) or the ceiling rakes upward (or downward) and we have a flat floor. In any case we are looking for the average height in this space. Note, however, that we are not looking for the floor-to-ceiling height, but rather the “ear-to-ceiling” height. For a seated listener, use 1 meter (3ft.) off the floor as the reference point. If the ceiling is high, this will be a small difference, but if the ceiling is low, the 1-meter subtraction will have a substantial effect in the calculation. Let’s consider what this ratio tells us. If the depth is twice the height, then our under-balcony listening area is a rectangle with a 2:1 aspect ratio. As a general tendency, the need for a U/B system rises with this ratio. Visualize an area with a 4-meter height above the listeners. If the depth were only 2 meters, we would not even consider a delay system. If the depth were 4 meters (1:1 ratio), what then? How about 8 meters (2:1 ratio)? Surely if it were 16-meters deep (4:1) we would need to drill some holes in the ceiling and run cable. At some point we are going to feel the need for multiple rows of delays.
The aspect ratio for the U/B space we will term the “shape factor,” which will be the most influential statistic for us. In my review of 40 or so design decisions, I found that if the shape factor was under 2:1, delays were rarely specified; whereas when this ratio was exceeded, they were commonly added.
The next factor is more subtle but still important. It can serve as a tiebreaker when the shape factor is in the gray zone. This is the ratio of the distances from the main speaker to the first and last listener under the balcony. In simple terms: the percentage of main system coverage not under the balcony. We have to bear in mind that the U/B-area listeners hear a combination of the mains and the U/B systems. In other words, unless blocked by the balcony structure, the house can be divided into two categories: (1) covered by mains only, and (2) covered by mains and delays. If only a small minority of the hall is shared between the systems, then it is reasonable to expect that the mains will leave it to the U/B system to cover a large proportion of the shared area. If a lot of the hall is under the balcony, then we expect the main system to cover a sizable part of the U/B area on its own before needing the delays toward the rear.
For illustration, let’s contrast two extreme scenarios with the same under-balcony depth of 10 rows. In the first case, the main speaker is 30 rows away from the balcony front, and in the other case, it is sitting three rows away. In the first case, we can expect the U/B to cover a majority of the rear seating. In the second case, since it is so close, we can expect that the main system can cover most, if not all the U/B area on its own.
To fully understand this we need to remember why we even need a U/B system at all. It is often mistakenly believed that the under-balcony area has high-frequency (HF) loss that needs to be compensated. Unless the path is blocked, the direct sound under a balcony does not travel any differently than outdoors, so there is nothing there that creates an extra HF loss. If this were the case, then we would need over-balcony speakers just as much as we need under-balcony speakers. Rather than HF loss, the challenge for the under-balcony listener is multiple strong early reflections arriving at different times. In the low end these can be highly constructive, leading to LF boost. In the midrange there will be boosts and cuts (comb filtering), and in the high end, there will be practically no constructive addition. Taken as a whole, this creates a sloped frequency response that favors the low end, hence the perception of HF loss. (Note that this sloped response can be fixed with a jackhammer taking away the balcony.)
Let’s summarize this down to two tendencies: For a given under-balcony height, the need for a U/B system increases, (1) as the length to the front of balcony rises, and (2) as the length under the balcony rises. When the mains travel a long distance, we can only tolerate a short run under the balcony. When the mains travel a short distance, we can handle longer underside depths.
Now we can add two more short subjects, the main speaker pattern control, and the room acoustics. A more directional main speaker will allow us to get deeper under the balcony before calling for help. Conversely, as room reverberation rises, the need for local sources under the balcony becomes more acute. Both of these stem from the same parameter: direct/reflected ratio. As this number falls, the need for delays rises.
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