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Under-balcony Delays

Oct 19, 2012 1:42 PM, By Bob McCarthy

When are they needed?


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Figure 2: Single balcony hall with short U/B and long O/B areas. (a) The U/B area has 3.5 shape factor, range 71 percent, and composite value of 2.5. It must have delay. O/B area has 3.4 shape factor, range of 47 percent, and composite value of 1.6. This is on the edge, but the hall is reverberant so we went with delays. Click here to see a larger image. (b) The U/B and O/B delays reduce the range of the mains and increase the average ceiling span on both levels. This reduces the shape factors and reduces both composite numbers to less than one. Click here to see a larger image.

MORE FIELD EXAMPLES

The next hall is another (see Figure 2) single balcony. In this case, the over-balcony area is much deeper than the under-balcony. Both levels have a shape factor of around 3.5 and therefore we see under- and over-balcony systems. Because the shape factor is so high we will need to cover as large an area as possible. The limiting factor is how far the delays can go before getting too loud in the closer rows. The over-balcony speakers are power scaled for the much longer distance they need to cover. After the delays are added, the shape factor for the mains is reduced to around 1.3, which means no second set of delays is needed.

Figure 3: Single balcony hall. (a) The U/B area has 3.5 shape factor, range 60 percent, and composite value of 2.1. Delay is needed but only for a few rows. O/B area has 2.4 shape factor, range of 47 percent, and composite value of 1.3. O/B delays were not needed, but the client wanted them anyway, so they were designed to cover just the last few rows. Click here to see a larger image. (b) The U/B and O/B delays reduce the range of the mains and increase the average ceiling span on the upper level. This reduces the shape factors and reduces both composite numbers to 1.3 and 1.0 respectively. PS: The budget got tight, the O/B delays were cut, and they were indeed not needed. Click here to see a larger image.

A third hall has an easy call for the U/B system (shape factor 3.5), but the over-balcony area is just over the edge at 2.4. The result is a small-scale delay system to cover the last rows.

FINALLY

These are not hard and fast numbers, but the trend lines can help to give the designer confidence in their decision on whether or not to go through the trouble and expense to add a U/B system. See more pictures of example systems and the complete chart that shows the ratios for the many designs I studied on my blog at www.bobmccarthy.wordpress.com.



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