Arraying Loudspeaker Systems
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Some people recognize the need for two loudspeakers to provide the proper coverage but fail to adjust the level of the amplifier channels that are driving each loudspeaker system. If the Near Field loudspeaker enclosure is not calibrated to be 6 dB lower than the Far Field enclosure, the Near Field enclosure will be the one to feedback first and limit the gain of the whole system.
I will remind you once again the concept here is to try to get as much of the audience within the pattern of the loudspeaker array, while trying to minimize wasted energy that is not reaching the congregations ears directly.
What if the room is even longer still? There is a point where trying to cover a large number of seats from a single cluster of source becomes much more difficult or even impossible. How far can one of these single enclosures throw with a 90 degree by 45 degree high frequency constant directivity horn? The answer depends on several variables, such as room acoustics, program material, and the actual distances involved. Most of our two-way loudspeaker enclosures designed for FOH (Front of House) sound reinforcement can do a good job up to 50 ft. or more. Under certain more ideal acoustical
Conditions, they may still perform well at 60 to 70 ft., but I can't tell you that you would be satisfied with their performance at distances beyond 80 - 100 ft., let alone at 150 ft. or even greater distances.
In times past and present, some sound engineers have employed what are called long-throw horns to increase the coverage with distance. These so-called long throw devices are far from perfect. Often, to provide the necessary sound pressure level at the farthest distances, they are operated at levels that are so high that they interfere with the array's ability to provide even coverage in the first place.
No matter how you slice it, sound drops in level -30 dB at 105 ft. (32 meters). (20 log D1 / D2 = 20 log 32 / 1 = 20 x 1.505 = -30.1 dB) or (20 log 105 / 3.28 = 20 log 32 = -30.1 dB). If the main FOH system has to be so loud in level as to cause pain to the listeners close to the front, what then are they accomplishing? There becomes a point where the best course of action is to use properly delayed loudspeaker systems to cover the rear of the audience. Until recent years, the digital delay lines were of too poor a quality to do the job effectively. This is no longer the case, and delayed loudspeaker systems are becoming more and more the viable solution to the problem of proper coverage with great distances.
In order to understand how the properly delayed remote loudspeaker works, you must know about the propagation of sound itself. Sound travels at a speed of approximately 1130 feet per second. In milliseconds that is 1.13 feet per millisecond (a millisecond is 0.001 seconds). In milliseconds per foot this becomes 0.885 or roughly 0.9 milliseconds per foot. So if you want to place a delayed remote loudspeaker system in a room and calibrate the delay to the arrival time of the FOH system, you would measure the distance between the main and delayed loudspeaker system locations, and multiply this distance measurement in feet by 0.9. As an example, let's say that the delayed loudspeaker system is 80 feet out from the main FOH loudspeaker array; then 0.9 x 80 = 72 milliseconds. Seventy-two milliseconds is the time it takes sound from the main FOH system to reach the location of the delayed loudspeaker, which means that without any delay, the secondary loudspeaker is 72 milliseconds ahead (in time) of the main FOH system.
It is not enough to set the delay for the signal sent to the power amplifier driving the remote loudspeaker at 72 milliseconds, as this will do nothing to maintain the image of the sound as having originated from the front of the sanctuary. There is a concept in audio that says that we pay attention most to that sound that arrives within the first 20 milliseconds. This is sometimes called the precedence effect or the Haas effect. Mr. Haas did testing of human auditory perception, and is more or less the father of Physco-acoustics.
It has been proven that if you add approximately 20 milliseconds to the actual calculated propagation time (when setting the delay for the remote loudspeaker), proper imaging is maintained, and the results are such that no one will even be able to tell that the delayed loudspeaker system is even operating.
It is not a hard and fast rule that the delay setting is set at exactly 20 milliseconds greater, I have seen it set any where from 16 to 26 milliseconds more delay than the calculated direct propagation time. With live music I have observed that certain sounds, particularly the cymbals, can appear as if there is a phase shifter on them if the delay time is exactly 20 milliseconds beyond the calculated propagation time.
Now let us take a look at applications where the room is wider than in these past examples.
The typical approach in wider rooms is to provide for left and right near field coverage along with a Far Field System. This can be best done with a three-loudspeaker array. The outside loudspeakers are turned upside down to cover the near field left and right seating areas, while the center loudspeaker is mounted right side up. The center loudspeaker is still angled downward somewhat while the outside loudspeakers are angled down about forty degrees farther.
Below is a scanned photograph of a three-loudspeaker array as we just described. This particular array is what we have in our auditorium at the Peavey Dealer Training Center in Meridian, Mississippi. There is also a complete article that covers how this approach can be done with a single power amplifier operated in bridge mode.
There are also some church sanctuaries where the actual longest distances are to the left and right rear corners of the church. These rooms are usually more octagon or even pie shaped. In the case of an application such as this, another type of a three-loudspeaker system array can be employed. This time, however, the two outside speakers are to be mounted right side up while the center loudspeaker is mounted upside-down. The outside loudspeakers now provide for the Far Field coverage to the left and right rear corners while the center loudspeaker becomes our Near Field center fill enclosure. However, this approach cannot be driven from a single amplifier operated in bridge mode. In this application the center (Near Field) loudspeaker needs to be operated -6 dB below the level of the two Far Field enclosures.
What about balconies and alcoves or under balcony spaces. All of the above are best addressed as separate acoustic spaces, which they actually are. Any time the Free Field is truncated (or reduced to a smaller space), the acoustics involved are totally different. The Free Field is that portion of the direct field not influenced by the boundaries. Anytime you introduce a new space with smaller dimensions; it is not a good idea to try to provide coverage from the main FOH array. The best approach for these special requirements is to use smaller loudspeaker enclosures with delay, as outlines above for delayed remote loudspeaker systems.
This paper is only intended to be a general guideline of the principles discussed, and is by no means intended as a cookbook solution. If you are to have utmost success with a sound system installation, you need to rely upon someone with solid experience in the design, calibration, and operation of such systems. To do otherwise is to risk the possibility of an expensive short-term experiment in audio. Prayer can be a powerful tool, but it can't fix a poor sound system design.
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