Arraying Loudspeaker Systems
By Marty McCann
This paper is going to address the issue of combining loudspeakers to increase the area of coverage from that of an individual loudspeaker. The concept is generally referred to as arraying the enclosures. Proper arraying insures even coverage with a minimum of mutual interference, and thus allows the sound system to perform as near to a single source as possible.
When dealing with multiples of loudspeaker enclosures, the main concern is to maintain the integrity of the high frequency devices. The modern highest quality sound reinforcement loudspeaker enclosures exhibit a high frequency horn that offers uniform frequency response with dispersion; they are said to be constant in their directivity or CD for short. There are still a lot of older horns out there that do not have constant directivity, or a well controlled pattern of coverage. These older horns are referred to as exponential radial horns. By exponential we mean that the rate of flare or taper of the horn increases with the square of the distance away from the throat or entry of the horn. Exponential radial horns do not direct the high frequency information very smoothly, with these horns the high frequency coverage is that of a very narrow beam (usually less than 20 degrees) directly on-axis to the horn. The taper or flares rates of the exponential radial horn are too rapid to allow the air molecules carrying the high frequency information to the cling to the side walls of the horn so they can be directed over a wider area of coverage. The reason constant directivity horns do a much better job is that their rates of flare or taper vary as the sound enters the throat area and moves throughout the horn's boundaries, they are said to be multi-taper and multi-flare. It is this variation of conditions within the sidewalls of the horn that allows for the much-improved high fidelity of the loudspeaker system. This paper assumes that the reader is going to utilize constant directivity high frequency devices in the design of the loudspeaker array.
It needs to be mentioned at this time, that all constant directivity horns require a special type of equalization, commonly called CD EQ, to maintain a flat frequency response. When we succeeded in directing the high frequency information into a wider pattern, we subsequently reduced the level of the highs as well. All constant directivity horns have a high frequency roll off rate of -6 dB or more. A compensatory equalization is employed to maintain a flat frequency response. This equalization is part of the systems crossover design. So this paper assumes that the reader will employ loudspeaker systems with constant directivity horns, with the appropriate equalization as the building blocks of the arrays we are going to discuss.
The biggest mistake or improper application involving systems with constant directivity horns is that when some individuals use them in multiples, they don't take into account the individual coverage patterns and therefore allow the high frequency devices (horns) to overlap in their areas of coverage. Some small or very modest amount of overlap is sometimes necessary and acceptable. However, severe overlapping of coverage (more than 10 degrees) results in interference patterns commonly called lobbing or fingering of the pattern. Remember sound is propagated through the medium of air by the vibrations of the individual air molecules bumping into one another in a pattern that exhibits a wave through the atmosphere. When air molecules bump into one another there is a reaction. For every action is there is an equal and opposite reaction (Sir Isaac Newton). If two billiard balls are traveling at an angle toward the center of a pool table, and are allowed to collide, will they not bounce off of each other at the same angle of their collision? What would make you think that solid air molecules would react any differently? The angle of incidence is equal to the angle of coincidence, or the angle of arrival is equal to the angle of departure.
It is the high frequency information that contains those components of speech that allows us to distinguish the consonant and sibilant sounds that make speech intelligible. Those portions of speech created with the lips and tongue are most important if the system is to have clarity, transparency, and general intelligibility. The word sibilance almost defines itself by the mere pronunciation of the word. It is the sibilant components of speech that allow us to distinguish words from another, words like float, tote, boat, and moat, or dog, log, and frog as examples. If the high frequency information is to be most transparent, i.e., intelligible, multiple loudspeakers must be placed with forethought as to the manner in which the horns will combine to retain the concept of constant directivity of the high frequencies emanating from the array of individual loudspeaker components.
Some definitions first. The Direct Field is that sound field emanating directly from a source and not significantly influenced by any of the boundaries within the room or acoustic space. Since all rooms have boundaries in the form of the ceiling, floor, and walls, eventually some sound will arrive at those surfaces. When sound strikes a solid surface, some small amount sound energy is absorbed due to the friction or heat created in the encounter with the boundary, but the majority of the energy is reflected off of the boundary. This reflected energy is called reverberation or the reverberant sound field, meaning that it is independent and no longer part of the direct field. The first concept I want to express then is that; any direct field that does not arrive at the ear of the listener is wasted energy. In other words, it is best to minimize that sound energy that arrives at the room's surface boundaries. Acoustic energy that does not reach the listener is wasted energy. So the first concept is, "Point the loudspeaker at the audience or congregation, and they will hear it better" (what a concept!). I am continuously amazed by the number of systems installed in churches and auditoriums where the directional components (high frequency horns) are not even directed to the listener's ear at all.
The next definition is for Critical Distance. Critical Distance is that point within a room or acoustic space where the level of the reverberant energy field and the level of the direct sound field are equal. Once you step beyond the point of critical distance, the reverberant level is greater than the direct sound level. The farther you move beyond the critical distance point, the reverberant field tends to mask or cover the direct field. A fairly simple and straightforward test can be conducted in any church to ascertain the approximate point of critical distance in any church sanctuary. You see the church has a critical distance point within its acoustical space, with and without the sound system turned on. It is a good idea to establish the point of critical distance without the sound system first, then conduct the same test employing the sound reinforcement system. The properly installed sound system should move the natural (unassisted) point of critical distance dramatically further out into the listening area. However don't be too surprised if after conducting both the assisted and unassisted tests, if the assisted or reinforced test exhibits an even shorter critical distance measurement. If this is the case, the sound system is of an inappropriate design for that room.
Finding the critical distance point in the church sanctuary can be done with one person acting as the speaking source and two to four subjects acting as the listeners. With the sound system off, have the speaker read a passage from the Bible while standing at the pulpit. (Note: It is best to use a speaker with a normal voice, like the actor, Richard Harris, who has a trained voice projected from the diaphragm and would be more easily understood at a distance than a normal talker.) Have the listeners stand a couple of feet in front of the pulpit, have them listen to the person speaking without looking directly at them (keep their eyes directed), and have them slowly back up the center aisle of the church. Instruct the listeners to raise their hands when they perceive that sound is no longer coming directly from the direction of the person speaking. As you slowly back away there will be a point at which the sound is still understood but it no longer appears to come directly from the source, it just appears to be there. If the listeners are of normal binaural hearing, i.e., both ears work equally well, they should come to within 12 to 18 inches of agreement as to the point in the room where the sound no longer appears to come from the pulpit. After this point is determined, turn on the sound system and repeat the test while speaking into the pulpit microphone. If the system is designed well, there should be a much greater distance from the pulpit to the critical distance point with speech reinforcement. Experiment with this test as it can show you a lot about the acoustics of your church sanctuary and the degree to which your existing sound system is effective.