Line Arrays for dummies
February 2nd, 2007 by Jeff
In recent years the audio design industry has fallen head over heels for a speaker design system known as line arrays. What are line arrays, and would they be the right solution for your audio needs? There are two basic approaches to audio system design. The first is the conventional point-source approach, also known as a delay speaker system. This approach usually utilizes a central cluster of speakers above the center point of the stage, sometimes left and right clusters are used in delay to complement the center cluster. This approach sprays the sound over a wide distribution area, in a diffuse nature. A line array system, on the other hand is a vertical array of speakers in a thin line. This approach focuses the sound like a laser beam in a very small cone, over a very long distance. Essentially the difference is the same as the “spray” and “stream” settings on a bottle of windex. Understanding it in this way it becomes clear that different configurations, or a hybrid of configurations are appropriate for different types of rooms. Since a point-source system sprays the sound out, the sound will then bounce and cause reverberation throughout the room which makes the acoustic design critical for this system. Line arrays are good for long rooms that require a lot of throw, and have difficult reverberation issues. They tend not to work well for small rooms with low ceilings, or obstructions. Line array Technology is actually quite old. It was used as far back as the 1930s primarily as a way to improve speech intelligibility. It really hasn’t found wide acceptance until the last decade for use as a single source of sound for both voice and music. The problem in the industry right now is the attitude that line arrays are the solution to all our problems. This is understandable since a line array system will provide a highly consistent and predictable coverage field with virtually no risk of feedback. Therefore it is easy to design, and predict its performance. The design of the acoustical performance of the room is not as critical since there is less sound bouncing off the walls, floor, and ceiling, and coordination with an architect and acoustician is not as critical. But is it the right solution, or just the easiest solution for an under qualified sound engineer? As always the best solution is a team of well qualified engineers and architect to collaboratively design a system to suit your individual needs. If you have doubts about the ability of your sound engineer simply tell him you are considering “…hanging a logarithmicspaced, articulated spiral array in a horizontally asymmetric configuration employing frequency tapering and divergence shading, which will include isophasic high-frequency and mid-frequency apertures, hyper-cardioid low-frequency transducer sections, is controlled by finite-impulse response filtering digital signal processing, and works well with a psychoacoustic infector.” If he knows what you’re talking about, he is the right person for the job.
[…] Every building acoustics issue can be broken down into the three basic concepts of source, path, and receiver. The source of sound is usually quite evident. Whether it be a desired sound, such as a speaker, or an undesired sound, such as a mechanical unit or a flushing toilet, the source is usually pretty evident. The nature of every sound source, however, is not. Every sound begins as a vibration, and just as a stone thrown into a lake produces circles of rings that propagate away from the source, that vibration will compress and expand the adjacent air molecules which causes a chain reaction of cyclical disturbance in the air molecules traveling away from the source. Subsequently, all acoustical design work is based on trying to either capture and trap these vibrating molecules (Absorption and Attenuation) or bounce them in a new direction (Reflection and Reverberation).This is probably best understood in the illustration of a simple tuning fork. Once the fork is struck it begins a vibration, which disturbs the adjacent air molecules. These originally disturbed molecules do not continue to travel away from the fork, they simply vibrate back and forth within a limited zone and transfer their energy on to adjacent molecules. This leads to the two ways of measuring the sound source which would be frequency and magnitude. Frequency is that limited zone of disturbance (measured in Hertz) that we just talked about in the air molecules. This back and forth wave is the frequency of the wave. A low frequency would be a deep bass or booming sound. A mid frequency would be speech. A high frequency would be a high pitch or hissing sound. Frequency is extremely important to understand in the control of sound, because the way to control sound depends largely on its frequency. The second measurement is magnitude. This is how loud the sound is, or in more sophisticated terms this would be how much energy is contained in the wave. When a stone is thrown into the lake how far will the ripples go? The unit of measurement for this is the familiar decibel (db) scale that has a human range from 0-140; the threshold of hearing (0) to threshold of pain (140). One item of interest at this point is that there is a subjective element in magnitude. The human ear will tend to judge mid and high level frequencies as louder than low frequencies, even if they are at the same decibel level. This tends to make the high and mid level frequencies more critical to attenuate then the low frequencies. The human ear tends to relate to this scale in that a 3 db change is barely noticeable, (even though it is a doubling of the sound energy) a 5 db change is clearly noticeable, and a 10 db change has the subjective sensation of doubling or cutting in half the loudness of the sound. Therefore corrective measures that are only going to improve the conditions by 5 db or less are not worth doing, in most cases. The process of amplification, which is a key design consideration in any worship facility, is simply copying and magnifying the source. The two most common design approaches for source in Worship auditoriums are speaker clusters and Line Arrays. See my blog on the issue of Line Arrays for a more detailed discussion on this topic. […]