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12 Nov 2011

Line Arrays vs. Point Source Featured

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Understanding the differences between Line Arrays and Point Source speaker configurations and which one to choose for your room.

 

There are several distinct differences between line arrays and point source loudspeakers. Sometimes, point source loudspeakers are comprised of simply full-range drivers. Many times, they include multiple drivers including combinations of horns and direct radiating cones that reinforce specific frequency bands. In either case, point source loudspeaker cabinets are generally built so that all the drivers function as one source. An example of point source type loudspeakers are EAW KF series boxes (KF650,750,850). Much like the behavior of a true acoustical point source, these systems usually have a fairly short near-field coverage; as the distance from the source increases, their SPL decreases according to the inverse square law. A line array, on the other hand, is typically very large in one dimension (usually vertical), compared to the wavelength of frequencies it radiates. This gives it superior directivity control for frequencies with wavelengths greater than twice the length of the line. Put another way, the length of a line array should be equal to or greater than one-half the wavelength of the lowest frequency over which directivity control is desired. At frequencies much higher than determined by L = A/2 (A = wavelength), a line array can have a very small coverage angle. Typically line arrays are oriented vertically. This enables a small vertical coverage angle, or opening angle as it is sometimes called, to be used to great advantage in reducing reflections and keeping sound off ceilings and other surfaces. This can be very beneficial in highly reverberant spaces. In the near-field of a line array, the SPL falls off at -3 dB per doubling of distance, instead of -6 dB as dictated by inverse square law. Line array systems tend to have a much greater near-field coverage distance than point source systems. This enables a line array to potentially offer higher SPL at a given distance than a point source system. However, there is a limit to the distance at which the line array can maintain this -3 dB SPL decrease per doubling of distance. It can only do this in the near-field of the line array. Beyond the near-field, the SPL from a line array will decrease at -6 dB per doubling of distance. The near-field can extend very far from a line array, and is dependant on the length of the line array as well as the given frequency. This means that for a fixed-length line array the near-field distance will change as a function of frequency. Thus the frequency response of a line array may change depending on the distance away from it. Another important point to consider is the difference in the size of the origin of radiation for these two types of loudspeakers. As the name implies, a point source has its origin at a single point. (While this is overly simplistic, it will help to illustrate this difference.) The sound from a line array, on the other hand, does not originate from a single point, but from a line. While a point is infinitesimally small and has no physical height, a line does. This difference can play a large role in understanding the radiation and coverage from a line array. As an example, consider a point source with a 5 degree vertical coverage angle. At a distance of 30 feet (9.15 m), this point source will cover a vertical height of approximately 2.6 feet (0.80 m). By comparison, a 3-foot (0.91 m) line array with the same 5 degree vertical angle will cover a vertical height of approximately 5.6 feet (1.71 m). This sets the origin for its radiation and will have a bearing on the overall coverage. (This is an extreme example but it does help to demonstrate this fundamental difference.)

When to use Line Arrays

A line array’s high degree of directivity control makes it particularly well suited for use in highly reverberant spaces, where it is imperative that sound be directed to the audience areas only and not excite other highly reflective surfaces. This will help to maximize speech intelligibility in these difficult spaces. Line arrays also work well in rooms with relatively low ceilings. When the ceiling is low in relation to the depth of the room, it may not be possible to position a point source system located at the front of the room sufficiently high enough to provide consistent coverage from the front to the rear-most point in the room. Additional point source loudspeakers would be required farther back in the room on a separate “delay” feed. Using a line array system at the front of the room can make it possible to achieve a consistent SPL distribution from the front to the back of the room, without the need for delay loudspeakers. The directivity control of a line array is generally only within the plane of the line. That is, if the drivers in a line array are arranged vertically (which is most often the case), the directivity control will be in the vertical plane. In the horizontal plane, directivity will be fairly broad. For this reason, line arrays cannot be rotated on there side and still maintain high vertical directivity.

Last modified on Saturday, 12 November 2011 11:55
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