Line Sources: Cylindrical Spreading
Figure 18.2 shows another extreme - the "infinite" line source - that, if it could be realized, would radiate a perfectly cylindrical sound wave, the area of which expands linearly with the radius. As a result, the sound level falls at the lower rate of -3 dB per double-distance. Practical line sources have finite lengths, so the critical issue becomes one of keeping listeners within the near field of the line, where the desirable -3 dB/dd (dd = double distance) relationship holds and out of the far field where even line sources revert to -6 dB/dd.
FIGURE 18.2 An illustration of a theoretical infinite line source and of a practical approximation.
Obviously the distance at which the near- far-field transition occurs is a function of frequency and the length of the line. Figure 18.2 shows a stereo pair of full-height lines, taking advantage of the ceiling and floor reflected images to make them appear to be even longer. A portion of one line has been expanded to show that it is a two-way system using conventional cone and/or dome loudspeaker drivers, densely packed (ideally spaced by less than about 1/2 wavelength of the highest reproduced frequency) to simulate a continuous sound source.
It is possible to use less than a full-height floor-to-ceiling array if one understands the variables and how they can be traded off. Lipshitz and Vanderkooy (1986) provide a thorough theoretical background to the behavior of "finite length" (not full height), truncated, line sources and they point out a number of problems, ultimately concluding that "there is little to recommend the use of line sources as acoustic radiators." They did grant that full-height lines had potential if the -3 dB/octave tilt in the frequency response is corrected.
There are advantages to collections of drivers: They share the workload and therefore can play loud without distress. However, most of the products casually referred to as "line sources" or "truncated line sources" in the industry are simply vertical arrangements of drivers that are too short to be useful even as truncated lines and with the drivers too far apart to be any kind of line. These loudspeaker systems obey the rules of collected point sources, with the disadvantage that, due to their size, the far field is a long distance away.