The cone-type class of loudspeaker produces a characteristic radiation pattern that is best
described as “point-source,” with output level diminishing rapidly as the distance increases
between the listener and speaker. The classic expression of this phenomenon is that sound
pressure level is inversely proportional to the square of the distance. The practical
acoustical effect is that there is a 6 decibel (4-fold) drop in level with every doubling of
distance.
Line-source radiators like the BG's “Radia” series ribbon devices do not suffer the same physical and psychoacoustic limitations, as they are characterized by sound pressure levels that are linearly inversely proportional to the distance, resulting in a 3 decibel reduction in level with every doubling of distance rather than the 6 decibel drop with point-source radiators. more...
This effect is obtained for distances up to roughly two to three times the height of the line-source driver itself and is applicable to the frequency range where the length of the driver is long relative to the wavelength of interest. Assuming that the listener is 1 foot from the left speaker, and 8 feet from the right, the right will be attenuated 9 dB with line-source radiators, and 18 dB with point-source radiators.
The 9 dB difference translates nearly exactly to the subjective sense of one-half, i.e., the right speaker will be subjectively judged to be one-half as loud with the point-source speaker as with the line-source.
At the user level, this means that as the listener moves closer to the left loudspeaker and further from the right, the left and right channel levels will remain relatively constant, with the left growing louder less rapidly, and the right growing softer less rapidly, than would be the case with point-source speakers.