77jovian, the main reason that SPL decreases as distance increases, in a reasonable home listening environment at least, is not related to atmospheric attenuation. It is the result of the sonic energy "spreading out" over a progressively larger cross-sectional area as distance increases, resulting in a given cross-sectional area receiving a progressively smaller fraction of the total energy.
In contrast to a typical box speaker, though, where a given frequency is likely to be emitted by one or two or three relatively small drivers, a large panel speaker will tend to "beam" most frequencies, resulting in a lesser degree of "spreading" as distance increases. Or putting it another way, as the listening distance increases the listener will tend to be "on axis" with a greater amount of the panel's area. And also more equidistant to the various sections of the panel's area, resulting in less difference in the arrival times of the sound emitted by the various sections, resulting in more coherent summing of the different arrivals.
Regards,
-- Al
In contrast to a typical box speaker, though, where a given frequency is likely to be emitted by one or two or three relatively small drivers, a large panel speaker will tend to "beam" most frequencies, resulting in a lesser degree of "spreading" as distance increases. Or putting it another way, as the listening distance increases the listener will tend to be "on axis" with a greater amount of the panel's area. And also more equidistant to the various sections of the panel's area, resulting in less difference in the arrival times of the sound emitted by the various sections, resulting in more coherent summing of the different arrivals.
Regards,
-- Al