What is the formula for calculating what the SPL would be at 3 meters (typical listening position) rather than 1 meter?
For a single speaker, and assuming it is a conventional box-type (i.e., it is non-planar and non-line source), and neglecting room reflections, the attenuation of SPL resulting from the increase in distance is:
20 x log(distance from listener/1 meter)
where "log" is the base-10 logarithm.
So in this case, and under those assumptions:
20 x log(3 meters/1 meter) = 9.5 db of attenuation relative to the SPL produced at 1 meter (rounding off slightly).
For Erik’s example that you quoted:
109 db - 9.5 = 99.5 db SPL at 3 meters.
The presence of a second speaker and the effects of room reflections will typically add several db to that.
The 20 x log formula is built into the online calculators which have been linked to in some of the posts above.
Regards,
-- Al