Originally Posted by bigguyca
Item 3 in the calculator says "If you know the distance in meters, divide by 3 to get an appoximation in feet." Given how wrong that is, it's hard to recommend the calculator. It's a grade school mistake.
Misspelling approximation as appoximation also unacceptable. This site isn't a hurried post on a thread. Spelling checkers are broadly available.
Most all these calculation sites drop the SPL by 6 dB with each doubling of distance. In a room a 3 to 4 dB drop is more likely. A few dB's difference translates into a lot of extra power required, and potentially money spent unnecessarily.
I'm just cruising by and noticed this. You are absolutely right about the approximately 3 dB/double-distance decline of steady-state room sound level - this is what is measured in the calibrations. -6 dB/dd is only for the direct sound. I haven't examined the calculators listed, but any of them that use -6 dB/dd are wrong.
The other factor that is commonly ignored is that 105 dB is not a steady-state goal, it is an instantaneous peak sound level goal - equivalent to a soundtrack hitting 0 dBfs, above which, in theory at least, nothing can exist. For this circumstance, using a pure tone with a 3 dB peak to average level difference, the equivalent steady-state objective is 102 dB SPL. So, we pick up a factor of 2 in amplifier power requirement for nothing. Right?
 Ooops, my mistake. Thanks to Roger Dressler for sorting this out. 0dBfs is a steady-state pure tone level (theoretically 105 dB in the room). This means that the peak level is 3dB higher, at +3dBfs (theoretically 108 dBSPL in the room). As this is digital clipping level, nothing exists above +3 dBfs (108 dB SPL). This is for a single channel, so multiples can of course generate much higher levels.
Something else that is relevant. I am working on the website to accompany my new book, and did a survey of about 60 consumer loudspeaker sensitivity ratings: published vs. measured. Not surprisingly the published numbers are higher, but the amount was disturbing: an average of 2.7 dB higher than those measured properly in an anechoic chamber (measured in the far field, 2 m, and calculated at 1 m). That is almost a factor of two in power. In fact there was one that was 7 dB too high - a factor of 5 in amplifier power - this is marketing out of control. So, question everything you read.
Loudspeaker impedance matters, especially minimum impedance, and this too is widely ignored. For those honorable manufacturers that state minimum impedances, the numbers were quite close to measurements. However the rest were all over the map. Average measured minimum: 4.3 ohms. Average specified impedance: 5.9 ohms. More power (current) is needed than might have been anticipated. One scrupulously honest manufacturer published curves.
And we haven't even touched amplifier power ratings, especially those in receivers.
We live in a world full of "alternative facts"