Very good, Gena. Keep in mind that in addition to the direct sound, which falls off at a rate of 6dB per doubling of distance, assuming omnidirectional dispersion as a sphere(the area of the surface of the sphere being 4pi[r(squared)],doubling the radius quadruples the area, leading to a SPL 6dB lower), the reverberant sound must also be taken into account. See
this analysis , especially fig.10.4.1, for an application of combined direct and reverberent sound. Linkwitz also has an excellent discussion of room acoustics on
his site . The information found by scrolling down to "Reverberation distance" and "Amplifier Power to Obtain Reference Level" is particularly in point. At the "critical distance"(aka "reverberation distance"), which is about 3-4' in typical home listening rooms, the reverberant sound is equal to the direct sound and the combined level is therefore 3dB higher. At longer distances, as the direct sound falls off, the reverberant sound remains relatively constant at the critical distance level in typical home listening rooms.
This may seem somewhat technical, but the practical effect is that far less power is used than is sometimes assumed if the combined sound fields aren't taken into account.
