In the real world, the inverse distance law p ~ 1/r is always an idealization because it assumes exactly equal sound pressure p as sound field propagation in all directions. If there are reflective surfaces in the sound field, then reflected sounds will add to the directed sound and you will get more sound at a field location than the inverse distance law predicts. If there are barriers between the source and the point of measurement, you may get less than the distance law predicts. Nevertheless, the inverse distance law is the logical first estimate of the
sound pressure you would get at a distant point in a reasonably open area. The reference sound pressure level SPL = 0 dB is the sound pressure of p0 = 20 µPa = 20 × 10−6 Pa
= 2 × 10−5 Pa or 2 × 10−5 N/m2.
Field sizes, as the sound pressure, will always be shown as effective values (RMS).