Perhaps I'm missing something but can you explain this in greater detail?

Once you exceed the critical distance (distance at which SPL of the direct sound = SPL of reflections), moving further away from the speakers has little effect on the total SPL.

Imagine your critical distance is 1m. Let’s say you measure 90dB here, where (by definition) the contribution from the direct sound will be 50% and the contribution from reflections also 50%. These two are mostly uncorrelated, so let’s oversimplify a bit and say that direct sound is contributing 87dB and reflected sound 87dB (SPL[direct] + SPL[reflected] = 10log10(10^(SPL[direct]/10)+10^(SPL[reflected])/10))).

The calculator does not account for reflections, so even at the critical distance we already have a gap of 3dB, that is, the calculator predicts 87dB, whereas the (closer to) reality is that we will get 87dB from the direct sound + 87dB from reflections =

**90dB**.

Now imagine moving back to 2m. The contribution from the direct sound will drop to 81dB (slightly oversimplifying by assuming the inverse square law perfectly applies, which is true only if the speakers are omnidirectional), while the contribution from reflected sound remains at 87dB (slightly oversimplifying by assuming a reverberant field - which is true only on average).

81dB + 87dB =

**88dB** (vs 81dB using the simple inverse square law used by the calculator).

Now do the same calculation at 4m, and:

75dB + 87dB =

**87.3dB** (vs 75dB using the simple inverse square law).

And so on..

Indeed, no matter how far one listens/measures from the sound source, the total SPL will (on average) never drop more than 3dB

*below *the level it is at the critical distance, which is already going to be around 3dB

*above* what the calculator predicts (give or take, depending on whether the critical distance is more or less than 1m).