the author did - from the article cited:
... the intensity from the sound would be in watts per square meter.
80dB would be about 1x10-4
... so he seems to have used an area of about 10m2
... which seems a little large. Perhaps the author has a big mouth?
The whole thing is suspect isn't it - I'm sure I can make myself heard in the front rows of a rock concert - well, by yelling in someone's ear.
If I yell at a cup of coffee the same intensity that is more like 0.1Wm-2
. Figure a mouth area of about a square decimeter
(0.1m x 0.1m = 0.01m-2
) and that's 0.001W directed at the cup.
So the final figure is about right.