Why the "20" in calculating db ratio?
According to this source, the sound power received by an aperture is proportional to the pressure squared, i.e.:
$$
P = \frac{A p^2}{\rho c} \cos \theta,
$$
where:


$P$ is the received power
$A$ is the surface area of the receiving aperture 
$p$ is the pressure
$\rho$ is the density of medium
$\theta$ is the angle between the propagation direction and the surface normal