Transmission loss

In some references transmission loss (hereafter TL) is introduced after dividing the acoustic pressure by the factor $P_0 = 1/(4\pi)$, while some models perform the actual calculations with the factor $P_0 = 1$. Additionally, TL is presented with either positive or negative values. The definition adopted by Trace and Traceo is that

$$\mathrm{TL}(r,z) = -20\log\left\vert P \right\vert .$$ (17)