The logarithm Energy measure in Speech
You compute the energy (before the log operation) as

$$\max\left\{\sum_i s^2(i),\epsilon\right\}\tag{1}$$

with some small constant $\epsilon$ (they suggest $\epsilon=2e-22$ which gives $\ln\epsilon\approx -50$) to prevent the energy to become too small. The reason for this is that the logarithm of zero is $-\infty$ and gives you numerical trouble. So you always want to take the logarithm of a strictly positive number, never of zero. As far as I understand, the choice of $\epsilon=2e-22$ corresponding to a log value of $-50$ is rather arbitrary.