log base in log-power spectral features
It's not particularly important since the different bases just result in a different overall scale factor, but the basic shapes are the same.

This being said, any actual numbers need to be defined with a specific bases otherwise it's ambiguous. The most common is the $dB$ which (for power) is defined as 
$$L = 10 \cdot log_{10}(P/P_0)$$

i.e. it uses base 10.