A decibel value.
Note: 1.0 linear gain is 0 dB in decibels. 0.5 linear gain is -6 dB in decibels. 0.0 linear gain is -inf dB in decibels.
Precision: This uses fast_exp which under normal conditions has a peak error under -135dB over the useful range.
Converts from linear gain (voltage) to decibels (dB).