A voltage value, linear gain.
Note: 0 dB is 1.0 in linear gain. -6 dB is 0.5 in linear gain. -inf dB is 0.0 in linear gain.
Precision: This uses fast_exp which under normal conditions has a peak error under -135dB over the useful range. However, keep in mind that different exp function sound differently when modulated.
Converts from decibels (dB) to linear gain (aka. voltage).