vol gain [type [limitergain]]
gain which is
interpreted, according to the given type, as follows: if type is
amplitude (or is omitted), then gain is an amplitude (i.e.
voltage or linear) ratio, if power, then a power (i.e. wattage or
voltage-squared) ratio, and if dB, then a power change in dB.
When type is amplitude or power, a gain of 1 leaves the volume
unchanged, less than 1 decreases it, and greater than 1
increases it; a negative gain inverts the audio signal in addition
to adjusting its volume.
When type is dB, a gain of 0 leaves the volume unchanged, less
than 0 decreases it, and greater than 0 increases it.
Beware of clipping when the increasing the volume.
The gain and the type parameters can be concatenated if desired,
e.g. vol 10dB.
An optional limitergain value can be specified and should be a
value much less than 1 (e.g. 0.05 or 0.02) and is used only on
peaks to prevent clipping. Not specifying this parameter will
cause no limiter to be used. In verbose mode, this effect will
display the percentage of the audio that needed to be limited.
See also gain for a volume-changing effect with different capabilities, and compand for a dynamic-range compression/ expansion/limiting effect.