The BASS_FX_VOLUME_PARAM type exposes the following members.
The current volume level... -1 = leave existing current level when setting parameters. The default value is 1.
The new volume level... 0 = silent, 1.0 = normal, above 1.0 = amplification. The default value is 1.
The time to take to transition from the current level to the new level, in seconds. The default value is 0.
The curve to use in the transition... 0 = linear, 1 = logarithmic. The default value is 0.