Audio rendered to the dedicated low frequency effect (AL_EFFECT_DEDICATED_LOW_FREQUENCY_EFFECT) is routed to a subwoofer if one is present. Otherwise, it is discarded.
Audio rendered to the dedicated dialogue effect (AL_EFFECT_DEDICATED_DIALOGUE) is routed to a front centre speaker if one is present. Otherwise, it is rendered to the front centre using the normal spatialisation logic.
Both effects support a gain control parameter AL_DEDICATED_GAIN, which defaults to 1.