I saw another question on here that asked how to set up a ducker. Can someone provide a detailed explanation of what a ducker is? I couldn't glean a specific meaning from the context of the linked-to question. In the context of a song, would it be reducing the volume of everything else when the vocals come in, to make the vocals stand out more?
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It is an effect that is created by using the sidechain input on a compressor.
Normally, a compressor will "turn down the volume" of its output based on increased volume at its input. But you can change that by sending a different audio signal into the sidechain input of the compressor; the compressor will then react to the sidechain signal instead, changing the volume of the input signal based on the volume of the sidechain signal.
I've heard this most commonly used when the sidechain input is connected to the kick drum of a drum set, and the main audio input of the compressor is connected to the cymbals. This causes a "swooping" effect on the cymbals. Visually, you can think of the cymbals "ducking" under the kick drum sound. This effect can be heard clearly on the song "Come Together" by the Beatles (near the end of the track), and on one of the tracks of Pink Floyd's "The Wall" (although I can't remember which one ATM).
You can also hear this effect a lot on trance albums.
To add to Robert's answer:
A dynamics processor/process that lowers (or "ducks") the level of one audio signal based upon the level of a second audio signal. A typical application is paging over background music: A ducker senses the presence of audio from a paging microphone and triggers a reduction in the output level of the music signal for the duration of the page signal. It restores the original level once the page message is over. Most dynamics processors (usually compressors are used) that give the user access to the detector circuit can be used for ducking. It is simply a matter of routing a copy/split of the second audio signal (the page in the example above) to the detector input such that it will trigger the gain cell to lower the level of the main signal (the music).