Volume, panning, EQ and other effects on an track can be changed mid way through a composition to add variety to the mix. Alternatively these effects can be changed over time to give further variation. This process is called automation.
An automation in volume is effectively just a fade, but automation applied to panning, will give the impression of the sound ‘moving’ from one place to another within the mix, in a smooth transition.
A common automation method found in almost any popular dance track is that of a change in frequency on a low-pass filter. A low-pass filter will be applied to the drums, only letting through the lower frequencies. Over time the low-pass filter will be altered using automation to allow more of the original sound to be heard, to give an impression of a ‘building’ sound, until the drop. Next time you’re listening to a dance/drum & bass record, listen out for this effect.
Here’s an example of automation in a low-pass filter.. Listen to the drums at 0:22