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A capacitor is basically a reservior with a very small internal resistance.
It is very effective at supplying a very high instantaneous current.

It's best placed as close as possible to load requiring high instantaneous
current. When there's a demand for more power, electricity comes from a
source with smallest impedance. In this case, a low internal resistance
capacitor placed within a foot of amplifier rather than from rather high
internal resitance battery connected a few feet away.

By letting the capacitor supply peak demand, you can reduce the sagging on
upstream.

Johan Wagener wrote:

I heard that the car battery cannot supply current at a fast enough rate
(because of chemical reaction neccesary) for subwoofers and that a cap is
needed.

At what power ratings should one start to worry about this?