Sooner or later, the plates of any capacitor will fill up with charge and repel any
additional electrons.
A small capacitor has less storage space than a large one and
so fills up faster. Hence,
small capacitors block longer duration, low-frequency
waves, allowing only shorter duration, high-frequency waves to
pass through.
Since capacitors filter signals based on frequency, they're
routinely used for tone control
in musical instruments and amps.
As shown below, a capacitor wired in series with a circuit has
the opposite tonal effect as
the same capacitor wired in parallel with the circuit :

Series-Wired Capacitor
This guitar will sound trebly because only higher frequencies can go through the capacitor to the speaker.

Parallel-Wired Capacitor
This guitar will sound bassy because higher frequencies can go through
the capacitor instead of the speaker.

Capacitance can exist between any two conductors, for example between the two wires in a guitar cable.
A long guitar cable has more capacitance than a short one and could
siphon off higher
frequencies.
|