As you might imagine, large capacitors can accept electrons for a longer time period
than small ones because there's more space on their plates.
Small capacitors only accept electrons for a short time before
additional charge is repelled.
Now consider low-frequency ac signals. Low-frequency waves span longer
time periods than high-frequency waves—the electrons
go one way longer before reversing direction. Small capacitors
fill up with charge quickly and can't pass these long, low frequency signals.
On the other hand, high-frequency signals have short wavelengths.
The electrons change direction quickly, letting the alternating signal cross from plate to plate.
Because a capacitor's ability to conduct an ac signal is related to the frequency of the signal,
capacitors are perfect for tone control
A capacitor wired in series with a circuit (see figure
below) has a tonal effect opposite to the same capacitor wired in parallel
with the circuit.
This guitar will sound trebly because only the higher frequencies can cross the capacitor
to the speaker.
This guitar will sound bassy because high frequencies can go through
the capacitor instead of
going through the speaker.
Capacitance exists between any two conductors
in close proximity, for example between the two wires in a guitar cable. For
that reason, an overly long cable will siphon off the guitar's high
frequency tones, as shown in the above figure.