Like coils, all capacitors represent an energy storage.

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The capacitor consists of two conducting layers (plates or similar) that are separated by an insulator. When voltage is applied to a capacitor an electrical is generated at the insulator. This electrical field holds energy. The bigger the capacitor's plates or the thinner the insulator, the bigger is the capacitor's stored energy - at a given voltage.

How is stored energy specified?
The capacitor's stored energy E is dependant on the capacity C of the capacitor and the given voltage U squared: E = 0.5*C*U2. The higher the capacity C the more energy is stored in the capacitor at a given voltage. The unit of measure is Farad F where usually only a millionth is needed: Micro Farad or uF.

What are the implications for loudspeakers?
When voltage is applied to a capacitor, current flows initially, but stops when the capacitor is charged. When the voltage changes current flows again until the capacitor has discharged and charged again. The more often and the quicker the voltage changes the more current is flowing through the capacitor. When a high frequency AC voltage is applied, the voltage changes continuously; current flows persistently. A loudspeaker connected in series is under current when the voltage applied changes often or quickly - meaning at high frequencies. The capacitor works like a filter, passing high frequency oscillation on to the loudspeaker that is connected in series.

When a capacitor is connected in parallel to a loudspeaker something else happens. If the voltage to the loudspeaker needs to change, the capacitor must be recharged first. The capacitor prevents quick voltage changes, therefore passing on only low frequencies to the loudspeaker.

What is the ideal capacitor?
The ideal capacitor show exactly above mentioned properties. Ideally a capacitor stores energy while not dissipating any - therefore, this capacitor doesn't generate any heat. However, in reality no electrical component is perfect. A capacitor's insulation is also conducting current, additionally, the plates' terminals are not perfect conductors either.

Therefore: lossy capacitors don't let the current pass through unhindered at high frequencies since the terminals represent an ohmic resistance. Besides, they are not insulating at lower frequencies, because the insulator shows a measurable conductance.

What types of capacitors are available and what properties do they have?
There are electrolytic and foil capacitors. The list of capacitors in ascending order of quality:

• Electrolytic capacitors consist of an aluminium foil and a wafer-thin oxide layer acting as insulator. To ensure that this layer is conducting to the next layer, a liquid, conductive electrolyte is placed in-between. There are unipolar (not suitable for loudspeakers) and bipolar, etched and un-etched electrolytic capacitors. Electrolytic capacitors are quite popular due to their small size and low cost. They are not very sophisticated and change their properties over time.
• MKT capacitors are foil capacitors with metal plated polyester foil. These capacitors are superior to electrolytic capacitors with the exception of size.
• MKP capacitors are foil capacitors with polypropylene foil. Regarding losses they are superior to MKT capacitors. An especially low-loss version is the tin or (Sn) polypropylene capacitor, like e.g. made by IT-Electronic or Mundorf.

When to use which capacitor?
The following table lists the typical properties of capacitors:
 

Capacitor type (Term as per I.T.)	typical loss (f=1kHz and 20 C)	size	cost	standard application	high-end application	dielectric medium
electrolytic capacitor, etched	0.12	small	low	impedance correction	-	oxide layer
electrolytic capacitor, un-etched	0.03	medium	low	in parallel to  bass driver	impedance correction	oxide layer
foil capacitor MKT	0.005	medium	medium	in series to tweeter	in parallel to  bass driver	polyester foil
foil capacitor MKP (Audyn Cap MKP QS)	0.0003	large	medium	-	in series to tweeter	polypropylene
tin foil capacitor MKP (Audyn Cap KP Sn)	0.00008	large	high	-	-	polypropylene

Additional important notes: If a crossover is optimised for a low quality capacitor, the use of a high quality capacitor often doesn't show any advantage. The crossover needs to be tuned again incorporating the high quality component.

What resistance does the ideal capacitor have?
The resistance of a capacitor is dependant on frequency. This resistance cannot be compared with the common resistance. For an estimate the calculation of reactance is quite helpful:
Z = 1 / (2*Pi*frequency*C)
where C=capacitance, f=frequency, Pi=3.141...

Example:

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for f=100 Hz and C=1uF (u - Greek letter for a millionth):
Z = 1 / (6.28 * 100 Hz * 0.000001 F) = 1591 Ohm = 1.591 kOhm
 

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