A capacitor stores electrical energy. It typically has two closely spaced metallic surfaces that are insulated from each other by a “dielectric” or non-conducting material. When a voltage is connected across the metallic surfaces, an electric field develops across the dielectric and a net positive charge collects on one surface and a net negative charge collects on the other. Thus, there is a store of electrical energy, similar to a battery.
Permittivity
There are many types of dielectric material, such as mica, glass, air, paper, mylar, and electrolytic materials. These materials have different permittivity ratings, which define how nonconductive they are.
Audio amplification applications
Capacitors couple constant DC signals with rapidly fluctuating voltages that drive speakers. This helps create a smooth sound. Further, capacitors can help limit the amount of noise between the power supply and the audio driving circuitry, resulting in a clearer sound.
The best types of capacitors for AC coupling include electrolytic and tantalum. Electrolytic offers the best performance since it is very linear when biased by Common Mode voltages. Further, their low ESR or equivalent series resistance makes them act like an ‘invisible’ component. However, they do have a large footprint and ‘dry out’ over time, especially in high temperature environments. While tantalum capacitors are more expensive, they do not dry out and have a smaller footprint.