Audio 101 - Electric Fields and Capacitors

The basis of an audio circuit is how difficult or how easy it is for a signal at a given frequency to travel around the circuit. If we make it easy for a frequency to get around the circuit, that frequency will have more power. If we make it more difficult for that frequency, it will have less power. It's all about how much we impede the signal.

Resistance always fights the flow of signal through the circuit, regardless of frequency, and it takes that energy and turns it into heat. But if we want to shape the frequency response of signals, we need to be able to impede the signal differently at different frequencies. We need devices that act like frequency-dependent resistors. Capacitors and inductors help us to do this, as well as some other components we'll discuss later.

Our current model/metaphor for things is electron guys giving each other a push. A bunch of electrons are lined up, passing that push from one to the other, and that is like a piece of wire — a conductor. How strong that push is, we call that Voltage. How many guys are pushing, we call that Current.

So, the “push” ...what is that exactly?

Gah! That requires quantum mechanics. Let's not go there today. Our brains will explode.

Let’s think of the push as a little unit of energy. The push is exactly what it sounds like: electrons naturally repel each other using that energy. Every electron wants other electrons to get out of its personal space. The strength of that repulsion is voltage. And that voltage travels down a wire as one electron pushes the next guy, who pushes the next guy, and so on.

In a capacitor, the plate is full of electrons — or, in our metaphor, a room full of electron guys — separated from another room (the other plate) by a wall (the dielectric). The electrons on one plate get pushed, and they try to shove that push straight into the next plate. But they can’t, because the dielectric blocks them. So instead, that push energy gets stored in the electric field between the plates.

Think of this field as an invisible closet where the electrons toss their leftover push. They can’t pass it through the wall, so they stash it in the space between the plates.

As more electrons shove on that plate, the electric field gets stronger. And the stronger the field gets, the more it resists being stuffed with more energy. It pushes back.

Now, if the electrons stop pushing into the capacitor, the field doesn’t just sit there. It starts returning the energy stored in the electric field back to the electrons on the plate — letting the push flow back out into the circuit.

There’s a kind of inertia to this give-and-take. Not literal mass inertia, but energy-storage inertia: the electric field resists changes in how much energy is in it. It resists being filled too quickly, and it resists being emptied too quickly.

The result is a natural “in-and-out” action, like pushing down and releasing a spring:

• when you push energy into a capacitor, it pushes back;
• when you stop pushing, it pushes energy out.

Because capacitors want a consistency to the amount of energy in their electric field, they tend to want the voltage to be consistent — they want a regularity to the pushes they store.

So, capacitors resist sudden changes in voltage, the same way a spring smushes and pushes.

Now, this isn’t resistance like we talked about with a resistor. A resistor simply dissipates energy into heat. A capacitor reacts to the energy it gets. It doesn’t waste the energy — it stores it in an electric field, or releases it from that field, in an effort to keep the voltage smooth and consistent.

We call this Capacitive Reactance.

A capacitor smushes to absorb the push, then it pushes back.

Now, if you push on a spring slowly, you can store a lot of energy in it. Storing a lot of energy means there is a lot of energy with which to react. Storing a lot of energy in a capacitor means it has high capacitive reactance.

But if you push and release a spring really quickly, more like wiggling it, you don’t have much time to store much energy. Likewise, with a capacitor, if the frequency is high, there isn’t enough time for the field to store much energy, so capacitive reactance goes down.

I'll repeat this differently in an effort to cement this into your brain because this is important.

As frequency goes down, capacitive reactance goes up — because the capacitor has more time to store more energy in the field and thus react with more energy — more forcefully, pushing back harder, storing more and releasing more. This is disruptive to the low frequencies. It gets in the way of low frequencies traveling around the circuit, so their energy goes down.

If the capacitor isn’t storing and releasing a lot of energy, the energy in the circuit flows more easily. The result is that as frequency goes up, the capacitor makes the circuit more efficient at passing the signal.

By carefully choosing capacitors, we can impede signals wherever we want in the frequency spectrum. Well, we need more than just capacitors, but I think you get the idea. We can have different impedances at different frequencies — remember at the beginning I wrote that we wanted a frequency-dependent resistor? We don't have that, but we come close: we have a frequency reactive capacitor, and for audio purposes, that works out really well.

Next week we have to dip more into Inductors.