Lesson 3

How Sound Flows as Electricity

Sound, as electrical energy, is represented through rapid fluctuations of voltage that correspond to the shape of the sound wave.
And how are these voltage fluctuations turned into sound that we can hear? Alright, it’s time for a science class, It all starts with electromagnet principles. 
An electromagnet is a coil of wires wrapped around an iron core. When electric current is applied to the coil of wires, the iron core becomes magnetized. By using this magnetization, through precise control of the voltage, it’s possible to precisely control movements of a speaker cone, and thus, force it to go in and out to the vibrations of the desired sound wave.
I’ll explain this further. If I take a 9v battery, and connect it to the speaker terminals, as you can see, the speaker protrudes. If I take 2 9v batteries, and connect them in series to produce 18v, the speaker protrudes twice as far. So, the distance the speaker moves is directly proportional to the voltage applied.
Now, if I take the battery and reverse the polarity, as you see, the speaker moves inwards. So, if we alternate from +9v to -9v, the speaker cone will consequently move in and out. And if we alternate it really fast, like 440 times a second, ithe speaker will vibrate at 440 hz, and produce an “A” note.
Wall power is typically 120 volts, and alternates from positive to negative, and back to positive 60 times per second. So, if we plug a speaker directly into a wall outlet, it would produce a tone at 60 hz. However, I don’t recommend doing this because 120 volts will try to force most speakers to excurt past their maximum point. Then the speaker would likely break up, overheat the voice coils, probably start on fire and blow the breaker. So, unless the speaker can handle extremely high voltage, it’s a really bad idea to try it.
So, if you were to draw a graph, of voltage over time, the graph would correspond to the in out movements of the speaker, and therefore correspond to the the sound wave generated.
Now, let’s take this to the next level, and learn how it works in reverse. For this lesson, we’re going to use electric motors to make an analogy.
If I take this 9v battery, and connect it to the motor, the motor will spin at 3400 rpm. If I reverse the polarity, the motor will spin at the same speed in the opposite direction. If I increase the voltage, the motor spins faster. Noticing a similarity? Well, electric motors also operate on electromagnetic principles. Now, if I’m able to spin the shaft, the electric motor now becomes a generator, and actually produces electricity. So, if I can spin it at 3400 rpm, it will produce 9v. 
Since a speaker works on the same principles as an electric motor, it behaves very much the same. If the speaker is moved in and out by an outside force, it will produce an electric current in which the voltage corresponds directly to the movement of the speaker. So, if a speaker were to be made with a cone that’s small and light enough that sound waves could make it move back and forth, it would produce electricity in which the voltage will correspond to the sound which produced it. This is exactly how a dynamic microphone works. And as a matter of fact, if you plug a line level signal, such as a headphone output, into terminals 2 and 3 of the microphone output, you will actually hear the microphone act as a speaker and play the sound.