Sound Waves, Transducers and Analog Audio - Part I

We all understand that sound is made of vibrations in the air: when an object physically vibrates back and forth at repeating frequencies it causes the air molecules around it to vibrate at the same frequencies. These vibrations in the air create a sound wave that travels outward from the object in all directions. When the sound wave hits your ear it causes your eardrum to vibrate at the same frequencies. Your brain then interprets your eardrum's vibrations as sound.

The vibrating object, the sound wave and your eardrum are all vibrating at the same frequencies

A transducer converts one form of energy into another. In the example above we have two transducers:

• The vibrating object: its mechanical energy created the acoustical sound wave's energy in the air
• Your eardrum: the acoustical sound wave's energy caused it to send electrical impulses to your brain

If we want to convert a sound pressure wave in the air to an analog audio (electrical) signal we'll need another type of transducer. But to really understand how this works it will help if we learn a little about how electricity moves through a wire or circuit first. So let's talk about AC/DC. 

• AC stands for alternating current
• DC stands for direct current

Current refers to electricity moving or flowing through something. In a direct current (DC) circuit charges only move in one direction.

Batteries supply DC. If you were to connect the positive side of a battery to one terminal of a light bulb, and connect the second terminal of the bulb to the negative side of the battery, you would have created a simple circuit utilizing direct current.

Note: the diagram above illustrates what it known as "conventional current" and is used for simplicity. Electrons actually flow from - to +. Here's a link if you want to know more.

You could make another simple (but not very useful) circuit by connecting the terminals on the battery to the two terminals on a speaker. Because the direct current from the battery only moves in one direction the speaker cone would push outward as soon you closed the circuit. And the cone would remain that way until you disconnected it. If you don't believe me watch this guy do it on YouTube.

For our purposes we want the speaker cone to vibrate fast enough to create a sound wave. So sending DC to a speaker doesn't seem very useful. Good thing we have AC.

In a circuit that employs alternating current (AC) the current flows differently: instead of moving ahead in one direction it moves back and forth at a repetitive frequency. What else have we talked about in this article that does that? If you don't remember go read the first paragraph again. It's important that you see a correlation here!

So what if we want to convert a sound wave in the air to an electrical signal? Remember we said that a transducer converts one form of energy to another. The transducer we're going to use here is a microphone.  

Inside a microphone is a very sensitive diaphragm, sensitive like your eardrum. When the energy of a sound wave hits the diaphragm it begins to vibrate at the same frequencies. Get a cable and connect the mic to a sound system. What's being transmitted down the cable is an AC signal that is alternating, or vibrating back and forth, at the same frequencies of the sound wave in front of the mic. 

I just looked up the word "analogy" and got this: 

"a thing that is comparable to something else in significant respects"

The AC signal that's present on the mic output is analogous to the acoustical sound wave you put in front of the microphone. They both vibrate at the same frequencies. 

When you hear the term analog audio that's what it refers to: alternating current moving back and forth in a circuit at frequencies and amplitudes proportional to a sound pressure wave. 

So now we have:

A microphone by itself is pretty useless. So in the next installment we'll talk about preamplifiers, amplifiers and speakers.





Karl Wenninger is an audio engineer, synthesist/sound designer, composer, guitarist and DIY audio electronics enthusiast. As an adjunct professor he has taught Pro Tools at The New School for Jazz and Contemporary Music, Computer Music at York College and Audio Post-Production for the Media Arts Program at NJCU. He was an program administrator and associate professor at the former Digital Media Arts program at Touro College in New York City for over a decade.