SciTech

# How stuff works: synthetic sounds

Everything is slowly becoming digital, including sound. However, it's difficult to synthetically create different sounds like dog barks, pianos, and human voices in bits and bytes without having premade recordings. So how do sound engineers do it?

Well, for starters, sound engineers have to understand musical pitch. Every note and sound at a given moment vibrates at a certain frequency. You can think of a given note "wiggling" through space in a series of high-pressure and low-pressure regions, known as a compression wave. These compression waves can then be visualized as sine waves. For example, the note “A4” is widely accepted to be 440 hertz, hertz being wave cycles per second.

But wait, the note A on the piano still sounds way different from when a human sings the note A. What makes these As different and how do you recreate that?

The answer to that is harmonics. When you hear the note A, you aren't actually only hearing the note A. You're also secretly hearing other notes overlayed on top of the A, and those notes are called harmonics. Any given note has a set of harmonics that you will hear with it, but different instruments and voices will play some harmonic notes louder than others, which is what gives you a different sound.

Harmonics are lots of fun to investigate. You can press the A key on a piano and try to pick out the notes you hear other than A. If you're a mathematician, there are lots to explore in the realm of harmonic series, a series that tells you what harmonics should appear with a given note. As a sound engineer, you can utilize this knowledge by altering wave formulas until you get the same harmonics as the instrument or voice you are interested in recreating. In the end, this should get you a digital sound that resembles the actual sound.

To alter the typical sine wave, you can subtract sine waves (subtractive synthesis), add sine waves (additive synthesis), or change the frequency of the sine wave (frequency modulation). This will give you very wonky-looking squiggles that will produce different frequencies and different subsequent harmonics to get you your desired sound.

While this is a very broad overview of the ideas in synthetic sound, there is so much to explore for a person of any field. And as virtual reality and synthetic voices like Amazon's Alexa and Apple's Siri grow in capabilities, there may come a time when every single sound in reality will be able to be reproduced synthetically.