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[Mixing] Audio and Sound Waves Theory

category: Music | course: Music Mixing | difficulty:

You’ve probably already seen some of this at high school physics class, and thought ‘well, what is this useful for?’ at the time. This is the moment you’ve been waiting for.

When mixing audio, you’ll be working with audio files. Audio files are files that have 0’s and 1’s in them that represent a sound wave. There’s certain aspects we can change with the computer, and some that we just simply can’t – or at least not without a lot of special effects. To understand why this is, and to understand why certain actions have a certain effect, you’ll need to know the theory behind audio.

Sound Waves

In its purest form, a sound can be represented as a sine wave. Sound is a longitudinal wave, which means it has spots where the air molecules are pressed together alternating with spots that have more space between them. These differences in pressure are interpreted by our ears as sound.

But, trying to easily represent a wave made of compressed and not-so-compressed parts is hard, so we use the normal representation of waves: a sine graph.

SoundSineWave-01

A sine wave has two important properties:

  • Amplitude: How far away from the centerline the highest (and lowest) points are. A greater amplitude means a louder sound.
  • Frequency (in Hz): How many times it loops through (or ‘completes’) a full period, per second. A higher frequency, means a higher pitch.

 

Decibel Scale

The decibel scale (dB) is used to represent how loud a sound is. However, it is not a convenient linear scale:

  • +3 dB means that the power of the wave is multiplied by 2
  • +10 dB means the power of the wave is multiplied by 10

Power of the sound wave? A wave basically transports energy. The power of a wave is the amount of Joules it transfers per second. Every sound therefore has a different perceived loudness which is based on its power. The formula is:

Power = log( W ) - 12

The take away from this is that you can’t easily relate a change in decibel to a change in power/volume. For example, say your track is at a level of 35 decibel, and you increase it with 3 decibel. Then the power of your sound isn’t multiplied with (38/35), but it increases logarithmically. You must therefore be careful with dB increases and decreases, and a track that’s too loud may permanently damage someone’s audio system, while a track that’s too soft can only be increased in volume by a limited amount until the recording becomes nothing but noise. Record and mix at appropriate levels.

Frequencies

A regular human being can hear frequencies between 20 and 20,000 Hz. This seems like quite a lot, but there’s one thing you should know – frequencies also are a logarithmic scale.

If you for example increase a sound’s pitch from 20 Hz to 40 Hz, you’re not just making it a little higher, you’re actually moving upwards a whole octave! Therefore, subtle differences in low frequency ranges are much more important (or disturbing) than ones in higher ranges.

In order to mix music properly, you’ll need to know what range a certain instrument can play, and you’ll need to be able to quickly guess what frequencies you’re hearing. If you’re listening to your recording and you’re hearing some notes in the low or high end you don’t like and want to cancel out, you don’t want to search for 30 minutes until you’ve found it – you want to be able to fix it within minutes.

Actual Audio

I’ve just described some very basic theory behind sounds, but that’s not how it works in the real world. You never hear only one pure sound wave.

When you strum a guitar, you’re pressing the strings with your fingers to create a certain note. You might think ‘hmm, I am playing an E, it sounds like an E, great!’, but you’re hearing much more than that. The guitar is one big resonance box, and what such a box does is create a space for your sound waves to bounce around in. When sounds are deflected off the walls of your wooden guitar, they increase in frequency and create so-called overtones or harmonics. Those are what create a full and beautiful sound. The same thing also happens with your voice: when you sing (with some proper vocal technique at least), you sing the note you want, and the sound travels into your skull and resonates there to create extra frequencies, and that creates a fuller, louder and nicer sound.

So in reality, every sound you hear is a collection of a basic note (the tonic) that is the most prominent, and a collection of overtones surrounding it to give it some more life.

What this has to do with mixing audio, is that you sometimes don’t want these overtones or you want certain ones to be more prominent. If you’ve recorded a guitar solo but it sounds too muddy or lifeless to you, you might have accidentally recorded some lower notes that you didn’t want. That’s what mixing is for: cutting out what you don’t want, and enhancing what sounds great to you.

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