Understanding Frequencies: The Foundation of EQ and Spectral Processing

Hey what’s up —
In this season, the focus is on spectral processors, and to really understand EQ, we first need to take a close look at frequencies.

This episode is all about building that foundation.

To make frequencies easier to understand, we start with the most basic example possible: the sine wave.

The Sine Wave: The Purest Form of Sound

The sine wave is the purest and simplest form of sound.
It’s not something you’ll find in nature, but it’s incredibly useful when it comes to explaining how frequencies work.

The sine wave is pure — nothing added, nothing removed.

What Is a Cycle?

When a sine wave oscillates 360 degrees and completes one full rotation, that is called a full cycle.

  • One complete oscillation = one cycle
  • Cycles repeat over time

Defining Frequency

The number of completed cycles that fit into one second is called the frequency.

  • Frequency is measured in Hertz (Hz)
  • One Hertz = one cycle per second

Low Frequencies and High Frequencies

Frequencies can reach extremely high values.

  • Higher frequencies can go up to 20,000 completed cycles per second
  • That equals 20,000 Hertz, or 20 kiloHertz (kHz)

We can divide frequencies into two simple groups:

  • Low frequencies → Bass
  • High frequencies → Treble

The Human Hearing Range

The frequency range of the human ear spans from:

  • 20 Hertz
  • up to
  • 20 kiloHertz

This range is not constant throughout life.

As we get older, we lose some of our ability to hear higher frequencies.

That’s why older people can’t hear very high frequencies as well as younger listeners.

Listening to Sine Waves Across the Spectrum

To get a real idea of what frequencies sound like, it helps to listen to sine waves at different values.

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Examples include:

  • A 500 Hz sine wave
  • A 1000 Hz sine wave
  • A 5000 Hz sine wave
  • A sweep dropping from 2000 Hz down to 75 Hz

These are pure sine waves.

They cannot be found in nature.

How Natural Sounds Are Built

Every sound we hear in nature — whether it’s speech or music — is not a single sine wave.

Instead, natural sounds are made up of:

  • Multiple sine waves
  • At different frequencies
  • At different amplitudes

Together, these sine waves form complex sounds.

The Example of Concert A (440 Hz)

A concert “A” is tuned to 440 Hertz.

If you play that A note:

  • On a guitar
  • Or on a piano

…it is still the same note.

Yet, you can clearly hear the difference between the two instruments.

Why?

Fundamental Frequency and Harmonics

The answer lies in harmonics.

  • The fundamental frequency is 440 Hz
  • But the character of the instrument comes from its harmonics

The harmonics together form what we call:

Timbre — the character of a sound

Timbre is what makes a piano sound like a piano and a guitar sound like a guitar.

What Shapes the Sound of an Instrument

Several factors determine how harmonics behave and how an instrument sounds:

  • The shape of the instrument
  • The way the notes are generated:
    • Plucked
    • Bowed
    • Finger-picked
    • Hit with hammers
  • The material the instrument is made of
  • Even the type of wood used

All of these influence the harmonic structure and, therefore, the timbre.

Working With Frequency Content in Audio

When working with audio, we’re constantly looking at the frequency content of a signal.

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This is especially important when:

  • Recording instruments separately
  • Mixing multiple tracks together

Each recorded track contains its own frequency content, and that content can be changed.

Spectral Analyzers and Visualizing Sound

Using a spectral analyzer allows us to see frequency content.

Typically:

  • The horizontal axis shows frequencies
    • From 20 Hz to 20 kHz
  • The vertical axis shows amplitude

This visual representation makes it easier to:

  • Analyze what’s inside a signal
  • Pinpoint specific frequencies
  • Decide which frequencies:
    • Need to be enhanced
    • Or should be cut

Applying Frequency Knowledge in Mixing

Understanding frequencies is essential for:

  • Shaping individual tracks
  • Creating space in a mix
  • Making informed EQ decisions

This knowledge becomes the foundation for working with:

  • Filters
  • Equalizers
  • Spectral processors

What’s Next in the Series

In the next episode, the focus shifts to:

  • Filters
  • Equalizers

Using everything learned about frequencies, the goal will be to:

  • Enhance sounds
  • Improve mixes

This season is all about frequencies, while the last season focused on dynamics.