The Frequency Spectrum

As audio engineers, having a comprehensive knowledge of the frequency spectrum and how to manipulate it is imperative to achieving well balanced mixes. We often visualize and manipulate the different frequencies present in a single track, or within an entire mix, to achieve a desired sonic effect.

Audio Frequencies are what we perceive as pitch, and are measured in hertz. The range of human hearing spans from 20hz to 20,000hz. While frequencies exist on a spectrum, we often categorize frequency ranges in order to hone in on and refine specific elements of a mix or individual track. The Low end (around 20-250 Hz) is associated with bass sounds, such as the kick drum and bass guitar. The mid-range (250-4000 Hz) contains crucial information, often considered the heart of the mix. The highs (4000-20000 Hz) give us the “brightness” in a mix. Though it is essential to understand the entire spectrum of sound, working in these subsets of frequency ranges allows us to focus on specific areas to achieve a better overall sonic balance.

The Low End
We often describe a track’s bass frequencies as the “feel” of a mix. A good thumping bass drum, or an active reggae bass line can help define how we react to a song on an intuitive level. Dialing in the low frequencies in a mix is essential for clarifying and defining that “feel”. While the initial desire is to crank the bass, too much low end can negatively impact a mix, so it’s important to properly treat bass instruments. It’s also crucial to make sure to cut unwanted low frequencies from tracks which may have picked up, for example, a low rumble from a room, a “pop” from a vocalist’s plosives, or a hum from a cranked guitar amp.

The Midrange
No matter where you’re listening to your music, or what speakers you’re listening on, you can’t escape the midrange. All of the energy and power in an audio mix comes from the midrange, but too much of certain frequencies can also feel “boxy”. Because of the way our brains and hearing have adapted, this crucial frequency range is also where the intelligibility of the human voice lives, which means dialing in this range is essential to a clear and present lead vocal sound.

The Upper Midrange and Lower Trebles
Working in the upper midrange and lower trebles involves resolving harshness in a mix, as well as refining edginess and grit. Taming sibilance, dynamically processing peaky frequencies, and sculpting distortion are some of the ways we work to balance these frequency ranges. Cutting in these ranges can also have the effect of “warming” or “sweetening” a mix.

Extreme Trebles
At the top end of the frequency spectrum are the extreme trebles. This frequency range is commonly associated with “airiness” in a mix. Adding extreme highs to an EQ curve can provide some shimmer. Some noise issues may exist in this range, and can be cut to “clean up” an audio source which has been recorded in less than ideal conditions.

Understanding the frequency spectrum is crucial for audio mixing and mastering because it allows audio engineers to identify and adjust specific frequencies to improve and balance a mix. With this knowledge at hand, we have the ability to shape an audio source in a number of ways, musical and other.