Frequency ranges are fundamental to the recording arts and sciences, and an understanding of them will help you make better-sounding recordings. You’ve undoubtedly read that human hearing extends from 20Hz to 20kHz. “Hz” is the abbreviation for Hertz, the industry-standard unit of frequency measurement, defined as one cycle per second (cps). It’s named after Heinrich Rudolf Hertz, the German physicist who first proved the existence of electromagnetic waves. Expressing Hz with a lowercase “k” (kilo) in front of it multiplies the value by 1,000 (e.g., 20kHz = 20,000Hz).
The Limitations of Human Hearing
The 20Hz–20kHz range is serviceable as a guideline for what most people can hear; however, it doesn’t take into account the fact that healthy human ears can sense frequencies above 20kHz and can certainly perceive when they are absent. This was particularly notable in the early days of digital. Initially chosen because it was roughly twice the assumed “cutoff” frequency for human hearing (and technically attainable), the original 44.1kHz sample rate specification, although adequate for dictation, proved marginal for music applications. And its shortcomings were exacerbated by the relative crudity of the analog-to-digital converters available in the 1980s. The technological limitations of the era tended to produce recordings with harsh-sounding high frequencies that sounded truncated, as if the overtone series (natural harmonics) were being lopped off with a chainsaw.
Full-range Audio Capture and Playback
Today’s pro audio gear is highly sophisticated. Modern audio interfaces (equipped with state-of-the-art converters) and DAW (Digital Audio Workstation) software can capture an extended high-frequency (HF) range, which is why many microphones and monitor speakers currently on the market can capture and reproduce frequencies beyond 20kHz. When recording engineers mic up a drum kit, acoustic guitar, grand piano, or vocal, they will typically choose a condenser microphone capable of capturing these frequencies — commonly referred to as “air” — along with any desired ambience. Focusrite interfaces even have a proprietary feature (appropriately named “Air”) that extends response to pull in more of these higher frequencies.
So, we know that humans are capable of sensing frequencies above 20kHz. Similarly, frequencies below 20Hz (such as the pedal tones from a pipe organ) can certainly be felt, if not heard. A loud playback through a powerful professional studio main monitor system, such as the ADAM Audio S6X, will confirm this, as you feel in your chest cavity the crush of a well-recorded kick drum or the full weight of a fat unison synth bass. On the high end, the S6X’s X-ART folded-ribbon tweeter extends all the way to 50kHz! We may not be able to hear all of these “dog frequencies” — but we tend to notice their complete absence. In order to record these frequencies, you must be able to hear them, so you obviously need a reasonably “full-range” reference. But not everyone has the space or budget for a pair of behemoth pro monitors. So, we would sensibly advise you to go as full range as your room acoustics (and budget) permit, perhaps by adding a subwoofer to your nearfield monitors. You may also be aware of the importance of checking your mixes on small, “real-world” speakers (like Auratones) that are representative of the way your recordings might actually be heard outside of your studio. Indeed, some engineers rely on multiple sets of reference monitors — including subwoofers — that they can switch between to ensure that their mixes “translate.”
Building a Mix
As you record, overdub, and make rough mixes, you’re developing a mental picture of your final mix — or at least, you should be. In the cognitive sciences, the ability to think in three dimensions is called structural visualization, and it’s an aptitude shared by architects, astronauts, and yes — successful recording engineers. You can think of your stereo mix in terms of a three-dimensional universe with depth, width, and height. Your sonic universe is populated with objects — instruments, vocals, and effects — that each have relative volume (depth), stereo pan position (width), and a frequency range (height). Thinking about “furnishing” your studio or live mixes in this way will yield more interesting mixes with dimension, detail, and impact. As you gain more experience, you will form a mental image of the way a raw, untreated source sounds — and how you want it to sound in the context of a given mix. This is vital for effectively applying equalization (EQ) and other signal processing as you build your mix.
To help you visualize common instrument and vocal sources in terms of their ranges in the frequency spectrum, we’ve provided our handy Sweetwater Music Instrument Frequency Cheatsheet for convenient reference. Download and study it and add your own embellishments (tuba or bagpipes, anyone?). With a basic understanding of frequency response and instrument ranges, you’ll be well on your way to creating great-sounding mixes!
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