The final section of the audible spectrum is the high frequency or treble portion. Humans theoretically are able to hear up to 20 KHz (that is, newborn baby girls can theoretically hear up to 20 KHz at normal listening levels; for the rest of us, considerably lower). So what could happen in the 16,500 Hz range if no new instruments can sound there?
It contains almost nothing but upper harmonics of treble instruments and room tone. This helps solo instruments and vocals sound present and full, but also adds brightness and clarity to a mix
Most telephones cut off around 3.5 KHz yet you can still tell whose voice it is on the phone. This tells you that practically everything needed to understand and distinguish any audio content pretty much lives below this range.
Pretty much only dog whistles operate in this range, so there is absolutely no need to worry about any more fundamentals or really any lower order harmonics getting in the way of any treatment you decide to apply.
Boosting in this range again helps with upper harmonics and upper harmonics are important to our brains in calculating proximity. The closer we are to something, the more detail we can hear in the sound. Similarly, the quieter an environment is when a sound is made, the more apparent that sound seems to us. The upper harmonics of a sound are generally very soft and are the first things to go when we are either far away from a sound source or it is sounding in a noisy environment. As such, the more upper frequency detail we can hear, the closer our mind perceives the sound source to be. Furthermore, we perceive upper harmonic detail as clarity and salience.
Many mastering engineers, as a final polish job, will use a very hi-fi shelving EQ and boost the frequencies from roughly 16 or 18 KHz up about 3 or 4 dB. The difference can be quite astonishing.
This range also gives you airiness and some pleasant room sounds. But boosting this range can also have negative effects like hissing, piercing, and sibilance.
Sibilance, which is an overemphasis on frequencies ranging roughly from 6-8 KHz, is by far the most apparent and troublesome. The beast way to deal with this is with a de-esser rather than an EQ so as not to sacrifice the harmonic content that you like that isn’t abrasive. A de-esser is a frequency-dependent compressor, it only compresses a narrow bandwidth, usually between 4 and 9 KHz to tame sibilance. It can be adjusted to work on cymbals or even hiss if it has the proper variables.
Next week, I’ll examine some of the commonly used terms associated with high frequency content and that will wrap up my series on the Audible Spectrum. I hope you’ve enjoyed it so far!