1
The Frequency Ranges (and What Lives Where)
Engineers use broad frequency bands as a shared listening map: sub and low carry weight, low-mids carry body and can accumulate mud, mids hold much of an instrument or voice, high-mids carry presence and attack, and highs add brightness or air.
The map is a starting point, not a fixed ownership chart. Listen for two parts masking each other, make one small move, then compare at matched loudness. Ear drills may use pink noise because it spreads energy across octaves and makes a changed band easier to isolate.
In practice: A boomy acoustic guitar and a bass both crowding 100-200 Hz sound thick and indistinct. Thinning the guitar's lows lets the bass own that band, and suddenly both are clearly audible.
Clarity = each part owns its band.
2
Fletcher-Munson / Equal Loudness Contours
The Fletcher-Munson curves describe a quirk of human hearing: how loud a frequency *sounds* depends on the playback volume. At quiet levels your ears go relatively deaf to bass and treble; turn it up and those extremes seem to return. The mids stay roughly constant.
This matters enormously for mixing. If you mix quietly you will hear too little bass and treble and overcorrect, boosting both, so the mix turns harsh and boomy when played at a normal level.
In practice: A mix done late at night at low volume sounds bass-heavy and bright the next morning at moderate volume, you compensated for ears that were not hearing the extremes.
Your ears hear less bass and treble when it is quiet.
3
Low-End Management for Small Speakers
Small speakers cannot reproduce frequencies below 100-150 Hz, but the brain still perceives bass, through the "missing fundamental" effect: it infers the low note from the harmonic series above it.
So a bass that translates needs rich harmonic content in the 200-800 Hz range. A gentle boost around 400-600 Hz brings out that midrange presence, making the bass audible on every system, not just full-range monitors.
In practice: An upright bass that vanishes on laptop speakers: add saturation for harmonics and boost 400-700 Hz, small speakers reproduce that, and the brain infers the fundamental.
Small speakers can't reproduce sub-150 Hz, bass translates through its 200-800 Hz harmonics.
4
Vocal Intelligibility Across Playback Systems
Vocal intelligibility: being able to make out the words, depends on consonants: the s, t, k, and p sounds that distinguish words, living in the 2-5 kHz presence range. If instruments compete there, the vocal becomes unintelligible even when it is loud enough.
The fix is a frequency pocket: cut competing instruments (guitar, mandolin, fiddle) in the 2-5 kHz range rather than just boosting the vocal, which would only congest the midrange further.
In practice: A bluegrass vocal loud enough but unintelligible over the mandolin chop and fiddle: cut those instruments ~2-3 dB at 3 kHz to open a pocket for the vocal consonants.
Vocal intelligibility lives in 2-5 kHz consonants, cut competing instruments there, don't just boost the vocal.
5
The Low-Volume Check: Fletcher-Munson at Conversation Level
Check your mix periodically at conversation volume, around 70-75 dB SPL. At low volume, Fletcher-Munson curves reduce your perception of bass and treble, so you hear primarily the midrange balance, exactly what matters most for translation.
If the vocal sits right, the guitar feels balanced, and the arrangement makes sense at conversation level, the core relationships are solid.
In practice: A mix that sounds punchy at 85 dB but loses the vocal and bass at conversation level: the high-SPL flattery hid real balance problems, fix them, then return to 85 dB for detail.
Check the mix at conversation volume (~75 dB): low level exposes the true midrange balance.
6
Reverb EQ: HPF and LPF the Return
Always EQ the reverb return. A high-pass filter at 200-400 Hz prevents low-frequency reverb buildup, every instrument's low end otherwise accumulates into mud. A low-pass filter at 6-10 kHz tames sibilance splash and keeps the tail from sounding harsh.
Real spaces never have a flat frequency response, so an unfiltered full-bandwidth reverb sounds fake. Filtered, the reverb supports the mix instead of competing with it.
In practice: A mix that is washy and muddy though every track is clean: the reverb return has no EQ, HPF it at 250-300 Hz and LPF at ~8 kHz.
EQ the reverb return as a habit: an HPF somewhere around 200-400 Hz keeps out mud, an LPF around 8 kHz tames splash.
7
Dynamic EQ vs Static EQ
A static EQ band applies its boost or cut constantly, no matter what the signal is doing. A dynamic EQ band only engages once the signal at that frequency crosses a threshold, it is, in effect, a frequency-specific compressor (or expander).
That distinction matters when a tonal problem is intermittent. A dynamic cut leaves the natural tone untouched whenever the problem is not occurring.
In practice: An acoustic guitar boomy only on hard strums: a dynamic cut at 200 Hz engages on the loud strums and stays inactive during fingerpicking, a static cut would thin the quiet passages too.
Static EQ acts constantly; dynamic EQ engages only above a threshold, a frequency-specific compressor.
8
Frequency Masking
Frequency masking is the ear's tendency to hide a quieter sound behind a louder one when both occupy the same frequency region (a "critical band"). The quieter sound is not just lower, it becomes genuinely inaudible.
This is why mixes get muddy: instruments pile into the same range and mask each other. And it is why subtractive EQ works, cutting the louder, masking sound is often more effective than boosting the masked one.
In practice: A strummed guitar can swallow a quieter instrument sharing its 1-4 kHz range. Cutting the guitar there "unmasks" the other part, without turning anything up.
A loud sound hides a quieter one in the same band, cut to unmask.
These lessons are the teach side of knowledge cards from the Studio Mix deck. In the daily plan each one is followed by a quiz, spaced over weeks, so it stays known rather than read once.