8. Psychoacoustics (Perception Layer) · Concept 6 of 18
Critical Bands
The fixed pitch zones your ear lumps together when deciding what masks what.
Two sources in one critical band fight (mud); a small EQ cut nudges one into its own bucket and clears it.
What it is
Critical bands are the fixed frequency zones your inner ear lumps together when deciding which sounds mask each other.
Key facts
Roughly 24 critical bands span human hearing from 20 Hz to 20 kHz (the Bark scale, Zwicker 1961).
Band 1 = 0 to 100 Hz; band 24 centres near 13.5 kHz; each band = 1 Bark unit wide.
Below ~500 Hz each band is a near-fixed ~100 Hz wide; above ~500 Hz it widens to ~20% of centre frequency.
ERB (Equivalent Rectangular Bandwidth) formula: ERB = 24.7 x (4.37 x f/1000 + 1), with f in Hz, ERB in Hz.
About 1 critical band = 1.3 mm of travel along the basilar membrane (whole cochlea is ~35 mm long).
One critical band maps to a roughly constant ~1.3 mm of basilar membrane, so the cochlea's ~35 mm holds about 24 bands end to end.
Masking is strongest when masker and signal fall inside the SAME critical band.
A critical band is wider than a semitone above ~500 Hz, so two close notes can share one bucket.
One critical band is roughly 1/3 octave wide across most of the spectrum (hence 1/3-octave EQ).
Speed of sound in air = 343 m/s at 20 degrees C (about 1235 km/h).
How it works
Sound enters the cochlea and travels along the basilar membrane.
High pitches peak near the stiff base, low pitches near the floppy apex (place coding).
Each spot responds to a RANGE of nearby pitches, not one exact frequency = one bucket.
Two tones landing in the same bucket excite overlapping hair cells and fight (mask).
Two tones in different buckets are heard separately with little masking.
A small EQ cut on one source moves its energy out of the rival's bucket and clears mud.
Real examples
Kick drum (60-100 Hz) and bass guitar (80-120 Hz) crowd the same low bucket and turn to mush.
Vocal and rhythm guitar both parked around 2-4 kHz fight for the same presence bucket.
Snare body and tom share the 200-400 Hz bucket and smear each other.
Two synth pads a tone apart at 5 kHz sit in one critical band and sound rough/beating.
Hi-hat and cymbal sizzle both pile into the 8-12 kHz buckets and mask sparkle.
How it helps in live sound
Carve, don't boost: cut bass 80-100 Hz to clear room for the kick fundamental.
Use a 1/3-octave (31-band) graphic EQ - each slider is roughly one critical band.
Fix masking by giving each source its own bucket: e.g. vocal 3 kHz +2 dB, guitar 3 kHz -3 dB.
High-pass everything that is not bass at 80-120 Hz to empty the low buckets.
Spot feedback: ringing usually sits in one critical band - notch it narrow (Q ~ 7-10), 2-4 dB.
Pan or arrange close-pitched parts apart so they don't share a bucket in the mix.
Everyday analogy
Your ear is a row of 24 buckets, each catching a band of nearby pitches; two sounds dropped in the same bucket splash over each other, while sounds in separate buckets stay clean.
Watch out
Myth: masking depends on exact frequency overlap. Truth: it depends on the critical BAND - sounds up to a band apart still mask, so close-but-not-equal pitches still fight.
Fun fact
Above ~500 Hz a critical band is wider than a whole tone, so two close notes (up to about a major third apart) can fall inside one bucket and sound rough - which is why tightly voiced low-register chords muddy faster than high ones.
Key takeaways
Your ear hears RANGES (buckets), not single exact pitches.
Same bucket = bad masking; different buckets = clean separation.