Equalization — or EQ — is the process of adjusting the balance between frequency components
within an audio signal. Every sound you hear is made up of multiple frequencies, and EQ gives
you the tools to shape which frequencies are louder or quieter. The foundation of EQ lies in
four filter types. A lowpass filter allows
frequencies below a cutoff point to pass
through while attenuating everything above it
— creating warmer, darker sounds by removing
high-frequency content. A highpass filter
does the opposite, letting high frequencies through while cutting the lows — useful for
removing rumble and mud from a recording. A bandpass filter isolates a specific range of
frequencies, attenuating everything above and below its passband. This creates focused,
vocal-like tones. The bandreject or notch filter is its inverse — it removes a narrow band of
frequencies while leaving the rest untouched. This is
essential for eliminating electrical hum at 50Hz or
60Hz. The cutoff frequency is where the filter begins
to take effect. The filter slope, measured in dB per
octave, determines how aggressively frequencies are
attenuated beyond that point. A gentle 6dB/octave
slope creates subtle transitions, while a steep 24dB/octave slope creates near-total removal.
Q factor — the quality factor — defines how narrow or wide a filter's effect is. It is
calculated as the centre frequency divided
by the bandwidth. A high Q creates a
narrow, surgical cut or boost. A low Q
creates a broad, gentle adjustment. Q and
bandwidth
are
inversely
proportional — as Q goes up, bandwidth narrows.
There are two main EQ architectures. A graphic
equalizer splits the spectrum into fixed bands —
typically 10 or 31 — each with its own gain
slider. The bands are spaced at octave or third-octave intervals, and filters are routed in
parallel. You get a visual "graph" of your frequency curve, but the frequencies are locked.
You can only adjust gain, not frequency or Q. A
parametric equalizer takes a different approach.
It offers fewer bands — typically 4 to 7 — but
each band gives you full control over three
parameters: centre frequency, gain, and Q. Filters
are routed in series, cascading through each band.
This gives you surgical precision for targeting
specific frequency problems. Shelving filters complete the toolkit. A high shelf boosts or
cuts all frequencies above its shelf point by an equal amount — unlike a lowpass filter where
attenuation increases with frequency. A low shelf does the same for frequencies below its
point. Peak and notch filters boost or cut around a centre frequency while leaving surrounding
frequencies untouched. These shelving filters are the building blocks of most parametric EQ
designs.