A filter is the simplest transformation you can do to a sound: make some frequencies louder and others quieter. But behind every EQ curve lies a second, invisible curve — the phase response — that shifts frequencies in time. You can see the amplitude change on an analyzer. You can only hear the phase change. Together, they are the complete picture of what a filter does, and the reason two EQs with identical frequency curves can sound completely different.
Key insight: Increase the Q and watch a resonant peak appear at the cutoff. The filter isn't just removing frequencies above the cutoff — it's actually *boosting* the frequencies right at it. This resonant bump is the 'squelch' you hear in synth filter sweeps. At extreme Q, the filter rings like a bell.
Where the filter acts. For LP/HP, it’s the −3 dB cutoff. For bells, it’s the center. For shelves, it’s the transition point. Measured in Hz.
How sharp or broad the filter’s effect is. Low Q = gentle, wide. High Q = sharp, narrow. On LP/HP, high Q adds a resonant peak at cutoff. On bells, high Q makes a narrower boost/cut.
How much boost or cut, in dB. Only applies to shelves and bells — LP/HP/BP/notch don’t have a gain control because they pass or reject, they don’t boost. ±6 dB is subtle, ±12 dB is dramatic.
Every analog filter shifts the phase of frequencies near the cutoff. A first-order filter shifts phase by up to 90°. A second-order shifts up to 180°. A fourth-order shifts up to 360° — a full cycle.
This matters because phase determines how waveforms combine. Two signals at the same frequency and level but 180° apart cancel completely. This is why multi-miking a guitar cabinet is tricky — the mics are at different distances, creating different phase relationships at different frequencies.
Digital EQs can be designed with linear phase — zero phase shift at all frequencies. This sounds “cleaner” but introduces a different artifact: pre-ringing, a subtle echo before transients. Analog EQs and minimum-phase digital EQs have phase shift but no pre-ringing. Neither is “better” — they’re different tradeoffs that suit different situations.
Filtering is the second oldest tool in audio, after amplification. The earliest radio receivers were just filters — tuned circuits that selected one station’s frequency and rejected the rest. Every bandpass filter in this explorer works on the same principle that let your grandparents’ radio pull a voice out of the noise.
What makes filters so fundamental is that they appear everywhere in disguise. The tone knob on your guitar is a filter. The body of an acoustic guitar is a filter. Your ear canal is a filter (it resonates around 3 kHz, which is why human hearing is most sensitive there). A room is a filter. The atmosphere is a filter — distant thunder sounds dull because the air has low-passed it.
Once you see filters, you see them everywhere. Every surface that reflects sound absorbs some frequencies more than others — that’s filtering. Every material that transmits sound changes its spectrum — that’s filtering. The entire journey of a sound from source to ear is a chain of filters, each one shaping the spectrum a little further. Your EQ plugin is just one more link in a chain that started at the vibrating string.