Four Filter Responses

What low-pass, high-pass, band-pass and notch filters pass, and how slope and Q change what you hear.

Related

EqualizerOrganAutomation

The question

Low-pass, high-pass, band-pass and notch are often treated as four EQ curves. They answer a different question, which part is allowed through, and slope and resonance shape the result.

How it works

A low-pass keeps content below the cutoff and a high-pass keeps content above. A band-pass leaves a middle slice; a notch removes a narrow strip. Each additional order adds about 6 dB per octave of slope, with 12 and 24 dB the common choices. The cutoff sits at the minus 3 dB point, and raising Q builds a resonant peak there, high enough at extreme settings to self-oscillate.

Settings and what you hear

Sweep a 24 dB low-pass below 200 Hz and a drum loses its body; 12 dB sounds more natural but leaks more highs. A high-pass at 80 to 120 Hz clears rumble, and higher up it thins a male voice. A band-pass around Q 2 to 6 sounds like a telephone; a notch above Q 10 bites one narrow line, useful on a resonance or a squeal.

What to listen for

Sweep the cutoff on a sustained chord and notice the resonant peak arrive before it disappears. On a noisy timbre, raise the high-pass and hear the hiss retreat layer by layer.

Common misconceptions

That a filter is just an EQ; that steeper is always cleaner, though phase and transients pay for it; that resonance equals warmth.

Sources

• Udo Zölzer, DAFX — Digital Audio Effects, filters chapter — background checked, not quoted
• Sound On Sound, synthesizer filter primer — slopes and resonance checked, not quoted

Used for fact-checking only; all prose is original.