A simplified yet thorough guide across mastering audio, mostly geared for musicmaking but also useful for audio books, voice acting, SFX, and improving sonic qualities for videos.
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Like an EQ but cuts off frequency from desired level. Useful for removing high or low end frequencies that don't do anything or reduce audio quality (e.g. wind, traffic, chatter, footfall).
Most filters have slopes stated in dB per octave. Set to 6dB per octave for soft adjustment. Set to 12dB per octave for moderate adjustment (is also most common).
24dB per octave is steepest setting; anything more totally cuts all frequencies below selected range and risks degrading audio quality.
Examples of good, free filters are HY-Filter4 Free, Flux Mini 2, and Filtron by Polyverse.
Note: Don't worry about memorizing graphs and numbers, focus instead how the shape of the line adjusts your audio!
High-Pass Filter / Low-Cut FIlter Allows higher sound to pass but blocks lows at chosen cutoff. Such filters have a fall-off called the slope, measured in dB per octave. Cutoff defined by frequency which falls to 3 dB, called half-power point.
A typical high-pass has a -3 dB cutoff at 125 Hz, slope 6 dB per octave.
If you cut off low frequencies at ~100 Hz or higher on all instruments except kick and bass and other bassy sounds, the entire song usually clears up.
Cutting off bass and drums 40 to 60 Hz sometimes helps add punch without affecting low end.
Low-Pass Filter / High-Cut Filter Allows low sounds to pass while blocking highs. A typical low-pass has a cutoff of 1 KHz, slope at 6 dB per octave.
Low-pass filters sometimes have Q, (Quality, though actually refers to Bandwidth). If so, cutoff usually around 2 KHz, with slope at 6 dB per octave, Q set to ~1.4 or ~6.0. To make things even more confusing, Bandwidth is reciprocal of Quality, meaning the more Quality, the narrower Bandwidth becomes.
Notch Filter Cuts a band of frequencies while allowing the frequencies on the sides to pass. Is obsolete by modern-day free plugins and digital instruments. Sometimes called a bandstop.
Shelving Filter / Shelf Filter When you boost or cut above or below cutoff, giving the response a shelf appearance. A typical shelf filter is somewhat an intermediate of high-pass and low-pass, but level out with a max boost or cut. A typical high-frequency shelving filter usually boosts from 3 dB to 12 dB. A typical low-frequency shelving filter cuts no more than 12 dB.
Peak Filter / Peaking Filter / Bell Filter When you cut or boost a range around the the centre frequency, either with a filter or EQ. Bell-shaped response, hence why it's often called bell filter. Consists of centre frequency, boost or cut of dB, and Q (Quality, though actually references bandwidth).
