What Those Other Filter Responses Mean to Recording

There’s more to filter life than peaks and dips

By Craig Anderton

Just about everyone knows what a parametric equalizer does: Boost or cut a specific range of frequencies. But today’s EQs are usually multimode, and have other responses as well. Are you taking full advantage of what your EQ can offer? If not, you will after reading this.

SHELVING FILTERS

The low shelf response is one of the preferred methods for adding “bottom” when boosted, and removing “mud” when cut. Similarly, the high shelf can add “sparkle” when boosting, and reduce “shrillness” when cut. The shelving filter’s distinguishing characteristic is that after reaching the maximum amount of boost or cut, that amount remains constant. For example, if you use a high shelf to boost a signal by +3dB starting at 1kHz, and it reaches the full boost of +3dB around 2kHz, the boost will remain at +3dB up to the filter’s high frequency response limit.

Using a combination of shelving and parametric can solve numerous problems. Suppose there’s a drum loop with too much kick drum, but not enough low end in general. Use a low shelf to bring up the low end, then use a parametric to “punch” a dip just in the vicinity of the kick (Fig. 1).

Fig. 1: Combining a low-frequency shelf with a parametric stage.

Shelving filters work with individual tracks, but because of their gentle and relatively benign effect, can be applied to program material as well.

HIGHPASS AND LOWPASS FILTERS

The difference between a highpass filter (which as the name suggests, passes high frequencies) and a shelf set to cut low frequencies is that the response continues to attenuate with frequency, at a rate specified in dB/octave. For example, with a 12dB/octave response, for each octave you go below the cutoff frequency, the response will be down roughly another 12dB compared to the previous octave.

The highpass filter is a fine way to get rid of subsonics, low frequency mud, room rumble, and excessive plosive sounds from vocals. One pole of rolloff (6dB per octave) isn’t really enough; if possible, dial in a sharper cutoff to solve these types of problems (Fig. 2).

Fig. 2: A highpass filter to the left rolls off lows at 48dB/octave; a lowpass filter (right) rolls off at a similarly steep rate for high frequencies.

Also note that with multiband EQs, you may be able to set each band to responses other than the traditional parametric. If you have to deal with subsonics, you can “gang” multiple highpass filters in series for a sharper overall cutoff.

High pass filters rarely control a signal’s tone; they exist mostly to solve problems. If you want to control the low end in a more general way, shelving is usually the better option.

As to lowpass filters, the traditional use is to attenuate hiss. Few signals have significant amounts of energy above 10kHz, so if necessary you can usually remove the very highest frequencies without degrading a signal’s integrity.

But there are other uses. Removing high frequencies can help put a signal further back in the mix without resorting to a change in volume, and a lowpass filter can also take away some of the “brightness” of digital signals (like synthesizers) if they clash with primarily analog tracks.

Lowpass filters are seldom, if ever, used with program material because even a slight amount of high frequency reduction can create a “muffled” sound. This isn’t as noticeable on individual tracks.

THE COMB FILTER

Some sounds demand more attention than they should. Conventional EQ may fix the problem, but an easy way to “dilute” a sound without altering its fundamental character is to apply comb filtering.

The comb filter gets its name because its frequency response curve looks like a comb — instead of being a straight line, it has a huge number of dips and peaks. Just as you can thin out MIDI controller streams by removing pieces of data, you can thin out sound by inserting lots of narrow notches in portions of the frequency spectrum.

This doesn’t work with everything, though. It’s best for overbearing pads, non-pitched sound sources, and instruments designed to sit in the back, like rhythm guitar. Otherwise, if the notches fall at, say, the resonant frequency of a tom, the sound may get too thin. Also be aware that comb filtering adds a subtle sense of pitch to the sound, although you can adjust this to some degree.

It’s unlikely that your host’s EQ offers a comb response, so you’ll probably have to construct one yourself (Fig. 3); it’s not difficult.

Fig. 3: Sony’s Simple Delay has been pressed into service as a comb filter.

1. Insert a simple delay plug-in — the simpler the better. Make sure that it can do short delays (1–20ms). A flanger or chorus may work if it’s not a multi-voice type and you can turn off the modulation.
2. Set the dry out and delay out to the same level (if there’s a blend or mix control, set it to 50%).
3. Adjust the delay time in the 100μs to 20ms range until you hear the desired amount of “thinness.” Times under 10ms have a major effect on the sound; longer delays get into the echo range.
4. If the sound is too pitched or “phasey,” reduce the delay level slightly. This reduces the notch depths.

DON’T FORGET ABOUT AUTOMATION

Most EQ plug-ins, as well as EQ integrated into a host, are automatable. This greatly extends the usefulness of “alternative” EQ responses; for example, you can cause the low pass to sweep down for just a fraction of a second to eliminate an annoying high frequency transient or reduce an overly friendly high hat, then return immediately to full frequency response. Or, if there’s a signal with some hiss, bring the lowpass frequency down in quiet sections, then sneak it back up again when no one will notice. In the example of setting up a comb filter effect, you could change the ratio of straight to delayed sound to adjust the sound’s “thinness.”

As long as you’re adjusting filter boost/cut, you probably won’t hear any “stairstepping” due to quantization of the parameter into multiple steps. You may or may not hear any when changing the frequency.

All of this comes down to one thing: Be creative with EQ. In the days of physical consoles, EQ tended to be set-and-forget devices because there just weren’t enough arms to run them and also ride the faders. But today, we have no such limitations . . . which means more options if you’re into creative recording.

Craig Anderton is Editor Emeritus of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.