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Mixing – Improve Your Mix with Bass EQ

October 10, 2013
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Fig. 1. Shelving and highpass filters are used at once in the Waves H-EQ Hybrid Equalizer plug-in to shape a bass guitar’s soun

STEERING THE bass-frequency band of a mix into good balance with midrange and high frequencies is arguably the most difficult task in music production. Boosting low frequencies—or the instruments that produce them—too much can make a mix sound boomy or muddy. On the other hand, minimizing their contribution to the mix might leave you with a thin, strident sound.

In this article, I’ll show you three ways to equalize bass frequencies to attain a perfectly sculpted bottom end. I’ll start with treating the bass guitar.

Shelve and Roll Off If the bass guitar needs to thunder more, try boosting its bass frequencies using low-shelving EQ. Keep in mind that the fundamental frequencies for virtually all notes on a four-string electric bass guitar lie in the bass-frequency band. Boosting with a bell-curve filter in lieu of shelving EQ would result in a peak sloping downward on both sides; that would boost some bass frequencies more than others, causing certain notes to pop out of your mix and others to be too quiet. A shelving filter, on the other hand, will boost all notes more or less equally below the frequency where its slope levels off.

Fig. 2. Bass-shift filtering is used in the Brainworx bx_digital V2 equalizer plug-in to simultaneously add punch and clarity to a kick drum track.
The only problem is that the shelving filter’s effect will also extend below where it’s musically useful, causing rumble and eating up valuable headroom. To prevent the attack of every bass note from sounding like concrete pylons falling off a truck, activate a highpass filter (HPF) on the equalizer’s lowest band and roll off its response below roughly 35 to 40Hz (see Fig. 1). Adjusted correctly, the shelving boost will moderate the HPF’s roll-off around its corner frequency, preventing low notes on the bass guitar from being attenuated. Be careful, though, when using an HPF modeled on a classic analog filter. Any overshoot (resonant boost) around 40Hz will boost notes on the bass guitar’s bottom string. A little extra boost there might sound good (or not); too much will make your mix sound uneven at times. Often, a linear digital algorithm is the best tool for highpass filtering on bass guitar.

Shift It Bass-shift filters and Pultec-style equalizers are fantastic tools for shaping a kick-drum track. The bass-shift filter in the Brainworx bx_digital V2 equalizer plug-in can be used to boost lower-bass frequencies while simultaneously cutting the adjacent upper-bass and lower-midrange bands (see Fig. 2). This prevents the reinforced kick drum from sounding too muddy and masking other bottom-dwelling elements of your mix. By clearing out a portion of the kick’s bass and midrange spectra—even as it adds punch and girth—bass-shift EQ gives the bass guitar room to speak in your mix.

Pultec-style passive equalizers—and the plug-ins that emulate them—create much the same effect. Simultaneously boosting and cutting an identical amount and at the same center frequency with Pultec filters doesn’t result in zero net gain. Because the two filter circuits have different slopes and bandwidths, you end up instead with a bass-shift response curve. The PSP NobleQ plug-in uses filters loosely based on Pultec curves. Incredibly inexpensive, NobleQ nevertheless sounds fantastic. Try using a little more boost than cut on the same bass-frequency setting to bless kick-drum tracks with thunderous lows and Windex-clear mids.

Fig. 3. The speed switch in Slate Digital’s creamy-sounding Virtual Tape Machines plug-in can be used at different settings on various instruments to preserve track separation while warming up a mix.
Change Speed
While not exactly equalizers, plug-ins that emulate analog tape decks can be used to bolster bass and thicken low-midrange frequencies—and do so in one particular way that the imitated hardware can’t.

Tape machines produce what is known as a head bump: a boost in frequency response that occurs in either the low or middle bass band depending on the speed of the tape machine’s transport. A tape recorder’s transport operating at 30 ips (inches per second) produces a head bump one octave higher than one operating at 15 ips. You can’t change the speed of a tape recorder to move the head bump (boosting different bass frequencies) without also pitchshifting tracks already recorded—but you can by using a tape-emulation plug-in.

The Slate Digital Virtual Tape Machines (VTM) plug-in is the most authentic and best-sounding tape-emulation plug-in available today (see Fig. 3). By setting VTM to 15 ips on a bass track and 30 ips on electric guitar, you set apart the head bumps imparted to each track, creating better separation in your mix. The bass and midrange bands still get that thick, creamy analog sound, but the instruments’ composite sound remains open. This is just another example of how selective reinforcement and attenuation of bass frequencies, on a track-by-track basis, can elevate your mix to perfection.

Michael Cooper is a recording, mix, mastering, and post-production engineer, a contributing editor for Mix magazine, and owner of Michael Cooper Recording in Sisters, Oregon (www.myspace.com/michaelcooperrecording).

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