Tech Tip:Channel Strip EQ
By Dennis Kambury
Continuing our journey down the channel strip, we arrive at the equalization section, commonly referred to as EQ.
EQ on a mixer is like an expanded version of the bass and treble controls on your stereo. In addition to bass and treble (or in channel strip parlance, low frequency and high frequency), channel strips commonly include midrange frequency controls.
EQ can be used to fine-tune the input signal from your mic or instrument, as well as to remove problem sounds such as rumble or hiss, eliminate feedback, or even create special effects such as "telephone voice" (sharply boosted mids) or "underwater muffle" (rolloff of all high frequencies).
This channel strip has three bands of EQ, starting with a high-frequency (HF) shelving filter that allows you to boost or cut frequencies above 12kHz by up to 15dB. By boosting this control, you add what's commonly referred to as "air," or a sense of openness. Add too much and the sound can become brittle or thin. By cutting at 12kHz, you'll remove high-frequency noise. Cutting at 12kHz is also useful if you're digitizing your audio at a low sample rate for game or web sounds.
The parametric mid on this channel strip has a sweepable frequency range from 100Hz to 8kHz and a 15dB boost/cut control. This lets you dial in select frequencies, and is useful for everything from accentuating the kick drum to killing feedback frequencies to increasing vocal clarity.
The low frequency (LF) shelving filter on this strip features a 15dB boost/cut of frequencies below 80Hz, great for cutting low frequency rumble that can muddy up vocals and higher-frequency instruments. The low cut switch on this strip is designed to remove frequencies below 75Hz at a rate of 18dB per octave. This means that 37.5Hz (one octave below 75Hz) will be 18dB lower in volume than 75Hz. (Normally frequencies are boosted or cut at a rate of 6-12dB per octave).
Some channel strips with parametric EQ include a bandwidth, or "Q" control. When you boost or cut a particular frequency, the frequencies above and below the primary band are also affected. With a narrow Q, fewer side frequencies are boosted or cut. Conversely, a wide Q will affect a broader range.
The illustrations below show what various EQ settings look like. The top graphs describe LF and HF shelving filters - in other words, all frequencies below or above a fixed point are raised or lowered (in this case, both are lowered). The bottom graphs show midrange band pass filters with two different Q settings.
In our next installment, we'll look at the pan control, from its place in the channel strip to how it's been used and abused over the years.