CHASING WATERFALLS

THE MAKEUP OF A MIXER,
Part 1

from Women in Sound #6
released February 4, 2019
text by Jade Payne
illustration by Maggie Negrete

This piece is intended for those who are new to audio mixers.
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The audio begins its journey at the top of the channel strip and trickles downward before it is carried over to the next stage.

Channel strips are the nearly identical vertical columns of knobs that take up the most space on a mixer. To understand the parts of a channel strip, you’ll need a basic understanding of signal flow, or the direction in which the audio signal travels through the mixer. Think of a channel strip like water flowing down a waterfall. The audio begins its journey at the top of the channel strip and trickles downward before it is carried over to the next stage.

First, at the top of the channel is the gain knob, which applies a huge amount of voltage to the preamp (the XLR socket where the mic is plugged in), allowing us to boost a low-level microphone signal in order for it to be audible at line-level. Notice how some mixers have an XLR socket at the top, while some have a 1⁄4-inch socket. You’ll almost always see a small numerical value next to the gain knob, which indicates the entire possible range of gain capable of being applied to the signal. And if you look even more closely, you’ll notice that this number is higher on the XLR channel than on the 1⁄4-inch line input channel. This is because microphone signals, which we connect into the XLR socket, are inherently low-level. They need a hefty amount of gain to become audible. On the other hand, line-level signals are almost always loud enough before they even hit the mixer, so they don’t need as much gain. Because gain knobs pack such a huge punch, it's easy to apply too much gain to the signal and cause clipping or feedback. You should always keep a steady hand and be careful when turning up the gain. A good rule of thumb is to start the channel fader at -5dB (decibels) and slowly raise the gain until your meters are hitting the “yellow” range, typically around -5 to 0dB. This can be more complicated if you’re also operating monitor mixes from the same channel, but I’m going to keep things simple here.

Underneath the gain knob, you’ll sometimes find a switch that says “+48v.” Shorthand for “48 volts,” this is the phantom power switch. If you’re running a condenser microphone through the channel, it will require phantom power in order to work. Always make sure to mute the channel before engaging or disengaging phantom power, otherwise you run the risk of damaging something along the signal chain and you’ll hear a nasty pop in the PA system.

Moving down the waterfall, you’ll find the EQ section, which gives you individual volume control over the high, middle and low frequency bands of a signal. Frequency, measured in hertz (Hz) is analogous to pitch (e.g., high frequency = bird chirping; low frequency = tuba playing, etc.). The human ear can hear frequencies across the spectrum of 20Hz to 20kHz.

First, you’ll most likely see a switch or knob that either says “low cut (100Hz)” or “HPF (100Hz).” This is the high pass filter. When this switch is engaged, all of the low or sub frequencies of the signal are bypassed from the channel. Many common sources going through a mixer do not contain audible frequency information below 100Hz. Although this information is inaudible, it still takes up space in the overall mix. Therefore, engaging the low cut switch is a quick way to create usable space in your mix early on. Some examples of instruments where you would likely NOT use this switch are bass guitar, kick drum, floor tom or any electronic signal that has bass-y or sub-y information. However, for a vocal, you’d almost always want to engage it, since it's rare that any human voice contains audible information below 150Hz. Instead of a switch, some mixers have a HPF knob, which allows you to fine-tune the specific frequencies you’d like filtered out. On a vocal, I would typically turn the HPF knob to about 150-180Hz, depending on my singer’s vocal range.

The most basic EQ section will contain three knobs that say “HI,” “MID” and “LOW,” each of which represent a particular frequency band. The “HI" knob will give you control over the clarity of the signal — the shimmer, the presence, the sizzle. The “MID” knob controls the nasal-sounding, honky, punchy zone. The “LOW” knob contains the boomy, bass-y, deep, sub frequencies. Consonants “p” and “b” exist in the low bandwidth, typically between 150 and 200Hz. These tend to have an intensely high attack, which can be distracting in your mix, especially if the vocalist has a particularly deep voice. To combat this, you might want to decrease the low knob a bit in order to soften their plosives. Conversely, if the speaker is soft-spoken and has a lighter voice, you could consider bumping the low knob just a tad in order to give their voice more body. Another common scenario in which you’d utilize the high knob is to control “s” and “t” sounds, which are often the culprits of high-frequency feedback in live sound. Careful though, as you’ll notice the more you decrease the high knob, the signal will sound duller and lose some of the clarity. A quick search on the internet will provide you with many common EQ techniques for a multitude of instruments and voices.

Another knob you’ll probably see in the EQ section is the bandwidth knob, commonly referred to as “Q factor” or “notch.” This knob will always correlate to either the high, mid or low frequency bands. You’ll usually see some kind of line connecting the Q knob and the knob for its corresponding frequency range. Usually, the most basic mixer will only have one Q knob, which is typically grouped with the mid frequency band. The Q knob is what determines the actual range of frequencies you will be decreasing or increasing. On some mixers, you’ll see a small graphic of a sharp, pointed peak on one end of the Q knob, and a wider, sloping hill on the other side. The shorter Q will cut or boost a narrower range of frequencies, while the more sloping Q will address a broader range. When I’m EQing a kick drum, I will usually employ a wider Q when decreasing mid frequencies, since I do not need many of them in my mix. On the other hand, upon EQing a cello, i’ll use a very tight Q to remove around 160Hz, which is typically where a cello tends to over-resonate.

Moving along, we find the aux send section of the mixer. Short for “auxiliary send,” this is where we introduce the concept of buses. A bus represents one of many destinations the signal can travel to. Sometimes we need a bus to carry the signal to an FX processor, or perhaps we need to send it to a monitor wedge onstage. You may have heard the terms “bus send,” “aux send,” “fx send,” “mixbus” or simply, “send” get thrown around — this is where things can get confusing. Different manufacturers love to use different terminology to describe the same thing. All of these terms indicate the signal being sent to a desired bus destination. The main left and right outputs of your mixer are buses. The mono output is a bus. Each FX master is a bus. The physical aux outputs that feed the cue mix or stage monitors are buses. There can be many or few buses on a mixer. The bus is the destination, while the aux send is the knob or switch that dictates how much of the signal gets sent to a respective bus. The two most common destinations for an aux send are a musician’s monitors and an effects or dynamics processor. Before it reaches its bus destination, an aux send will always pass through an aux master control, which we will go over in the “master” section at the end.

Next, we find the pan knob, which applies to mixing in stereo. We use the pan knob to allocate a certain proportion of the signal to either the main left bus or the main right bus, which allows us to create a stereo image of the main mix. Every engineer’s approach to panning will vary, especially when mixing live versus recorded sound. You may have heard the terms “hard right” and “hard left.” When a signal is panned hard right, it means the pan knob has been turned fully clockwise and there will be none of that signal in the left bus. This is something extremely noticeable, especially on headphones. The only situation where you’d always want to hard pan is when mixing a stereo instrument such as a synthesizer or sampler, which often contains sounds that fluctuate between left and right. The left channel would be panned hard left, while the right channel would be panned hard right. One of my favorite song examples of hard panning is “Such Great Heights” by The Postal Service. The song begins with a sample that you can tell is hard panned — I first realized it because my old ‘94 Honda Accord’s left speakers would often fail, giving a brand new rhythm to the sample, as I was only hearing half of it from the right speakers. Sometimes, if we’re struggling to get a vocal to stand out in the mix, we can slightly pan all of the other sources around it, leaving us with a stereo image of the vocal sitting in the middle. In a traditional mix, you’d probably never want to pan a lead vocal very far outside of the default 12 o’clock position on the pan knob.

The mute switch is what you’ll most likely encounter next. This is pretty self- explanatory. The mute switch cuts the signal off before it can go any further.

Nearing the bottom of the waterfall, we find the channel fader, which is probably what we’re all most familiar with. The channel fader is similar to the gain knob — it contains another stage of gain to the signal. The channel fader will have ticks and incremental numbers along its path that indicate the amount of volume in dB being applied or taken away from the signal. At the very bottom of the fader path you’ll see the minus-infinity symbol (-∞), which completely decreases the level of audio passing through the fader. So if you have your gain knob cranked up and you’re not hearing anything, you may want to check to see if the mute is disengaged and whether your fader is up to 0dB, meaning it’s neither attenuated nor amplified. Note that even when the fader is all the way down, the signal may still be passing through your aux sends and out to their respective buses. Remember, we’re following the path of a waterfall and the fader has no say over what happens to the signal above it! That is, unless we’re dealing with pre- and post-fader switches, which perhaps we’ll cover another time.

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Jade is a live sound engineer based in New York City. She has toured internationally with Khruangbin, Amen Dunes, Girlpool, Lucy Dacus and many more. Read her feature in the third issue of Women in Sound from 2016 here.

jadepayneaudio.com