The Basics of Mixing Live Sound: a Brief Tutorial
Learn how your mixing console worksIf you have been asked to run sound for a buddy's band, but you're not sure what all the knobs, push buttons, and sliders (faders) do, you've come to the right place. Most likely someone in the band will set up the P.A. system for you, or at least show you how everything plugs together. Your main job will be to man the control center of the sound system: the mixing board.
All too often a band will put a novice in this position to, as they say, "babysit" the board. By this they mean to occasionally turn up something at their direction, or just be there in case of feedback, to turn something down. That is unfortunate, because a sound tech can often either make or break a band. You are as important to the band as any musician in it.
Since every mixing board is different, this webpage is not designed to replace the manual for your mixer, rather, to help you understand it. For example, the manual may tell you that a button selects between pre-fade and post-fade, and here you will learn which one you need for what. It is recommended that you read this first, and then look up the online manual for the mixing board you'll be using, and familiarize yourself with the specific layout of that board. That said, let's get started.
Understanding channel strips from top to bottomEach audio input, from a microphone or an instrument, for example, has it's own unique channel, and every full-featured channel has it's own vertical strip of controls that either modify or route that channel's audio signal. Once you know how one channel strip works, you'll understand all of them, making a large mixing console seem far less intimidating. We will discuss the channel strip's controls in the order in which they usually process the audio signal: starting at the top and working down to the bottom. Obviously, not all mixers will have all of the common features mentioned here.
Input types, Insert, and Phantom PowerThe two most common inputs are the 3-pin balanced XLR type, and the unbalanced TS (Tip/Sleeve) 1/4 inch jack. Some microphones, condenser mics in particular, require external power to work. For this you turn on Phantom Power, a switch that is usually next to the mixer's main power switch. Turning on Phantom Power will have no effect on inputs that don't need it.
The insert jack allows you to insert an external effects processor, so that the audio signal will run out to and through the processor, then back to the mixing board. A common example of use would be to add dynamic compression to the kick drum. The insert jack accepts a 3-contact TRS (Tip/Ring/Sleeve) 1/4 inch jack with a cable that splits into two TS jacks for the processor's input and output.
Gain, Input Sensitivity, or TrimThe gain control, also sometimes referred to as sensitivity, input level, or trim, is perhaps the most misunderstood, and yet perhaps the most important, control on the mixing board. Often mistaken for a volume knob, the gain is used to level out, or set, the input signals from the various sources to fairly equal strengths that are strong enough to provide a good signal to noise ratio without clipping or distortion. This will be discussed in more detail below where we talk about setting levels before a sound check.
Low Cut/High Pass FilterThis button will filter out low frequencies. This is often used for vocals and guitars, where frequencies below 100 hertz are mostly stage rumble and noise, and are not part of your targeted signal.
Graphic Equalization (EQ)It has been said that there are no rules when it comes to mixing audio, only quidelines. When it comes to equalization, trusting your ear is probably the best guideline. Another good one to try is cutting frequencies around 130-140 Hz on vocal channels. This may help clear up muddy sounding vocals.
Aux sends and the pre/post fade optionIn live sound reinforcement, pre-fade auxiliary sends are typically used to send a custom mix to each stage, or an in-ear, monitor. If the bass player, for example, is using a monitor plugged into the aux 3 send, and wants just the guitar and lead vocals in his monitor, simply turn up the aux 3 knobs in the two channels where the guitar and the lead vocal microphone are plugged into.
You will want to find out ahead of time how many monitors the band uses, and who's monitor is hooked to which aux send. Then you can ask each musician what they they want in their monitor mix. Before setting the level for each channel, starting with all aux knobs turned all the way down, go through the horizontal aux 1 row, turning each desired channel for that aux send half way up, repeating for aux 2, and so on. Doing this will give you a substantial head start on the level set process. Also, remember that each aux send usually has it's own master volume that will need to be adjusted.
Some aux sends allow you to choose between pre and post fade. Choose pre-fade when you use your aux sends for monitors. This will prevent your monitor mix from changing when you use the faders to adjust the front of house mix. Post-fade aux sends are typically used for effects sends.
Effects (FX)The use of effects is too broad of a topic to cover here in this discussion of basic concepts. If your mixing console offers on-board effects processing, it would be to your advantage to read that section of the manual. Some of the more popular effects include reverb, delay, chorus, flanger, and compression.
Stereo PanningStereo sound can only be effective when the listener is positioned to hear both left and right channels equally, but in many live sound situations, mono, or center-panned sound, is recommended. To people on one side or the other in a small venue, stereo panning would just sound like the speaker closest to them is cutting in and out.
Main and Subgroup routing buttonsIn most cases you will depress the Main button to route that channel to the front of house speakers. Alternatively you can route a group of channels, all of the drum mics for example, to a subgroup, and then send that subgroup to the mains. This way, not only can you control each drum mic channel independently, you can also turn the entire drum kit up or down with one subgroup fader.
Setting the sound levelsOur goal here is to adjust the input level one channel at a time, using the gain control, until the level meter tops out around the zero mark, occasionally jumping into the yellow LED's, while staying out of the red. Start with all of the gain controls turned all the way down (counterclockwise), and all of the faders slid to the zero (unity gain) position. Depress the solo button for the channel you are about to set, and depress the PFL (pre-fade listen) button on the level meter. In PFL mode, the meter will work, and you can listen through headphones if you wish, with the main volume fader all the way down.
Now have the musician who is plugged in to the channel to be set sing or play at their top volume while you turn up the gain and watch the level meter, stopping when the meter is hitting where you want it, and you're done with that channel. Turn off the Solo button for that channel, and on for the next. Repeat for each channel until all channels are set, then press off the PFL button. You are now ready for the band to do a sound check.