How to Master the Art of Digital Audio Mixing for Hybrid Events
Mixing audio for a venue is a well-documented practice. At its most basic level, it requires microphones, prerecorded audio sources, a soundboard, loudspeakers, and, most importantly, a skilled operator. The two most common challenges with live-venue audio are avoiding and eliminating feedback and ground-loop hum.
Mixing audio for a video recording or live stream involves many of the same techniques and equipment, but each has different production standards and you don’t have to worry with live sound reinforcement using loudspeakers. The two most common challenges with audio for a live stream are attaining a clean signal and mixing audio within a smaller range than you might if you’re only mixing for the venue.
Neither of these two is very complicated to explain and implement, although, in practice, mixing live audio can be very challenging because of room dynamics, electrical grounding issues, and, most importantly, microphone technique.
As a live-stream specialist, my perspective is going to be different compared to how a live audio technician would approach venue audio. The main reason is that I’m considering both the in-person audience and the live-stream audience equally and simultaneously.
In this article, I explain my approach to mixing audio for both of these environments at the same time. In my business, I refer to events that have both an in-person audience and a live online audience as a hybrid event, and the majority of the work that we produce is for hybrid events with dual audiences. Over time, we’ve found that we prefer to handle the audio for both audiences, rather than trust part or all of this to an external A/V company. The biggest reason is that the needs of the audience in the room are different from the needs of an online viewing audience, and if an audio technician is mixing audio only for the room and is not involved in or monitoring the audio for the live stream, the audio levels may not be within a desirable range.
So, with that lengthy preamble out of the way, let’s start with the basics of mixing audio for a live venue.
Step 1: Connecting audio sources
Connecting microphones and prerecorded audio sources to a soundboard is best done with balanced connections like XLR or ¼" TRS cables. The use of balanced connections helps reduce external noise from electromagnetic interference, which is especially an issue with longer cable runs.
Step 2: Setting the pre-amp levels
When you connect a cable to your soundboard, regardless of whether it’s a digital board or an analog one, you need to set the preamplifier levels for each input. The pre-amp levels can also be labeled as gain or trim. Some boards have a levels indicator that is green when you’re in the correct range and red if your signal is too hot. With all audio signals, you never want to clip your audio anywhere in your signal flow. This is similar to protecting your highlight when shooting video.
Step 3: Confirming clean audio
You can confirm clean audio with headphones or speakers. This is when you listen for obvious problems like a ground loop hum or a noisy or distorted audio signal. Ground loop hum can occur when you’re connecting A/C-powered audio devices together. Normally, equipment grounds to the A/C plug it’s directly connected to, but sometimes, due to differences in grounding potential, the device chooses the path of least resistance through your audio cables, not to the ground on the plug. This causes ground loop hum.
This electrical interference hum needs to be eliminated. The solution is to add a direct box (aka a DI, Figure 1, below) into the workflow to lift the ground and force the devices to ground, as intended, to their respective plugs. Using a DI should be done automatically when you are connecting audio from a remote laptop both to lift the ground and to convert the unbalanced signal to a balanced XLR connection.
Figure 1. A Behringer DI
Step 4: Setting PA speaker and soundboard volume levels
I start with my PA speakers set to one-quarter volume and then I increase the volume levels for each input and the main output. If your levels are set properly, you should be operating in a range where the levels for the inputs and output are just below the parity level of 0 on the level faders. If your fader levels are too high, then you don’t have any ability to increase the levels as needed, and if they have to be set too low, this means that your loudspeakers are set too high and the signal for the live stream won’t be within a desirable range.
It might take a bit of back-and-forth to find the right balance between the levels on your loudspeakers, input volume, and main output volume, but this is all a part of mixing professional audio. In this step, you are generally getting the audio into a proper range so that you can move on to the next step; you can then return to this step to get your final show levels set.
Step 5: Equalizing your audio
Generally, this step is part of Step 4 because, most of the time, you will find that when setting your audio levels, you get feedback before the audio gets to show levels. This is especially relevant when you have multiple microphones and especially with omnidirectional microphones like lavaliere mics. Feedback is what happens when certain audio frequencies bounce around the room and back into the microphones at a higher level than the other frequencies. This creates a feedback loop that instantly turns into a mounting squeal. It has to be terminated immediately by manually turning down the levels.
Every room is different in terms of which frequencies will bounce around more than others. Some audio technicians claim they can hear and identify the feedback frequencies, but I prefer to rely on simple frequency spectrum analyzers to identify exactly which frequencies are ringing. If I am using a digital soundboard like the Behringer X18 (Figure 2, below), I can consult the 100-band Real Time Analyzer (RTA) to monitor and identify the problem frequencies, then use the equalizer to turn down the levels only on those specific frequencies. This is more precise than turning down an entire range of frequencies, like you do on most analog boards, and is one of the reasons I prefer digital soundboards.
Figure 2. The Behringer X18 digital soundboard
Audio equalization (EQ) can be done on individual inputs or the main output as a whole. Normally, you would add a bit more EQ on those previously mentioned omnidirectional microphones that are more prone to feedback and less on a cardioid handheld microphone. You also want to be careful not to go overboard with carving out too much when you are EQ’ing, or the resulting signal may sound too much like it was recorded in a fishbowl.
On an analog soundboard, you’ll be more limited with how surgically precise you can get. Most boards limit you to only three controls: high, medium, and low. I find this too restrictive, but this is what most small-venue operators use, and it reinforces my preference to manage my own audio.
My favorite line of analog mixers is the Mackie ProFX V2 line (Figure 3, below) because the eight- and 12- channel models that I use have a seven-band stereo graphic equalizer, and this allows me to be more precise than with a traditional three-channel equalizer. Unfortunately, the new V3 line eliminated the graphic equalizer in favor of a compressor.
Figure 3. The Mackie ProFX V2 analog mixer
Before I moved to a digital mixer with a 100- band graphic equalizer, I used the Behringer FBQ1502HD 15-band EQ. Although it does feature feedback detection that lights up the frequencies that have the strongest signals, I found that there were too many false positives, and I got better results using an external spectrum analyzer. This might sound like a fancy piece of tech, similar to an expensive light meter for exposure, but I’m talking here about a free, downloadable cellphone app called Advanced Spectrum Analyzer PRO for Android (Figure 4, below) that is also ad-free.
Figure 4. The Advanced Spectrum Analyzer PRO app for Android
EQ’ing a room is as simple as slowly increasing the volume on your soundboard until you start to hear feedback. Then, using the RTA or a Spectrum Analyzer, you determine which frequency is spiking and causing feedback. Next on your graphic equalizer, lower that specific frequency. You can then increase the volume again to target the next-worst frequency.
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