How to Master Audio Mixing for Live Production and Streaming
Live audio is challenging. Unlike many of the issues that arise on the video side of the equation, none of the problems that affect audio quality—RF interference, feedback, ground loop hum, hiss, over-modulation—can be seen. All of these challenges take a knowledge and understanding of the problem to assess and fix them properly. On live productions, more often than not, the time to fix audio issues is as soon as you hear them starting to happen.
You might find yourself dealing with over-modulation on the input to the mixer, or bad equalization in your mixer. Your potentiometer or fader might be too high or too low, and you’ll find yourself trying to compensate for it later in the mixer. Or your master audio may be distorting, which means you need to try to tamp it down.
Finally, there’s the dreaded feedback. Most of these are problems you can learn to address by mastering the controls and features of a good audio mixer (Figure 1, opposite page), or— better yet—instructing presenters, talent, and other members of your crew on how to make proper use of the technology in their hands so that they send you a clean and usable signal.
Figure 1. The Behringer Eurorack UB1222FX-PRO professional audio mixer.
One of the most important things you can do to ensure good audio is proper microphone placement. For example, you can use a good lavaliere microphone on the body of the speaker, as opposed to a podium microphone he or she can walk away from, or a handheld microphone that he or she can wave around the air. You might even try a headset microphone if the venue is particularly noisy. These have become much more discreet and more socially accepted when the situation warrants it.
Also, be sure your source audio won’t overwhelm your microphone. Usually, this happens with music, for instance, when your performer is using a vocal microphone for drums or a harmonica and the audio distorts at the microphone. But it can also happen if you set up a microphone for someone who speaks softly, then hand it to a football coach who shouts everything to the back of the room. You can adjust the input level of your wireless microphone so that it picks up a good signal, but does not overmodulate.
With handheld mics, the as-near-the-mouth-as-possible rule applies. If you ever do weddings or other events, watch the DJ. DJs know how to work a microphone. A DJ might stand in front of the speaker—which will often cause feedback—but hold the mic extremely close to his or her mouth to avoid talking very loudly. The system will also be set so that unless the microphone goes into the speaker, it’s not going to create feedback.
When it comes to an instrument mic, naturally, the best approach is to put the mic on the instrument. On some productions you could encounter, say, an acoustic guitarist who wants to play to a mic on a stand. That’s a lot farther from the instrument than an on-instrument guitar mic, which will pick up the strings and the resonance of the chamber, and give you much better leeway in the rest of the chain to avoid feedback and other issues.
These are all areas in which effective mic placement—and aggressive mic guidance from you—can make a big difference in the quality of the audio you capture and deliver.
Overwhelming Source Audio
In situations where audio recording issues arise from source audio overwhelming the microphone in use, the first thing you want to check is the sound pressure level (SPL) of your microphone. Microphones have sound pressure level ratings, which vary depending on the type of microphone, the manufacturer of the microphone, and the size of the microphone.
One microphone that a lot of people use is the Shure SM-58. It’s a classic vocal mic that’s great for certain tasks and not so great for others. For instance, the SM-58 has an SPL rating of 94dB. To put that in perspective, some common approximate sound pressure levels include 110dB for an airplane taking off, or 100dB for a rock concert. If you’re working with the SM-58 on a live event and recording a loud instrument such as a kick drum or trumpet blaring right into it, that sound can easily overwhelm this mic and distort the audio.
With wireless mics, you’ll also find potential for distortion in the transmitter. You’ll need to adjust the input trim. This is one strategy many people either don’t know about or simply don’t like to fiddle with. But in theater productions, we do this a lot because we match a microphone to a performer. Whether handheld, lavaliere, or instrument transmitter, it will have an input trim. For instance, on the Sennheiser wireless lav system, you go into the menu to access the sensitivity of the microphone (Figure 2), and you can bring it down or bring it up depending on what your situation requires.
Figure 2. Sensitivity adjustments on Sennheiser wireless transmitters
If you’re working with a performer or presenter who’s going to sing or speak on stage, you’ll need to adjust the gain of the transmitter to his or her voice. If you’re going to clip that microphone to the horn of a trumpet, you’re going to need to set it differently.
If you’re adjusting input trim for a guitar, the setting will vary depending on how the microphone is used, whether you’re attaching an acoustic microphone or jacking into the guitar directly, as with an electric guitar. You need to adjust the sensitivity of the transmitter appropriately; otherwise, it can distort right away and there’s absolutely nothing you can do to fix it down the line.
To adjust input trim, have the presenters/performers speak or sing in a normal tone. If the level starts to bounce a little bit while the person is talking or singing, that’s good. If it sits, or if it just goes up and hits the top of the audio level indicator, it’s too loud. If they’re talking and it doesn’t light up, you’ll have to add so much gain later that you’ll raise the noise floor and hiss. The same principle applies to instrument mics. Have the performers play they way they will during the performance and set the transmitter’s input levels accordingly.
Dealing With Hum
Another very annoying problem with audio is hum. Usually it’s a 60Hz (in the U.S.) hum from AC power. It comes when the ground used for the audio is coupled to different things in different places. This happens, for instance, when the venue is mixing sound behind the stage and run a couple mic lines to you at the back of the house, and you’re also plugged into a circuit that’s not grounded in the same way the house audio is. Suddenly, the 60Hz hum crops up and invades everything. If you hear it when you plug in, or even touch, the long audio lines into your mixer, and it goes away as soon as you remove them, you’ve pinpointed the problem.
The first thing to try is a ground lift adapter. You should always have at least one of these in your kit, because they can help you solve so many different problems. That said, it’s a temporary fix, and not one you should rely on in a permanent setup.
If a ground lift adapter doesn’t solve the problem, try to work on the audio cables themselves. There are multiple ways of doing this. Simple accessories such as an XLR adapter can be put inline on the XLR cables to try and find which pin is carrying the offending voltage. Alternatively, there are more substantial transformer devices that completely isolate the in and out, while still passing audio. These devices can usually resolve the most stubborn hum problems.
Lastly, make sure power cables aren’t taped down along with the audio cables. That’s also a great way to pick up extra noise. They should be separated by at least a few inches if not a foot.
If you’re still working with old 700mHz wireless microphones on frequencies that have been allocated for other uses—including public service—you should replace these systems as soon as possible.
If you’ve moved on to systems that use approved frequency ranges, you’re less likely to encounter interference, but it can still happen under certain circumstances. For example, if you’re filming a lecturer who has his lav belt pack just a few inches from his cellphone, you’re going to get RF interference from his phone. He needs to leave it on the podium, or on a table. The farther the phone is from the microphone, the less it will affect your signal.
The same goes for your own crew. Don’t put your phone on the audio mixer, even if you’re using it to play music into your system. The RF noise can get into any part of the signal path.
Overmodulated Mixer Input
Another problem that arises frequently when you’re mixing audio for live events is overmodulation (or overmod) on the mixer input. Several factors can cause overmodulation, some easy to avoid, others a bit more subtle. To begin, if you’re feeding line-level input, don’t plug it into the mic jack. It will distort. Some mixers have a universal gain that you can dial down to avoid clipping. If yours doesn’t, you’ll need an external PAD to knock the line level XLR down to mic level. These should be in your kit and always stay with the audio mixer.
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