How to Become an Audio-Mixing Ninja

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, and they 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 microphone that’s great for certain tasks and not so great for others. For instance, the SM-58 has an SPL rating of 94dB. (Some common approximate sound pressure levels include 110dB for an airplane taking off, or 100dB for a rock concert. One reason certain jobs require mandatory ear protection is that your ears are only meant to withstand certain SPLs for limited amounts of time before you risk hearing damage.) If you’re working with the SM-58 on a live event, and recording a loud instrument like a trumpet, kick drum, or harmonica blaring right into it, that source can easily overwhelm what this mic can capture without distortion.

With wireless mics, the next potential for distortion is in the transmitter. , You need to adjust the input trim. This is one strategy most people don't know about, or tend to forget because it's difficult to do. But in theater productions, we do this a lot because we match a microphone to a performer. Most lavalieres, such as the popular Sennheisers, have an input trim. On the Sennheiser, you go into the menu to access the the sensitivity of the microphone (Figure 1, below), and you can bring it down or bring it up depending on what the situation requires.

Figure 1. 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 their voice. If you're going to clip that microphone to a trumpet, you're going 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 feeding it an acoustic microphone or jacking into it directly with an electric guitar. You'll have to adjust the sensitivity of the transmitter appropriately; otherwise it will start to distort right away and there's nothing you can do to fix it.

To adjust input trim, have the presenter/performer speak/sing in their normal tone. When the level starts to bounce a little bit while the person is talking, 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, then you'll be adding so much gain later, that you'll raise the noise floor and hiss. Whatever wireless system you use should include information on this in the manual.

You’ll also need to see if you can make an adjustment on the audio out of the wireless receiver. Each of these settings has to be right so that you have clean audio all the way through the chain.

Over-Modulated Mixer Input

Next, we’ll look at over-modulation (or over-mod) on the mixer input (Figure 2, below). First things first: If you're feeding line-level input, don't plug it into the mic; otherwise, you'll have to bring the gain way down. On the other hand, if you feed a mic level into the line-level jack, you’ll find yourself cranking everything up and wondering, "Why am I not getting any sound out of this?"

Figure 2. Over-mod on the mixer input.

Bad Equalization

If you're adjusting the EQ (Figure 3, below)--for instance, you want someone to have a big booming presence in the room--like it's resounding through the speakers--and you increase the gain of the low frequencies, you'll notice that the input level trim you had previously set is now considerably higher.

Figure 3. Sound mixer EQ.

Once you crank the bass up, you've got to turn the input gain back down so that you'll have the proper level again. The same goes for highs--although you won't notice them as much--but they can get kind of sharp and shrill if your high frequencies are too bright.