Sound Advice: Mixing Multiple Mikes

Many of us have employed external microphones to isolate a single subject’s voice and get better sound quality. This is as easy as plugging a mike into your camcorder’s microphone jack. But what if you need to mike a panel of four people for a TV-style talk show? If you want to know what you’ll need and how to get it done, then you’ve come to the right place.

A multiple-microphone setup is necessary to cover interviews, panel discussions, talk shows, stage performances, sports commentary and news-style videos, but you can’t just plug all those mikes into your camcorder. To make the jump you’ll need an audio mixer and a recorder (not to mention some audio cables). To go with the tools, you’ll also need a few tips.

Cabling Up

Hooking up for multiple miking is relatively simple if you have the right tools. Simply connect the output of each mike to an audio mixing board. Each mike level is adjusted at the mixer, combined with the audio from the other mikes, then sent out to the recording device. The mixed output of the sound board routes back to your camcorder, or to a VCR or dedicated audio recorder (see Figure 1). If you use a VCR to record, it will be helpful to select a high-quality deck with VU meters, so you can monitor the record levels. To really know what is being recorded to tape, the recorder should also have a headphone jack. It is important that you monitor your audio at the recorder, not the mixer. It is possible for a bad cable or faulty connection to corrupt a signal, which is strong and clean at the mixer, before it reaches the recorder. The only way to know what you are actually recording to tape is to monitor with headphones at the recorder.

The Mixing Board

You don’t need to purchase an expensive mixing board to get good results when recording the human voice for video, but if you plan to record musicians, it’s worth a few more dollars to get a well-equipped board that offers a greater degree of control. Whatever you’re miking, you’ll need a board with enough inputs. A four-channel mixer will be sufficient if you’ll never need to mix more than four mikes, but it never hurts to have an extra input or two. An eight- or 12-channel mixer may be a better long-term investment.

Your mixing board (and hopefully your record deck) will have some kind of meter to monitor record levels. The VU meter provides a visualization to the amount of sound and the degrees of differences in the ongoing level changes. Some VU meters have a needle pointer that remains in motion as it indicates the varying sound levels. Some components feature a VU meter that consists of a bar or vertical column of light emitting diodes (LED).

Gain Structure

Each microphone input generally passes through three stages before it reaches the final output to the recording device. The setting of these three stages so that they work together to produce a strong clean signal is "Gain Structure."

The first stage is a small rotary knob directly after the cable input and in most cases labeled "Trim" or "Mike Level." The Trim is an amplifying stage that boosts the signal of the microphone itself up to a level that the mixing board can work with. Too much level at the trim stage can cause input distortion, a rattle or fuzzy sound that ruins an audio signal for good. Too little level at the trim and you must boost the other stages to get enough level, which adds hiss from the electronics in the mixer.

The second stage is the channel Fader. The channel Fader should, for all intents and purposes, be set at nominal (zero) so that the signal from the microphone is neither attenuated (lowered) nor boosted (raised) at the channel fader stage. A great misconception about the fader is that you are raising the level as you approach zero (nominal) when in fact the signal is attenuated until it reaches zero. The key is to let the signal pass the fader unaffected by setting it at zero.

The third stage is the master Fader where final output of the mixed signals leaves the mixer. Generally this fader (as with all faders) should be set at nominal (zero), but, depending on the recording device, the master may have to be attenuated so as not to overdrive the recording device input. This is especially true when the recording device is of consumer grade, with an input level of -10dB, and the mixer is a pro device with a much higher output level of +4dB.

The easiest way to achieve proper gain structure is to set the faders to zero and trim each microphone at the input in relation to the sound that is being picked up. You can always lower the faders of individual microphones or the master if needed but knowing that zero is the optimum place for the cleanest strongest signal is just good live audio mixing technique.

Monitor Carefully

We recommend using both meters and headphones to monitor your audio mix. Be careful not to lean exclusively on either of them. Headphones will let you know the quality of your signal (whether it is clean or noisy), but meters will tell you if the signal is strong enough to give you good level to tape. It is important to use meters and headphones together to insure good sound.

VU meters generally can bounce into the red by as much a +3dB and not distort. Most distortion appears at the mike input because of an overdriven trim setting.

Set Your Levels

Assign one microphone per person. This allows more effective monitoring and lets you adjust levels to evenly balance loud and soft talkers. During your pre-production setup, take a sound check from each participant. Have each person speak in a normal tone and set a beginning level for each mike at the mixing board. Because people have a tendency to talk louder when they know they are doing a sound check, don’t ask them to count aloud while you set your levels. Instead, start a conversation with each person, asking them to tell you what they do for a living, or to tell you about a pet or hobby. This will help your subjects relax and will give you a more accurate representation of each person’s speaking voice.

Don’t make the mistake of setting levels then walking away and leaving the sound board unmanned. It is important that someone stay on the mixing board for the duration of the shoot. If you know a person is not going to speak for a while, pull his mike level down. This prevents recording yawns, finger tapping, lip smacking and squirming sounds while an individual is waiting for another turn to speak. Be alert, however; you don’t want to ruin a take because a person suddenly does speak while his mike wasn’t up. "Riding gain" is the process of using the slider to maintain a consistent level of sound.

Another tip for successful mike mixing is to not have your switcher person be your sound person. Dedicate a full-time sound person. Nothing is more frustrating than getting to the end of a shoot only to discover that all your sound levels are mismatched, too low, or that a passing siren, flushing toilet, bad cable or dead battery ruined your sound track.

Just Do It

Don’t allow fear of technical wiring or components keep you from trying multiple microphone shoots. It’s surprising what an impact quality sound has on viewers’ overall impressions of your video. As soon as they hear exceptional sound quality, they perceive a much more professionally-produced video project. Sound is a production value that truly has an immediate return in your end product. So take the plunge and start mixing that sound.

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