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When I watch videos of live performances, like rock concerts, I notice that when the vocalists use the microphone the distance to the mouth will vary somewhat, sometimes significantly, and I would expect this to affect the volume levels.

I have a very high quality Shure vocal microphone. When I use it, I notice I have to hold my head exactly the same distance from the mic, otherwise the volume level will vary significantly.

How do sound engineers maintain constant levels with the vocalist moving the mic around? Do they have some kind of automatic electronic system that keeps the levels balanced?

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This is probably a better question for Sound.SE, but anyway:

A good vocalist will be aware of their mic placement (it becomes habit pretty quick). In "loud" genres, rock for example, as an engineer, you typically want the mic very close the vocalist's mouth. The closer the vocalist (and louder the vocalist), the less gain you require, which improves your gain-before-feedback, which is typically very important in loud live concerts. So, in short, most vocalists are pretty good about this (and it's a nightmare to work with someone, vocalist or presenter, who doesn't know how to hold a mic). Also, the way humans project sound when singing loudly or screaming is very different than speaking. You'll find that when you're speaking into a mic or singing into a mic, the directional behaviour of your voice is drastically different.

Do they have some kind of automatic electronic system that keeps the levels balanced?

Compression is an audio process which reduces gain above a threshold, and sort of does this "level balancing" you speak of. However, heavy compression on live concert music is suicide for a sound engineer, as it drastically reduces gain-before-feedback, which is bad. While I might use a tiny hint of compression (or more likely, limiting) on a vocalist, it wouldn't be applied to such a degree where it would sound like a "level balancing" effect.

There are some special cases to consider:

If you are watching a video of a well run concert, and everyone on stage has in-ear-monitors (IEMs), the "stage volume" (how loud the stage area is) can be reasonably low. The audience has huge directional line arrays blasting their direction, but hopefully the stage volume is low because everyone has IEMs. In this case, the gain before feedback margin is higher, and a sound engineer has a lot more freedom to compress vocals, etc. In this case, the sound in the video might be a board mix, which is directly from the mixing console, not a camera microphone in the crowd, which gives a further boost in audio quality and (if well mixed) can let the vocals "shine" far more as they aren't overpowered by instrumentation.

Also, if you are watching a video and it sounds absolutely wonderful, perhaps it's not actually the sound from the concert. This is common in music videos and such.

Sorry for the wall of text, but I don't think there's a concrete answer. Hopefully this helps.

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  • ...and sometimes if you are watching a concert live, and the sound is great, you can watch carefully to see if the actions of the performers fail to coordinate to the sound you hear, and raise a fuss about being sold tickets to a live concert when what was offered was a recorded performance with performers pretending to play/sing.
    – Ecnerwal
    Apr 17, 2016 at 2:09
  • It's also worth adding that if you are doing pre-dynamic (compression) monitors, then you can much more safely use compression, even in an environment with wedges. It will limit the levels in the room, but won't really impact stage levels that much. That said, it needs to be carefully used in that case as the engineer is taking over responsibility for relative levels at that point. It isn't so key in a typical single vocals basic rock mix, but larger mixes can become messy without matching dynamics.
    – AJ Henderson ♦
    Apr 19, 2016 at 18:55
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What they said - plus:

If it is well done foldback* is provided to the performers, the variation in distance to the microphone is compensated for by a feedback loop via ears brain and mouth - often unconsciously.

A usually effective way to get vocalists to reduce their sound level is to increase foldback output relative to amplified output to the audience.

*Foldback is the provision of a monitor(or headphones) to the performer so they can hear their own 'output'. The performer will tend to maintain their output level such that the level "sounds good" to them. Increasing foldback gain decreases the performers level required to provide the fold-back monitor's "sounds good to me" level.

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  • Interestingly, I've never heard the term "foldback" used before. Everyone I know in the US just calls them "Monitors" whether using wedges or IEMs (in-ears). We do, however, user the term "talkback" for a line sent to monitors from the board operator, so I'd guess foldback has a similar origin.
    – AJ Henderson ♦
    Apr 19, 2016 at 18:52
  • @AJHenderson I don't know which countries use the term. Wikipedia knows it without specifying regionality of use. They do focus on different aspects of its use than level control. They also comment usefully on the term 'monitor' as opposed to main speakers - a distinction which is sensible but not usually made afaik. I'm not a "sound man" (despite having had too much to do with it over the years) so my knowledge base in this area is not to be relied on :-)
    – Russell McMahon
    Apr 20, 2016 at 0:23
  • Mains is normally the term used for the speakers facing the audience, monitors is the term used for sound routed to the stage/performers. Every production company I've ever worked with uses those terms.
    – AJ Henderson ♦
    Apr 20, 2016 at 1:04
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Considering that the above answer by @unit128 is a good one, a couple of extra tricks are used. Sound absorbing foam on certain walls plus clear shielding around the drum kit reduce echos from the walls and help trap the sound of the drums so only drum-specific mic's pick up sounds from the kick-drum, hi-hat, etc.

Outdoor concerts have no walls to contend with, but dampening the drums still helps. A small amount of reverb can improve the sound by removing brief periods of silence between guitar rifts, etc, making the sound 'warmer'.

Noise gates are used on drums and other percussion instruments so background noise does not 'leak' into an un-used but active mike.

Most of the sound volume comes from walls of speakers in front of stacks of 5,000 watt amplifiers. Because they are away from the stage and face the audience, direct feedback is not an issue so the sound levels can be really high.

Digital signal processing (DSP) can be set to detect any hint of feedback and adjust signal phase delays to help cancel it out. DSP is also used to partially compensate for the singer not having the mic in the right position. Little compression is used because with today's ultra high-power, ultra wide-band equipment it wound cause wierd sound level shifting.

Beyond a certain range from the singer the DSP MUST cut the sound level or a screeching feedback will occur. The singers must learn the fine art of mic position verses pitch and power.

For each location the sound engineer, musicians and singers must work out these details before the show starts.

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