Home > JoeRants > Audio stuff, pt 2

Audio stuff, pt 2

October 19, 2012

The other day, I started talking about recording audio on set (and here’s a link to that article).

I’m starting this series with the technical audio stuff – what settings should you set for your recorder. Then we’ll get to the aesthetics and how to get good sound. But that’s few posts away.

In part one, I talked a lot about line level, and today will pick up with mic level.

Surely mic audio is all the same level, right? Of course not. Mics are even worse than line level. Basically, whatever the output of a mic is, that’s the level of that mic. It runs a certain gamut, which is “mic level,” but there’s a huge difference between the levels output by different mics.

Some mics are really quiet and need a lot of boost (most mics without batteries are like that), while others (usually the ones with batteries) are more powerful and don’t need as much boost. A mic is almost never as powerful as line level, not even close, but they can vary pretty wildly.

So let’s talk about a “pad.” You hear that term a lot amongst audio guys, and might even have seen it on your audio gear or upper-level cameras.  The pad is actually named fairly well – it’s a pad that keeps the audio signal from being more powerful than the equipment can handle.

This is a good time to bring up what can happen if you record any audio at the wrong level. Can’t you just turn down the volume with the level knob? Maybe, but usually no, not really.

Here’s why: your recorder is set to record audio that’s within a certain set of levels. Anything too low won’t record well, and anything too high will overload the circuits. Even before the audio gets to the volume level knob where you can adjust it, the level might be too powerful, and it will overmodulate. You’ve heard overmodulation – it’s that scratchy sound when the audio level is too high, and the signal breaks apart. So even though the meters may tell you the signal’s fine, the sound will “crunch” and overmodulate before it gets to your knob.  The signal’s just too powerful for the recorder to handle, no matter where you have the level set.

So you have to set the level as it comes into the recorder so it’s in the right levels.

Many audio recorders either let you select the difference or have a “Pad” setting, which will automatically lower the input level, letting you safely record from a more powerful mic. For you camera guys, think of the pad as an ND filter – it takes a signal that may be too bright/powerful, and gets it down to a level that the circuitry can handle it.

An overly simplistic rule of thumb is that if the mic needs a battery (or phantom power), it probably has some power, and the pad should be on. If it doesn’t need a battery, turn the pad off. By the way, I’ll briefly mention phantom power in the next post, and why it’s a nice feature, but if you don’t know what you’re doing with it, just leave phantom power off.

Another name for a pad is an attenuator. But when you see “attenuator” on your equipment, it often comes with more options than on or off.When they use the more technical term, you tend to get more technical options.

When you have more options, that’s where it gets tricky. The controls are usually measured in dB (again!), and those settings just mean “how much do you want to lower the signal by?” Generally, 0 dB means no attenuation – the pad is off. The other settings need explanation.

In the Marantz PMD 661 (my current-favorite portable recorder that isn’t ridiculously expensive), there are 4 levels: 0, -6, -12, and -18. So what do you do with that?

So the 0 setting is off, and is for unpowered mics, recording things that aren’t ridiculously loud. If you’re recording explosions, space shuttle launches, or half the cars that drive by my house, then you might want to add a bit of attenuation (maybe -6, possibly even -12), because even though the mic may not be that powerful, the sound you’re recording is, and the attenuator keeps the level in check. But most of the time, except in extreme situations, 0 is the right setting.

For other mics, experimentation is best. Sometimes the spec sheets with the mic will tell you pad or attenuation that you should set for. Then set the recorder’s level to the mic’s level, or a bit more. You want them to match as close as possible, without going over, like the Price is Right.  So if your mic specs say pad -10, then you should set for -12, which is the closest, but won’t allow the mic to overmodulate.

Setting for -12 seems to handle a large number of boom mics, but check your specs (or google them) and also experiment. A lot of lav mics don’t need any padding, but again, experiment. Do some sample recordings with your equipment and listen for that “crunched” sound of overmodulation.

And again, take your source into consideration. A scene between two character whispering may need a different level attenuation than a scene between two characters screaming. A door closing and a door slamming -world of difference. An acoustic guitar recording and an orchestra need greatly different settings.

That’s how you set audio levels for a mic. There’s still about another day’s worth of tech crap to get through, then we’ll start talking about how to USE the equipment, and how to get good sound. We’re getting there, I promise.

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  1. October 24, 2012 at 10:03 pm
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