Let’s imagine you’ve just gotten the most perfect audio recording ever, with no effects or filters having been added to it. The audio compression is therefore non-existent, and all the levels are great. The quality is awesome, and there are no unwanted background noises.
Now unless your talent are doing their lines completely deadpan and monotone with no inflection or dynamics changes, some sounds will be louder than others unless you use audio compression to fix the problem.
In post-production you can use the technique known as audio compression to make dialogue and other vocal noise easier to hear above the other sounds going on. Most digital editing programs have some kind of plugin that performs compression for you.
While you can use audio compression effectively in your indie projects, it’s also used in lots of ways in the recording industry. Listen to any major-label song you own, and you’ll notice that even in songs where the vocalist’s voice goes from very loud to very quiet, the loud parts aren’t ear-splitting and the quiet parts aren’t impossible to hear. This phenomenon is caused by the use of compression.
How Compression Works
In order to get the quietest parts of an audio recording loud enough, you need to boost the signal during
those parts. But trying to boost only the quiet parts would be tedious and time-consuming, so a compression filter boosts the signal for its entire duration.
The problem now is that the loudest parts are also louder by the same amount. To keep these loud parts
from clipping or distorting, the compressor employs a certain ratio of limitation at a threshold. In other words, it tones down the loud parts by cutting them off above a certain loudness that you specify.
Ratio and Threshold
To do this, you set the point where you want the compressor to start restraining the audio, say -15dB. Then you set the ratio of limitation; it might be 3:1 or 5:1 or infinity:1. Three-to-one, for example, means that for every three decibels the sound signal pops up above the threshold, the audio compressor only lets one decibel “get past.” A 1:1 compression ratio, therefore, isn’t really compression at all, and an infinity:1 ratio is called a hard limiter because it doesn’t let even one bit of sound get louder than the threshold.
Attack and Release
The attack and release settings are measured in ms (milliseconds), and are simply
measures of how quickly the compressor acts on audio that gets too loud and how quickly it stops acting after the audio has gone back below the threshold. Attack and release settings can be as little as 0ms, but setting them to act too quickly or with too much delay can cause the compression to be ineffective, so each piece of audio is different.
Going to Extremes
So now you may be wondering why you shouldn’t just use a hard limiter with 0 attack and 0 release, the most stringent settings you can push on your compressor. Well, it’s no different than any other time you alter a digital file, whether it be audio, video, an image, or another piece of media. When you do something that changes audio dramatically from what it wants to do naturally, you run the risk of making it sound “canned” or artificial.
Just try some extreme settings with your audio compression plugin and you’ll see what I mean. If you pull off the attack too much, loud audio will get through before the limiter turns on. If you take the release out too far, the limiter will stay on when it doesn’t need to be on and cause muffling or echoing. Using a hard limiter on audio that’s too loud already can make the loud parts crackle and sound over-modulated.
With a good audio compression setup, you can take lines of dialogue – or entire scenes worth of it – and smooth it out so that the volumes are consistent throughout. Audio is a tricky medium to work with, so there will never be a hard and fast rule on when to use audio compression and when not to. Each time you tweak a piece of audio you’re changing the fundamental signal, so use effects and plugins such as audio compression sparingly when possible, and with good judgment when necessary.
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