Audio Compression Guide
The following guide will explain some simple things you can do to improve the audio of your movies.
There a few reasons why you may want to process your audio track. Sometimes a movie can have some very quiet parts and other parts that are too loud.
This happens a lot for school plays and recitals. In this case you may want to apply a little compression to the audio track.
Compression or limiting, is a process that manipulates the dynamic range of an audio signal. This helps make all audio appear to be at the same volume.
The following guide will use Audacity. Audacity® is a free, open source software for recording and editing sounds.
In my editing software, Sony Vegas Studio I can easily export the audio portion of my movie into a WAV file.
- Click on Make Movie / Save To Hard Drive
- Click on the Advanced Render Button
- Select Wave (Microsoft) .wav from the “Save as Type” drop down box
- Click On Save
- In a few minutes you will have rendered the audio portion of your video to a wave file
For other editors consult you owners manual or online forums.
Compression
Let’s take a look at the sound wave in Audacity. In the following example there were two presenters at a presentation.
The first speaker was quieter than the second speaker. You can see this in the sound wave.
The easiest way to understand how a compressor works is this. Think of it as an automatic way to adjust the volume control on your TV. For the quiet part you may turn up the volume. For the loud parts you may turn down the volume.
There are two primary parameters that are adjustable when you run a compressor on an audio track. The compression ratio and the threshold at which the compressor starts to process the signal. For video I usually use a ratio of 3:1 or 4:1. That is, with a ratio of 4:1, if the input level is 4 dB over the threshold, the gain will be reduced so that the output level will only be 1 dB over the threshold. If you use higher compression ration you can start to make the sound unnatural or introduce “pumping”. Let your ears be your guide for your situation.
For the threshold I usually change the scale in Audacity to DB’s an look at the waveform.
From the waveform I can see that most of the large volume spikes occur above -20 to -30 DB. In this Example I am going to set a threshold of -30DB.
- In Audacity click on Effect>>Compressor from the menu.
- Set the threshold and compression ratio.
- The attack time is how quickly the compressor will react to the signal once it hits the threshold. I usually set it to the default or the minimum settings.
- Leave Normalize to 0dB checked. If you don’t the overall volume of your audio will be reduced and you will need to normalize it afterwards. This will save that extra step.
Here is what the final waveform will look like. You can see that the overall volume level is a bit smoother. I didn’t overdo the settings so it still sounds natural.
2 Comments »
RSS feed for comments on this post · TrackBack URI
Glenn said,
December 19, 2009 @ 1:23 am
I can’t get the same results.
Some of the largest peaks seem unaffected while lower ones are reduced.
Settings = Thresh -17, Ratio 4:1, attack .1
Also the scale is different from the initial to your final example.
Mine is like the early ones with the ” in the center.
Somehow in the final example you have the 0 at the top and 36 at center?
Glenn said,
December 19, 2009 @ 12:49 pm
Ok it’s likely I didn’t have the latest update.
I’ll try with the full pallet of Comp. functions.