Audio Post 5: Compression

Posted by bsteel on
Monday, March 3rd, 2008 at 2:57 am

A crude definition of a Compressor would be to say that it’s an audio signal processor that acts similarly to a volume control knob. When interviewing a subject sometimes the subject speaks softly and other times loudly. The subject may move closer to the mic or farther from the mic therefore the audio signal level changes. Compression helps to compensate for these dynamic changes from your audio source by keeping the source audio signal at a constant volume. Thus, eliminating the inconsistent volume or dynamic changes throughout the interview. The purpose of the compressor is to make the loud parts softer and soft parts louder.

Be extremely careful when using compression, because excessive compression will rob the interview of expression, vocal inflections, and natural tonal qualities thus leaving your interview subject soulless. Remember compression also affects all of the additional noise within your source audio tracks. So, when compressing an audio track with excess environmental noise keep in mind that you risk raising the volume of this noise in addition to your target audio track. One way to prevent this from phenomenon from occurring is to filter out excess noise as best as you can before compressing.

There are four parameters common to most compressors Threshold, Ratio, Gain Reduction and Output Gain control knobs. Here is an explanation of each parameter:

Threshold determines the level at which the compressor will begin to process input audio signal. When the audio input signal rises above the set threshold, the compressor begins to work.

Ratio controls the amount input level to output level. Thus, 2:1 ratio means for every 2dB (decibels) entering the compressor above the set threshold, 1dB will be output. This feature instructs the compressor how much to compressor or the amount of gain reduction.Gain Reduction/Peak Reduction is the number of decibels that the input level is reduced by the compressor, and is sometimes substituted for ratio.

Output Gain enables the user to set the output level of the compressor to the desired volume level. This feature also enables the user to compensate for volume loss due to compression.

Two more common features you will see on compressors are attack and release times. These features are far too advanced for our purposes so I will refrain from discussing them in detail. Although if you encounter attack/release controls on a compressor, I suggest leaving these controls on the ‘auto’ or ‘default’ settings. Below are two pictures of some excellent digital audio compressors.

In my next post, I will combine both the audio theory and tools from this post into a lesson on useful tips and tricks to sweeten audio content. I will apply the material for my next lesson in context to iMovie and Final Cut Pro. Stay tuned!

-Brian Steel, Director, Spielberg Group