Audio Compression
The techniques and use of compression in audio recordings can be one of the heaviest labored facets of studio music production. It is one of the essential ingredients in a recording’s ‘sound.’ As true of everything else, the use of audio compression requires lots of practice. The ear must train itself to identify and prescribe certain compression techniques. As you progress, the practices of these techniques will become more elaborate and, over time, you will gain confidence in experimentation and practice of these audio compression techniques.
It is important to understand the basic function and principles of the audio compressor. Audio compression, also known as dynamic range compression, controls the volume output of a given sound wave amplitude. By reducing the loudest volumes (measured in decibels), you can increase the overall volume (gain) and literally squish, or compress, the original sound into a smaller range of volume.
The aural effect of this compressed sound is commonplace in audio recordings. In fact, the radio, particularly AM radio, can be a great tutorial in heavy sound compression. A quick study of the various frequencies (particularly the high and low) should help reveal its effects. Audio compression is ever-present in television and very often music recordings are compressed specifically for television play. Compression also exists on a practical level in the form of mp3, CD, cassette tape, LP, 78, etc, each with its own specific natural compressions. No matter the format, compression is vital in expressing the character and dimension of a sound, in both its individual tracks and on the entire audio recording.
Audio Compression Parameters & Techniques
The point where volume limiting is set to begin is called the threshold. Volumes softer than the threshold will be unaffected, while the decibels above the threshold will be reduced. The amount of reduction assigned post-threshold is called the ratio, because it uses a ratio format to measure the amount of reduction. A 2:1 ratio indicates that any volume over the threshold will be reduced by half. A higher ratio will reduce the volume (over the threshold) by a greater percentage. It is common for an engineer or producer to use compression ratios between 3:1 and 8:1, though there are many settings and uses for heavier and lighter compression.
There are a number of other factors that play into using compression techniques. The knee, attack, and release will all determine how the compression will react to a signal. A soft knee, quick attack, and slow release will result in a more gradual compression than a hard knee, slow attack and quick release. There is an endless amount of compression techniques; including side-chains, which can serve to compress specific frequencies, or a compressor can be setup to be triggered (keyed) from a different signal than the signal it affects.
As usual, a user of audio compression will spend many hours listening and adjusting, refining his or her techniques. Understanding the basic principles, however, is the first step.
