Compressors And Expanders For Recording Music

Compression and expansion are necessary tools for the music recording enthusiast and not as complicated as they seem.

Compression and expansion are key principles to audio mastering, and can be handled in a variety of ways by a variety of devices, analog and digital, hardware and software. The key principles remain essentially the same, regardless of the tool used to achieve the desired effect, and it is perhaps best to explain compression first, as it is the effect more often found useful.

Reduced to simplest terms, audio compression involves a reduction of volume as it becomes louder, namely after it passes a particular threshold which may be indicated by the user. This can be done for a variety of reasons, namely that it has the effect of making a track sound more even and consistent. A particularly animated conversation may vary greatly in volume, and while this does not sound unusual to human ears, a microphone only a short distance from either speaker can record sounds varying incredibly in volume. This is neither pleasant in a recording nor always technically practical. Broadcast media have strict limitations as to what volumes can and cannot be transmitted, and a recording is similarly limited by its maximum volume.

Compression actually works by dynamically changing the volume routed through the device according to its changing intensity. The rate at which this change occurs is determined by two variables the user can usually set: "attack" and "release". Attack is the speed at which the compression sets in, while release is the speed at which it returns to normal, or a state of no compression after making its change. These are used to fine-tune the results of the effect placed upon the audio sample.

As I mentioned, compression is valuable in part because it can be used to make a track's volume more regular. This is especially valuable when one is making the very most out of an allotted range of volumes. For instance, when recording music, one is often confined within a decibel range, while one may have a bass track, a drum track, and one or more guitar tracks, as well as vocals. A drum produces short bursts of sound, while a rhythm guitar track might maintain a steady volume, while a vocal track tends to become loud in places and very quiet in others. The rhythm guitar track might not need compression, already being "compact", but the vocals and drum tracks may need to be limited in volume so all the tracks can fit into the same piece without it being very quiet overall. Another approach is to mix all the tracks together, and then compress the entire track, allowing the drums to be comparatively louder when the guitar or vocals become quiet, for example. Usually this change in volume is unnoticed to the human ear, which is accustomed to making up for subtle changes in volume.

A similar effect is often employed in advertisements. The FCC has guidelines which limit the volume at which a recording may be broadcast, but some advertisers may compress their audio heavily so as to achieve the highest overall (and thus perceived) amplitude of sound.

Expansion is essentially a negative degree of compression""quieter sounds become quieter, while louder ones become louder. This is useful in achieving certain percussive effects.

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