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What Is Dynamic Range Compression?
Dynamic range compression (often shortened to just “compression”) is a process that limits the volume range of a piece of music. This means that rather than have passages that are almost inaudibly quiet of ear-splittingly loud, a piece of music will slot entirely into a preset volume range.
Many people believe that an audio compressor both dampens loud sounds and amplifies softer sounds so. Traditionally, however, this is not the case. A traditional compressor only targets the loudest sounds and makes them softer. By comparison, this means the softer sounds will seem louder, even if they haven’t actually been touched.
However, there’s no doubt that some compressors really do make the soft sounds seem louder. They achieve this by doing two things:
- They raise the overall volume of everything in a musical performance. This process will make the softest sounds louder. (Many compressors include a gain control for this function.)
- At the same time, they will limit the volume peaks of the loudest sounds. The combination of these two effects shortens the distance between the softest possible sounds and the loudest ones.
How Do Compressors Work?
Compressors have four parameters that determine the sounds they produce:
- Ratio. This determines how loud a signal would have to be for the compressor to reduce its signal. A common compression ratio is 8-to-1. This means that for every 8 decibels of audio signal coming into the compressor, there is a 1-decibel increase in output volume. So the “loud” signals are still louder than the “normal” signals, but the difference between “loud” and “normal” has become compressed.
- Threshold. The point at which the compressor kicks in. Audio signals that are softer than the threshold will not be limited; audio signals louder than the threshold will be limited. Audio engineers sometimes call the threshold the “knee.” A “hard knee” is where there’s an abrupt cutoff between where a signal does and doesn’t get compressed. A “soft knee” means that the compression gradually kicks in, becoming stronger as the signal gets louder.
- Attack. The amount of time allotted before the compressor begins limiting a signal. If the attack time is very short, the signal will be compressed almost the moment a note is played. This will make the attacks sound muted. Increasingly the attack time will let a note sound loudly for an ever-so-brief moment before the compression kicks in. This creates a “snappy” sound that’s great for funk and country.
- Release. The length of time that a compressor engages upon an audio signal. When release times are long, the compression takes a while to fade away. This can produce an effect that’s close to reverb and is favored by instrumentalists who want to stand out in a band, such as a lead guitarist.
4 Types of Audio Compression
Compressors take on many forms in music, and these vary in popularity depending on genre.
- Traditional compressors are in regular use in music studios, and most recordings have some degree of compression. This allows engineers to make sure that all instruments will be recorded at levels conducive to mixing, such that a producer can make artistic decisions as they balance individual tracks. Typically these machines are multiband compressors, which allow engineers to isolate specific frequencies for more or less compression.
- Peak limiters exist to make sure that very loud audio signals don’t pollute a recording, causing distortion and unusable tracks. Engineers place microphones and set amplifiers in a way to minimize the need for a peak limiter, but most will keep one on as an emergency valve.
- Stompbox compressors are used by instrumentalists—particularly guitarists and bassists—to set their own individual compression levels. These devices are typically focused on the specific frequency range of the instrument they’re designed for. Depending on their settings, they can drastically alter the tone of an instrument and are perfect for some genres (funk, country) but wrong for others (blues, classical).
- Sidechain compression is a type of compression where the effect level on one instrument is controlled by the volume level of another instrument. A common example would be making the compression level on a bass controlled by the output volume of the kick drum. So when the kick drum sounds, the bass gets more compressed so it can keep cutting through the mix. Sidechaining is frequently used by dance, pop, and EDM producers, using software plugins like Nicky Romero Kickstart.
How Is Compression Used in Music?
Compression is on nearly every professional recording you’ll hear. It’s typically most subtle on classical recordings since classical music relies heavily on massive dynamic ranges. It’s most present on Top 40 pop recordings, where music is expected to hover near maximum volume at all times.
Here are some great ways to add compression to your own music:
- Vocal compression almost always improves a singer’s performance. Setting a relatively low compression ratio of around 4-to-1 can provide a nice subtle effect that brings out all aspects of a person’s vocal range.
- Guitar compression, using a pedal like the Ross Compressor or the Wampler Ego Compressor, is great for many pop, rock, and R&B styles. Try boosting your attack time for funky passages like the guitar lick in “Thriller” by Michael Jackson. Try boosting release time for epic guitar solos in the style of David Gilmour.
- Sidechain compression on a kick drum or bass brings out the throbbing pulse of an EDM song. The beat must cut through at all times in EDM (apart from the drum-free breakdown section that’s in nearly every EDM track). Sidechain compression assures this will happen.
- Overall compression is a key part of mastering—the final stage before a recording is considered “complete.” Mastering is done to a final stereo mix and so the compression applies across all instruments.
Learn more about music production in Timbaland’s MasterClass.