Sound Principles:
Compression

By Jay Frigoletto

In the art of mixing audio, one of the most frequently used processors is one of the least understood by aspiring engineers. The compressor belongs to a family of tools known as dynamic range processors. Some other members of the dynamics family include limiters, expanders, noise gates, de-essers, maximizers, and companders.

Dynamic range is the range from the softest to the loudest level. Compressors are designed to reduce the dynamic range of signals by continually detecting the input level and applying a set gain reduction at the output. It’s like having somebody riding the fader up and down on your console. The quiet sounds get louder and the loud sounds get quieter, and the difference between soft and loud gets smaller. This can help with an erratic bass player, or a vocalist who occasionally sings a loud passage in an otherwise moderate track.

One of the early uses of dynamics processors was for radio broadcast. As the transmitter power for stations is limited, compressing the broadcast signal allowed for subjectively louder broadcasts and increased modulation that helped the broadcast carry. It didn’t take long for audio recordists to realize the benefits of adding compressors to recording chains. It helped get the signal above the high noise floors in early recording formats. Many units also had an interesting sound apart from the dynamic range control, and some engineers sought out certain units for those pleasing characteristics. One such unit is the Teletronix LA-2A (pictured), which is still highly prized today.

Several controls can be present on a compressor including input level, threshold, ratio, attack, release, and output level (sometimes called makeup gain). The input level controls how high a level you send to the unit. The threshold will control at what level the compression will start to take place. Below the threshold, no gain reduction takes place, but when the signal crosses the threshold, the compression kicks in. The ratio is how strong the gain reduction will be. A 2:1 ratio means that an input signal above the threshold will be reduced by half.

The attack controls how fast the gain reduction will begin. If you set it to slow attack, brief transients will be allowed through without attenuation, and only longer, more sustained sounds will have gain reduction applied. The release controls how quickly the gain reduction is backed off after the signal falls below the threshold. Set this wrong and you hear “pumping and breathing” as the level fluctuates. Finally, the output control lets you make up for the gain reduction that took place.

Some of the classic professional compressors include the Urei/Universal Audio 1176, LA-2A, and the dbx 165. Popular newcomers include the Empirical Labs Distressor and the Avalon 737, which also includes a mic pre-amp and EQ. Many mastering engineers use the Manley Variable-Mu. However, you don’t need to sell your car to get a decent compressor. Some very affordable units have come to market that sound remarkable for the money, like the RNC (Really Nice Compressor) from FMR audio; dbx also has several high-quality units that won’t break the bank, including the 1066, 160A and 166XL.

Compression can add the “glue” to a mix, level out an erratic performance, or enhance “punch.” It can also suck the life out of a performance, squashing the human element, destroying the rhythm, and obscuring details behind a wall of homogenized noise. Don’t use compression to avoid having to move the faders, but don’t be afraid to experiment. Once you master compression, your mixes will sound more polished and professional, and your palette of creative sounds will greatly expand.

A Few Good Compressors:

Universal Audio 1176 (mono) $1,995 list, $1,595 street
Universal Audio LA-2A (mono) $3,195 list, $2495 street
Empirical Labs EL-8 Distressor (mono) $1,899 list, $1,300 street
Avalon 737 (mono with mic pre & EQ) $2,295 list, $1,995 street
Manley Variable-Mu (stereo) $4,000 list, $3,600 street
FMR Audio RNC (Really Nice Compressor) (stereo) $199 list, $175 street
dbx 1066 (stereo) $599 list, $399 street
dbx 160A (mono) $599 list, $399 street
dbx 166XL (stereo) $359 list, $249 street