Compression

Everything you need to know.

Compression is one of the most common processing techniques used during recording and mixing. At the same time, it is one of the most difficult processes to understand, both in terms of its function and its significance and usefulness, especially for new sound engineers.

In short, compression reduces the transients in sound peaks, meaning it lowers the volume of the louder parts of the music, and as a result, the quieter parts of the music are heard louder overall.

Often, the compressor is used to shape the sound in complex and creative ways.

Settings

Most compressors have the following settings, though they may have slightly different names:

Threshold

This is the level above which the compressor starts to work. It is usually measured in dB. Lower threshold values increase the amount of compression, as a large portion of the signal exceeds the threshold, causing the compressor to work continuously. For example, with a threshold of -20 dB, the signal would be almost continuously compressed. With a threshold of -3 dB, only a few peaks would exceed the threshold, and the compressor would work minimally.

Ratio

This is the signal ratio between the input level and the output level once the signal passes the threshold. For example, a ratio of 4:1 means that if the signal exceeds the threshold by 4 dB, the output will only increase by 1 dB instead of 4 dB.

Attack

The amount of time it takes for the compressor to start working when the signal exceeds the threshold. It is typically measured in milliseconds (ms). Use a slower attack when you want more transients to pass through without compression (for example, allowing the initial hit of a snare drum to pass through).

Release

The amount of time it takes for the compressor to return to the normal signal once the signal drops below the threshold level. It is typically measured in milliseconds (ms).

Makeup Gain

This restores the signal to a similar level as before compression. This results in raising the lower-level signals and allows you to compare the effect of compression on the signal by comparing the pre- and post-compression levels at the same volume. Naturally, some compressors have more or fewer controls.

For example, the classic Telectronics LA-2A has only an input gain (which essentially controls the threshold), peak reduction, makeup gain, and a switch for choosing between "compression" and "limiting." Attack, release, and ratio are not adjustable.

Compression & Limiting

Compression and limiting are more or less the same thing. The term limiter is usually used for a device that compresses with a ratio greater than 10:1. In general, with compression, low-level signals are raised, while limiting restricts the peaks.

A Brick Wall Limiter is a special type of limiter with an extremely high ratio (50:1 or more), usually used with a high threshold.

The Look Ahead Limiter is a digital Brick Wall Limiter that continuously "looks ahead" at the signal and "predicts" the peaks that exceed a given threshold. This allows algorithms to be constructed that ideally control the music peaks in a way that makes the compression less audible, though within certain limits and depending on the music. Unfortunately, this technology in our time has created intense competition, pushing CDs to sound very loud, with disastrous effects on music and sound quality.

Classic digital limiters, and the first to appear, are the Waves L1 and L2.

Soft Knee and Hard Knee

The knee refers to the shape of the compression curve around the threshold. With a soft knee, the compressor works in a more musical and natural way, without abrupt fluctuations in signal limitation. In hard knee mode, the curve is sharp, providing immediate compression, which is sometimes desired, but it requires great caution, as improper settings can often result in an unpleasant sound.

Sidechain

It is a technique that uses an equalized version of the original signal entering the compressor to "guide" the compressor to work more or less based on certain frequencies.

A characteristic example is using a compressor as a de-esser, or by emphasizing, for example, the low-frequency areas, we guide the compressor to work more when the signal has a lot of bass and less in other areas.

How much compression should we use?

Compression largely depends on the original material, so there isn't a specific amount of compression that will work for all material. Some compressors have presets for certain types of sounds, and these can be a good starting point for beginners, but remember that you need to adjust the input/threshold to make them work properly because every recording has different headroom and dynamics. Generally, you'll get the best results by learning how to use compression and understanding how the settings affect the audio signal. Experiment and listen!

Potential Problems

Noise Increase

When you reduce the peaks of a signal and then add the same relative amount of makeup gain, you increase not only the music but also the noise that exists at lower levels. That ventilation fan you forgot to turn off in your live room could go from being non-existent to an annoying hum if you overcompress and add too much makeup gain.

Pumping

This is the "swelling" and "deflating" of the sound due to poor compressor settings unless you want that effect. For example, after a snare hit, if the compressor has too fast a release and the rest of the mix comes back too quickly after the hit.

To avoid this effect, you can:

Set a slower release,
Use a lower ratio,
Set a slower attack,
Use a higher threshold.

Each of these adjustments has different effects on the signal, so listen carefully and decide which sounds best and gives you the result you want. If you train your ear, you'll notice that almost all radio signals have an "acceptable" amount of sound with the characteristic "pumping" effect.

General Compression Settings Guidelines for Various Musical Instruments

In general, the compressor is used as follows for various instruments, although the rules are not always the same, as the needs and recordings vary significantly. Additionally, depending on the type of compressor (VCA, Opto, etc.), you get a different character with similar settings.

Vocals: Fast attack and release; the ratio depends on the recording and vocal style, usually set to soft knee mode.

Guitars: Typically, fast attack and a somewhat fast-to-medium release.

Bass: Slow attack and fast release, allowing the first note to pass and emphasizing the transition of the hit. Of course, if you want to smooth out the bass and blend it into the track, use a faster attack.

Drums: Compressing drums is an art form. Different amounts and styles of compression can completely change how the drums sound. If you want huge snare drums or a more delicate performance, knowing how to compress can save you from a weak-sounding mix. Attack and how long the transient peak (decay) lasts is the most defining part of a hit. If you use a slower attack that starts compression right after the transient peak, the hit will be emphasized. Combined with the right release for the track, this can make the drum sound last longer, fuller, and more impactful.

This is a very general overview, so you need to experiment and listen to find the desired sound.

Different Types of Compressors

There are different types of compressors that determine their construction and operation, which are directly connected to the resulting sound.

Opto (Optical Compression)
Opto Tube (Optical Tube Compression)
VCA (Voltage Controlled Amplifier)
FET (Field Effect Transistor)
Multi-band Compression
Digital Compression

Conclusion

The only true judge of whether a compression setting is correct is our ear. Listen carefully to how the attack and release settings work, and go to extreme settings before dialing back to a good sound—it's the only way to start truly hearing how a compressor works. If you're unsure about a setting, it's better not to use compression at all, as it's very difficult to fix bad compression in mixing or mastering. Of course, high-quality, revealing speakers are needed to provide an accurate picture of what's happening. Unfortunately, affordable speakers used in home studios often can't capture the issues caused by bad compression.