Compression Guide

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    Avatar of djdunamis

    Here are two compression guides/articles I found, thought would be beneficial to everyone.

    What is Compression?

    1. The process of reducing dynamic range of a given audio signal by making the loud parts quieter and the quiet parts louder. Compression literally squashes the sound. It works by making quiet parts of the music louder, and loud parts quiet. By using compression and reducing the dynamic range, you can smooth out the sound by finding a medium between the lowest and highest peak volumes.

    Terms to know:

    Threshold: Threshold level determines which signals are subject to compression. With a threshold of -5db, all signals above this level (-4db < ) would be compressed by the set ratio.

    Decibles (db): Measure of sound pressure.

    Attack: How fast a compressor will react once the threshold is breached. 0ms will result in immediate action.

    Release: How fast the compressor will return to its normal state after the signal has moved below the threshold. 0ms will result in immediate return.

    Ratio: How much a signal is compressed. With a compression ratio of 3:1, a signal which is 9db over the threshold level would be reduced to 3db. A signal of 3db over the threshold would be reduced to 1db.

    Gain: Used to increase or decrease compressed sound. (measured in DB)

    Knee: A compressor characteristic that affects the way a compressor behaves.

    Milliseconds (ms): Attack and Release times measured by milliseconds.

    Before beginning, you’ll need a large decible meter, preferably with a digital readout. As a general rule, your mix before mastering should fall around or below the 0db mark. Leaving a ceiling will allow you to compress and boost, without having to do too much limiting.

    Drums: Perhaps the most important element in a track. What exactly does compression do to help? Fatten, thicken, louden, and sharpen. Deep, rumbly kick drums and sharp, snappy snares. Ah, the wonders of compression.

    Threshold: -10db to -15db
    Ratio: 6:1 to 8:1
    Attack: 3ms
    Release: 10ms
    Knee: Hard
    Gain: +5db to +7db

    Percussion: Percussion doesn’t require a lot of compression because usually, the percussion track rests behind the drum track. Bongos, congas, and the likes usually have an immediate popping sound that doesn’t need compressing, so the attack should be set slower than drums.

    Threshold: -3db to -7db
    Ratio: 3:1 to 6:1
    Attack: 5ms to 7ms
    Release: 15ms
    Knee: Hard
    Gain: +2db to +4db

    Bass: A common problem with bass is that the low notes seem to disappear into the mix while the higher notes stick out like a sore thumb. With many instruments, reverb could solve this problem. However, using reverb on the bass track usually gives it an undesirable effect. By using compression, you can bring up the lows, and submerge the higher notes into the mix. Often times, there is an initial “pluck” to the bass sound, and it can be more beneficial to let this sound slide through uncompressed.

    Threshold: -4db to -9db
    Ratio: 4:1 to 8:1
    Attack: 3ms (if there is a plucking sound, use an attack closer to 7ms)
    Release: 100ms on short bass sounds / 300ms on long bass tones
    Knee: Hard
    Gain: +2db to +4db

    Brass / Wind instruments: Brass and wind instruments require a “transparent” type compression. Any obvious processing can noticably ruin the sound. Brass and wind instruments have a lot of variety in playing styles. Trumpets can be played expressivly loud, and a smooth, mellow flute will need much different processing.

    Threshold: -2db to -4db
    Ratio: 6:1 (lighter instruments) to 15:1 (deep brassy instruments)
    Attack: 3ms (If a transient sound needs through uncompressed, use 6ms)
    Release: 300ms
    Knee: Hard
    Gain: Varies

    Guitars: When working with acoustic guitars, compressors tend to reveal themselves more so it’s a good idea to use a very “transparent” compression. If working with electric guitars, make small increases to the ratio and threshold.

    Threshold: -2db to -3db
    Ratio: 3:1 to 4:1
    Attack: 3ms (If there is an initial pluck, use 5ms)
    Release: 30ms to 60ms
    Knee: Soft
    Gain: 0db to +1db

    Samples: If you’re a sampled based producer (specifically, phrase sampler), chances are you don’t get to compress several instruments in different ways. Using the following numbers, you’ll be able to smooth out the entire sample without too much limiting.

    Threshold: -2db to -4db (If the sample is recorded bad, and there’s lots of peaks, use a higher threshold around -8db)
    Ratio: 2:1 to 3:1
    Attack: 2ms
    Release: 400ms
    Knee: Hard
    Gain: +1db to +3db

    Full Mix: The final mix doesn’t require much compression, although some songs have been compressed with up to a 4:1 ratio, most aren’t needed that much. A final compression should act as a limiter, keeping the signal close to the 0db mark.

    Threshold: -4db to -7db
    Ratio: 1.5:1 to 2.5:1
    Attack: 5ms
    Release: 200ms to 500ms
    Knee: Hard
    Gain : Varies

    Written by: silexz

    Here’s the big secret of compression:

    You should *barely* hear it working except as increasing your overall volume within the parameters you need. The average person may not even hear it working much. THAT is how the pro’s set 75% of their compression, the other 25% is super squish city reserved for things like submixing drums in stereo and mixing it back in at low levels to beef stuff up.

    Jim’s rules for compression:

    First let’s define what a compressor does–which is to affect the amplitude of a signal by selectively reducing it. Compressors tend to have the following controls:

    Compression ratio: this determines how ‘hard’ the compressor is supressing the signal. Usually described as a ratio such as 2:1, 4:1 and so forth. What this means is, after you cross the threshold setting, how many db’s you have to go over to effect 1db of volume change. Thus a 4:1 ratio means that once you go over the threshold for every 4db over you will only get 1 db of amplitude change.

    Threshold: this sets the decibel level that the compressor starts to work. Signal underneath the threshold will be unaffected–signal above it will be hit by the compression amount determined by the ratio. Needless to say, setting the threshold above the peaks of the signal will NOT do jack to the signal. You gotta set it in the path of the signal, so to speak. This is always expressed in negative db, thus a -24 threshold will compress any audio above -24db, and leave everything below it alone. (*Note, soft knee compressors start to work a bit before the threshold!)

    Attack time: how long, in milliseconds, it takes for the compressor to kick in. This keeps your transient peaks unaffected and is the trick for getting a “punchy” kick or snare (the front end crack will be uncompressed and thus louder than the following signal).

    Release time: once the signal falls below the threshold how long, in milliseconds, it takes for the compressor to ‘let go’ of the signal. For vocals and other similar instruments you want this to be fairly long like 200-250ms. For drums 75-125ms is great.

    Special note on soft-knee compressors: some compressors have a soft knee function. What this does is start compressing the signal lightly as it approaches the threshold, and as you get closer to the threshold it will compress harder and harder until you reach the threshold and the full compression ratio will be utilized. This provides for fairly transparent compression and is great on vocals. Personally it sucks for drums unless you are squishing a stereo submix of drums.

    Another note on stereo compressors: you should *always* link stereo sides of compressors when processing stereo signals. Once a side reaches the threshold BOTH sides get the compression. Failure to do this can lead to, for example, drums that leap in volume on one side but not the other.

    Moving right along…..

    Here are some guidelines off the top of my head:

    2:1 ratio–overheads, distorted guitar, soft vocals, most synths
    3:1 ratio–clamping down on overheads, acoustic guitar, most singers
    4:1 ratio–bass, snare, kick drums, toms, crap singers
    8:1 ratio–bad bassists, screaming vocalists, squishing the life out of stuff
    12:1 ratio–out of control peaks or when you want to sound like limiting but still keep some life to it

    Compression ratio and threshold are intertwined, so set both accordingly!

    If you need dynamic range–LOWER the ratio (ex. 4:1 to 2:1)
    If you need more regularity in levels–RAISE the ratio (ex. 2:1 to 4:1)
    If you just need to shave off some peaks–RAISE the threshold (closer to 0 db)
    If you want to affect a lot of the signal–LOWER the threshold (further from 0 db)

    Here’s the tricky parts that require hard decisions:

    If you want more smooth sounds–LOWER attack time (under 6ms)
    If you want more punch–RAISE attack time (between 7-50ms)
    If you need “more” compression–LOWER attack time more (more transient will get compressed)
    If you need “less” compression–RAISE attack time (less transient gets compressed)

    If you need ‘invisible/natural’ compression–RAISE release time (or go to soft knee)
    If you need ‘audible/percussive’ compression–LOWER release time (watch for pump-n-breathe)

    Now pull out yer ears:

    If it pumps and breaths–RAISE release time (unless you want that)
    If the compression seems to disappear–LOWER release time

    Finally the number one rule for compression:

    ALWAYS match relative volume levels (by ear) before and after compression using makeup gain–meaning that they should be peaking about the same. If you record using my “-15 to -12dbfs with peaks no greater than -6dbfs” rule this is easy; if you tend to record sloppy and “hot” you may need compression to keep you out of the red. Don’t do this to yourself.

    The idea for this is that LOUDER often equates to sounding better to us, fooling us into setting duff and mookish compression settings. When dialing in compression make sure that the before and after levels are identical so you can hear the compression and not the jump in volume.

    Here are some guidelines on setting makeup gain:

    The lower the threshold the more makeup gain you need.
    The higher the threshold the less makeup gain you need.
    The higher the ratio the more makeup gain you need.
    The lower the ratio the less makeup gain you need.

    Further modified by:

    The faster the attack the more makeup gain you can get away with.
    The slower the attack time the less makeup gain you can use.
    The faster the release time the less makeup gain you can use.
    The slower the release time the more makeup gain you can use.

    Written By: James Meeker

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