Compression
what does compression do?
Compressors are tools that compress the audio that it receives into it, many people associate compressors with making the volume louder and quieter. This is because a compressor decreases the volume of the loudest peaks in the audio file, but a compressor also has the ability to bring the master volume of the audio up. A compressor is used to avoid audio from getting loud in the mix of the track, and to make all the sounds blend together. Using a compressor is essentially the same as telling your DAW to decrease the audio on a track, however with a compressor we have more accuracy and precision on where we want to increase or decrease the volume of the audio file. When we are using a compressor we can tell it how quickly we want the compressor to reduce the volume, this is called the attack time of the compressor. We can also tell the compressor how long we want it to take when releasing the gain reduction as the music returns to its usual level, this is called the release time of the compressor. And of course we have control over how much we want to decrease the volume by, this is called the ratio of the compressor. We can also tell the compressor what dynamic level we want to compress within the audio file, this is what we call the threshold of the compressor. And last but not least the compressor can control the boost of the output to match the original signal of the music, this is called the Make Up gain of the compressor.
Threshold
This decides when the compressor will engage. we can control the threshold by assigning it to a decibel of sound, when the sound goes above the threshold of sound it will begin to get compressed. If you set the threshold to high none the audio in the music will be compressed, because they will mostly likely fail to reach the threshold. But if the threshold is too low all of the audio will be compressed. When making a threshold you most likely only want to catch out the peak in audio, in this case you could start your threshold at -24. This is generally a good starting point because it will catch out the top 6 decabels of peak.
Ratio
The ratio in a compressor decides how much the volume will be decreased by when it hits the threshold. An example of a ratio in music is 2:1, this means for every 2 decibels we go over the threshold we only hear one decibel come out as half of the signal is being compressed. Another example of a ratio is 4:1, this means for every 4 decibels we go over the threshold we can only 1 decibel as %25 percent of the signal is being compressed. An example for if this ratio went 12 decibels over the threshold the output would only be 3 decibels due to the heavy compression ratio on the audio signal. Mathematically this is because 12 is 3 times of the threshold in the ratio 4:1 so we times divide 12 by 4 to get 3 decibels. This is equal to 11 decibels of reduction. So if you wanted the audio to be twice as quite the ratio would be 2:1, if you wanted it to be 4 times as quite the ratio would be 4:1 and so on.
Gain (Make up)
The gain of the compressor increases or decreases the volume of the output, this is because the ratio and threshold can make the track quieter in it’s output than it was in it’s input. If you have a gain reduction of 3 decibels, you should try and increase the gain by 3 decibels. A good way to decide the gain is to try and make the output and input gain match. The gain generally start around -18 and be peaking around -6.
Attack Time
The attack time is in control of how long it takes for the compressor to decrease the volume of the audio above the threshold. For instance if the attack time was set to 10 milliseconds it would take 10ms for the compressor to fully engage after hitting the threshold. This would allow 10 milliseconds of transient (short lasting) audio to exist over the threshold. If the attack time was 0 milliseconds their would be no transient audio slipping through because the threshold would instantly detect it. If the attack time is around 40 milliseconds the transient slips through the threshold but also delivers the rest of the note to sounds natural. If the attack time is around 10 milliseconds the transient is almost immediately compressed and sounds very unnatural, sometimes you might want this to capture an aggressive sound. However on vocals fast attack times tend to sound more natural around 2 – 1o milliseconds, this is because the voice will be slowly compressed with a slowly attack time to sound unnatural.
Release Time (Decay)
Release time is often referred to as decay, decay is the exact opposite of attack time as it controls how long it takes for the compressor to disengage. If the release time is too slow the compressor is always engaged because of how long it will take for it to release and will sound over compressed . But if the release time is too fast the compressor will sound very unnatural and will again sound over compressed. It is best to adjust the release time until the music breathes and fits with the rhythm of the music. The release time sounds most natural when the sound isn’t getting compressed before the note finished, and also when the compressor is not always engaged . With release time in most cases it is better to be slow than fast, with around 60 milliseconds.
Knee
Knee is a setting that makes the transition from uncompressed sound to compressed sound gradual after the threshold is hit. The knee does this by gradually working it’s way up to the ratio marking, unlike without the knee when the music will go into the ratio at an angle. The knee gives the sound in the compressor a more musical and natural sound, this works very well on vocals.
Tips for Compression
- Starting with a low threshold will make the audio sound very over compressed, this will allow us to clearly hear what the compressor is doing to the audio it is receiving . This a good technique for seeing what the compressor is doing while mixing vocals, when you are done with the low threshold make sure to bring it back up to it’s usual level.
- Start with a attack time of 40ms and an attack time 60ms and adjust them from there
- For vocals start with an attack time of around 6ms for vocals so the transient doesn’t get through to make vocals sounds unnatural. However if the vocals are spoken word it is best for the attack time to be around 12ms as a starting point.
- Do not use compression for the sake of it, only use compression if it is needed.
- Subtle changes in compression can add up to form professional mix of music
- Try compressing the bass drum and the kick drum together, this can bond them together musically
- When starting to learn compression try an stick to one compressor, in this case you will begin to learn your way around it really well.
- Keep an eye on the gain reaction meter, this can tell you if your mix is over compressed. However you should mostly rely on your ears.
Reading List
Mayzes, R. and Mayzes, R., 2021. How To Use a Compressor: The Easy To Follow Guide (10 Top Tips). [online] Musician on a Mission. Available at: <https://www.musicianonamission.com/how-to-use-a-compressor-plus-10-top-tips/> [Accessed 20 March 2021].