Compressors are a type of signal processor and can be thought of as a kind of “automatic fader”. They are used either to fix problems, for example, too great a dynamic range in the recording; or to enable creative mixing for example, by adding presence to the soundtrack.
So how does this work?
When an audio signal passes through a compressor, it can:
- lower the volume of the loudest parts of an instrument or mix
- raises the volume of the quietest parts
- reduce the dynamic range of the signal overall
This means you can increase the signal’s overall level, without clipping.
In simpler terms, the loud parts of an instrument or mix are quieter and the quieter parts are louder, giving a better balance, and suiting the recording for a wider range of listening environments. You can enjoy the string quartet in your car, amongst noisy city rush hour traffic, instead of having to wait until you’re in your home theater.
Consider Compression to be an art
Do you feel the track would benefit from compression? Are the quiet sections too quiet or the loudest sections too loud? Then introduce it – a little at a time.
But take note – compression is not always necessary. If applied incorrectly it can result in your track or mix sounding awful instead of awesome!
So, compressors are used to enhance your soundtracks.
There are two main types:
- Upward- raises the volume of the quieter sound sections
- Downward- reduces the louder sections
The term “compressor” arose because they originally only reduced the level of the louder sounds.
Compression settings are typically characterized by these parameters:
- Output Gain – When you compress an audio signal by a certain amount (measured in dB (Decibels)), the louder parts become quieter. The output gain allows you to compensate for the reduction in levels in the loudest section of the signal that has been compressed.
- Threshold- The volume level at which compression starts to take effect. You set the threshold to where you need it, depending on the mix.
- Compression Ratio- A measure of how much compression is applied to the sound signal.
- Attack time- This refers to how tolerant the device is when a signal exceeds the set threshold. The attack determines how rapidly the compressor reacts to the signal. Depending on this setting, shorter sections exceeding the threshold will be ignored, but longer sections will be compressed.
- Release time- This controls how long the compressor will stay in effect once the signal falls below the set threshold value.
- Knee- This parameter determines how quickly an audio signal reaches full compression once over the set threshold.
When a compressor is fed an audio signal, it measures the signal level to determine whether to apply processing or not. If the signal remains below the threshold, compression remains off, and the signal is not affected.
When the signal level does exceed the threshold, then compressors react using the attack speed parameter and reduce the signal level according to the settings of the ratio and knee parameters.
The Black Art of Applying Audio Compressor Settings That Every Producer Must Master
Using a compressor effectively is a bit of a black art – It needs a good understanding of the problems arising in your recorded tracks, along with well-developed judgment as to where compression could help, and how to apply it.
This article is for you if you’re unfamiliar with the principles of using compression, or would like a reminder of how to approach this aspect of music recording.
We’ll touch on setting the various parameters – threshold, compression ratio, attack and release times, and of course, the knee, as well as where to place the unit in your signal path.
Step One – Setting the Threshold level
First time using a compressor? Start by listening to your track or overall mix to find the threshold where you would like the compression to start. Now, adjust the parameters and listen for how they affect the signal.
Start by adjusting the threshold parameter – using one of the suggested methods:
- High Threshold: This setting allows the compressor to lower the loudest part of the signal or peaks only.
- Low Threshold: This setting allows the signal to be almost continuously compressed.
Dave’s Tip: Choose either method (the first is more common). You can come back and make further adjustments later.
Step Two – Compression/expansion ratio
Here you need to determine how much you want to raise the lower level signal to be more present in the track or lower the higher level signal to moderate the loudest peaks and avoid clipping on playback.
You achieve this by adjusting the ratio parameter, in the range 3:1 â€“ 6:1. Then the louder tracks will be slightly lowered and the quieter ones slightly raised.
Start with a ratio of 3:1 – it will help produce a more natural result.
There is a maximum setting of 10:1. This will really crowd the track and make the overall volume very uniform throughout.
This is referred to as Limiting, which is another class of signal processor all together. You should not need to go to this extreme.
Limiter: A Limiter has a much higher compression ratio with a faster attack time.
Step Three – Setting the Attack and Release (Decay) Times.
This can be a bit difficult and involves some artistic judgment.
When setting the Attack, it’s important to consider the instrument in question. For example, does its natural tone start fast with the loudest part at the beginning of the sound or does it build slowly to get there?
Dave’s Tip: Measured in milliseconds (ms), start with a fast Attack – around 1ms – and a release time of 500ms to give you a smoother sounding starting point.
It can be useful to tweak the threshold setting to compensate for the attack and release settings at the loudest part of the track
Step 4 – Setting the Knee Parameter
Properly setting the Knee parameter determines how long it takes the compressor to get to full compression.
One misconception many beginners have is that for a sound “punchier” you need a fast attack setting and a slow release setting. In fact, it’s the exact opposite.
- Aggressive / Punchier sound â€“ slow attack, fast release
- Gentler / Smoother Sound â€“ fast attack, slow release
Hard Knee: When the signal reaches the set threshold, the full set compression ratio is applied instantly all at once to shape the sound. Use when you want the compression effect to be a part of the resulting sound – for example, with a drum track
Soft Knee: When the signal approaches the threshold, compression is applied gradually until the threshold is reached, then the full ratio is applied. Use when you don’t want the compression effect to be noticeable – for example, on a vocal track.
Dave’s Tip: Soft Knee compression is better for overall mixes, and for gentler, more subtle sounds.
The Signal Chain
It’s quite common in recording, especially when mixing, to use a compressor and an EQ unit together to help shape that perfect sound. But which should be placed earlier in the signal chain?
It comes down to personal preferences and what result you are trying to achieve.
Putting the EQ before the compressor is a more common method – using the EQ to remove the frequencies you don’t want, then boosting the overall signal with a compressor.
Similarly, if you have already compressed a bass track during the initial recording, adding EQ and another compressor after could be beneficial.
This is like having a whole bunch of compressors working at once because they allow different parameters to be applied to each frequency band.
This can create a highly effective, extremely detailed result – which is why it is mostly used for the fine tuning required during mastering.
In summary, Compressors provide two main functions;
- to fix problems
- to create effects
Technically when you are using stationary faders during recording you are exercising compression to find the right balance of tone. However, this approach is effective only when you are recording individual or a small number of sound sources.
When you have multiple different instruments, with continuously changing signal levels, your faders will not do the trick.
Again, it is important to remember that there is a lot to learn about this technique. As you practice it more and more, you’ll find it easier and become more skilled as to where, when and how much compression to apply.
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