Learn to use Compressors to make the sound of your vocals, non-keyboard instruments and the overall mix, better.
What is a compressor?
Simple definition
A compressor is a processor designed to reduce the dynamic range of an audio signal by applying gain reduction when the input exceeds a certain level.
In other words, when the sound gets too loud, the compressor turns it down.
What does a compressor do?
We set a threshold, we set a ratio. The signal level is continuously compared with the threshold that we set, and when the signal reaches or exceeds the threshold, the level of the signal is reduced according to the ratio.
Threshold is just a upper limit we set for the signal. When the signal goes higher than that, compression takes place. The signal above the threshold is divided by the ratio before sending to the output.
Why compress?
Compressor is like an automatic level rider. Instead of a human manually controlling the volume fader to level out the volume fluctuations, the compressor does it, easier and at a more minute level.
Compressor on vocals
Compressors are useful to smoothen out the vocal level fluctuations.
There could be obvious level differences when the when the high level parts are not sung with control, making it sound much louder than the rest of the song.
The vocal performance can fluctuate in levels from word to word where some sung words and phrases come out as obviously loud while others get drowned in the instrument tracks.
Some words and phrases are just easier to sing and they come out louder than the rest. Some vowels and consonants come out naturally louder, due to the way the mouth forms when they are pronounced.
Using compressors makes the vocal more intelligible, and avoids unintended changes in level, thus making the recording more comfortable to listen to.
The pop mix
A degree of routine compression can make the vocal sit more comfortably at the correct level in a pop mix, because pop music dynamics are not as wide as other genres like classical music.
In addition to vocals, most acoustic instruments, electric guitars and basses work better in a pop context where the dynamic range is deliberately restricted.
How to connect the compressor?
A compressor is an insert effect. A compressor should be patched into a mixer via an insert point, or connected in-line between one piece of equipment and another or used in the insert slot of your software sequencer
Compressors should not normally be used with aux sends or as send effects (in software sequencers).
The next section explains the parts of a compressor.
Threshold
The threshold is the level above which the compressor starts to cut down the signal.
We get more compression as we turn the knob down.
With the threshold set to maximum, there is a lot of room for the signal to get loud, very little is considered too loud, so very little compression occurs. But when we reduce the threshold, the higher limit is brought down and most things will be too loud.
Bring the threshold really low and only the softest sounds are left untouched.
The simplest compressor would do this: if the signal is below the threshold set by the user, nothing happens to it, but as soon as it reaches the threshold, it is turned down by a specific amount.
In the case of a 'hard-knee' compressor, the threshold level is well defined, but in a 'soft-knee' compressor, the gain reduction is introduced more gradually.
Ratio
The ratio decides by how much the signal should be reduced once it crosses the threshold.
The higher the ratio, the more gain reduction is applied and the stronger the compression effect.
Ratio is defined as the number of dB by which the input level needs to increase to cause a corresponding 1dB rise in output level. If, for example, a compression ratio of 5:1 is set, an input signal exceeding the threshold by 5dB will cause only a 1dB increase in output level
Suppose the input signal goes up by 4db above the threshold, and the ratio is set as 4:1. Then the 4db extra signal is divided by 4 (ratio being 4:1) and a signal with 1db above the threshold is passed to the output.
If the ratio was 2:1 and the input signal has gone 4db above the threshold, the 4db is divided by 2 (2:1 ratio) to get 2db. The output signal would have 2db above the threshold instead of the 4db in the input signal.
Knee
Hard knee
The hard-knee is also called the hard-ratio compression. The compression is sudden and occurs as soon as the signal reaches the threshold level.
A compressor with a hard knee setting has no effect on signals that are below the threshold, but as soon as they reach the threshold, gain reduction is applied at the ratio set by the user.
Hard-knee compressor can sometimes sound abrupt and the compression can be very obvious, to avoid which, the soft-knee compressor was developed.
Soft knee
With the soft knee type of compression, gain reduction starts a few dBs below the threshold, but at a very low ratio. As the signal gets close to the threshold, the ratio increases, until at the threshold the ratio is that set by the user.
This kind of compression is useful in some situations because it can sound smoother and more musical.
Using soft or hard knee
Soft-knee compression is often used when the compression needs to be 'invisible', such as when you're keeping a mix level under control.
Hard-knee compression is used when the sudden onset of compression being heard is not a problem. The audible side-effects of hard compression are often used as production devices to make vocals or specific instruments stand out in a mix.
Attack and release
A conventional compressor can't start to pull the signal level down until it has reached the threshold. In some cases we would like to let the signal to overshoot.
There are cases where an occasional overshoot may sound better than a perfect compression.
And there are instruments which need the initial part to pass through and not be compressed. Percussions, drums, any instrument with a clear strike or hit need the attack time set carefully.
The attack and release settings give us control over how fast the compression takes effect.
Attack
The attack setting lets us choose how fast the gain is pulled down once the threshold is reached. In other words, how much of the signal is allowed to pass through without compression even after the threshold has been reached.
This is particularly important in the case of percussive instruments, as mentioned before. Letting the signal overshoot the threshold by setting an attack time of several milliseconds, emphasizes the sound of the percussive instrument, its percussive nature.
Compressor on snare drum track
The snare drum is an example of an instrument where the attack settings have to be set carefully in order to get the best out of the signal.
When applying compression to a snare drum, you want the initial hit to pass through without compression, while compressing the tail of the snare drum sound to reduce it from adding to the mix as noise.
By setting a slow attack, letting the initial hit to pass through and then using the compression, the hit is heard with all its punch while the remaining unwanted part of the snare signal is compressed and kept from affecting the overall mix.
Setting the best attack and release values for a given type of material can take a certain amount of skill and experience. More about settings in the setting up section.
Release
The release settings determine how fast the compressor stops acting once the signal falls back below the threshold. In other words, how much time before the gain raises back to normal.
Too short a release time can result in level 'pumping', where the compressor taking immediate action is obvious in the output.
If the release time is too long, the compressor takes time to stop its effect even after the signal has fallen below the threshold.
If a loud sound causes the compressor to take action by crossing the threshold, and if the release times are long, the compressor will take time before it stops acting on the signal and reducing the gain of the signal. This makes the quieter sounds following the loud sound also to be compressed, making them even quieter.
To avoid this, make sure that the release times are not too long, so that the compressor stops acting on the quieter sounds once the signal has gone below the threshold.
Hold
Hold time is a short delay that prevents the compressor from going into its release cycle until a certain time has elapsed.
A very fast attack and a very fast release time make the compressor work on individual cycles of the input signal rather than on its overall envelope. This can result in an audible distortion in addition to the ‘pumping’ of the output signal level.
In the case of bass instruments, this can be very obvious since the cycles are long enough (bass frequencies, longer the wavelength, longer the time to complete the cycle) for the compressor to start acting on each of the individual cycle.
Increase either the compressor's release time or its hold time to solve this problem.
How to set the hold time?
Set a hold time longer than the wavelength of the lowest audio frequency and the problem of distortion and pumping is solved.
With a long enough hold time setting, the compressor will be released only after the individual cycle of the lowest frequency has passed through, thereby not making the compressor to work on each of them individually.
If distortion is audible at fast attack and release settings, and you don't have a hold control, increase the release time until the distortion stops.
Practice
Get familiar with the controls available on a modern compressor. If you work with a compressor, try changing the parameters to listen for yourself what has been written in this article.
If you are using a software sequencer, chances are there is a compressor that came along with it. Start a project, add a channel, record something, add the compressor plugin as insert effect, work with the parameters and see how the sound changes.
The image of the compressor shown above in this article is of Sonalksis, which is an excellent softward compressor effect plugin. Waves CLA Classic Compressors Native Bundle
As an additional exercise, reading through the descriptions of any of these compressor effects will tell you what makes a good compressor good and what they are constantly working on to make it better.
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