Compression is certainly one of the most exciting topics in music production, as it is for the students of HOFA-College, our online academy for audio engineering. Using a compressor, however, can be quite challenging, especially for beginners. We have a few explanations and tips on what an audio compressor actually is, what options for adjustment and application it offers in a mix, and how the use of compression can be purposefully trained in mixing and mastering.
What does a compressor do and why do you need one?
There is a widespread myth that a compressor makes audio signals louder. But that is precisely what it does not do. Quite the opposite: a compressor usually makes an audio signal initially quieter.
But how does such a myth become established? To answer this question, we need to take a closer look at the concept of dynamics in audio engineering. In a nutshell, the term dynamics describes the difference between the quietest and loudest levels of an audio signal. This difference can also be identified visually in the waveform of an audio signal.
In itself, a dynamic signal is not bad at all, and it reflects the sound source as close to the original as possible. In music production and mixing, however, we always have to consider the relationship between a signal and other elements in the mix. In this context, too much dynamics is usually perceived as unstable, which is why it is necessary to reduce the dynamic range of signals.
This is exactly where the compressor steps in. It ensures that the louder parts of a signal are attenuated, bringing them closer to the quieter parts of the signal. The result: a lower dynamic range that allows the producer or mix engineer to place a signal in the mix more consistently without the level peaks standing out too much or the quieter signal parts getting lost.
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Parameters of a compressor
So that’s what a compressor does. But how does the compressor know when to make a signal quieter and how much to reduce it? To accomplish this, compressors offer various adjustable parameters, starting with the threshold. This sets a value in dB above which the compressor starts to reduce the signal. The lower the threshold, the more of the signal will be reduced by the compressor.
Furthermore, the ratio can be set in which the compressor attenuates the signal above the threshold. You can use the parameter ratio for this, which is usually displayed as Value:1.
As an example: if the threshold at the compressor input is exceeded by 8 dB and the ratio is set to 4:1, the compressor reduces the output to only 2 dB.
This example clearly shows that the compressor itself can only make the signal quieter. This inevitably changes the balance in the mix, which is why the loss of volume caused by the compression should be compensated. This can be done using the makeup gain. To restore the original balance in the mix and ensure that the uncompressed and compressed signals can be compared objectively, the makeup gain should be set so that the uncompressed and compressed signals are perceived as equally loud. The specified parameters can be displayed in the typical compression diagram:
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More InformationThe compressor’s control times are not visible in this diagram. While attack determines how quickly the compressor starts working after the threshold is exceeded, the release time determines the speed at which the compressor returns to the zero state after the threshold is undershot.
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Applications
Whether producing, recording, mixing or mastering: compression is an indispensable tool in all areas of audio engineering and music production. By understanding the parameters of compression, we can decide which signals require compression and what we want to achieve by using it.
Let’s get one thing clear: there is not just one correct compression setting. The possible applications in mixing are too diverse and the needs in producing are too individual. Nevertheless, there are a some landmarks regarding the parameters that can be used for various applications in the mix, and a look at a signal’s waveform is often helpful.
Let’s assume we want to control level peaks without reducing the dynamics too obviously. In such a case, a rather low ratio and a rather high threshold are an obvious choice. This approach is conceivable, for example, for group or even sum compression.
It is a different matter if we deliberately want to limit the dynamics, for example to stabilise a vocal recording; in this case, a significant reduction and respective settings of the threshold and the ratio can achieve the desired result.
By adjusting the control times, we can further shape the waveform according to our specific needs. This way, transients and the decay phase of a signal can be specifically emphasised by adjusting the attack and release times. If, for example, we want the transients to remain unaffected by the compression, the attack time must be set to a slightly longer value. If, on the other hand, we want the transients in a signal to be controlled as much as possible, the attack time can be set to a shorter value.
This approach can be transferred to the release time in reverse. If you want to emphasise the sustain of a signal or make it appear longer, a short release time is recommended. A longer release time, on the other hand, ensures that the compressor returns to its initial state more slowly, making the decay appear more organic or even shorter.
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More InformationPractising compression
Theory is all well and good, but how can you train your ears to recognise compression when mixing? To do that, you first have to get to know the compressor. The stock compressors of the common DAWs are very suitable for this purpose, as they offer the common parameters and the compression diagram mentioned above.
From here on, we highly recommend experimenting. Common musical signals such as a snare or vocals are well suited to immediately understand the effect of compression in the mix. Try out extreme values to find out how the settings affect parameters such as transient, decay, background noise and, above all, signal stability.
In the HOFA online courses in audio engineering, you will find a large number of tutorials, comprehensive knowledge and tips on the subject of compression, as well as a wide range of training material. The HOFA AUDIO DIPLOMA online course in particular prepares you for all the tasks involved in music production.
