Compression can be a bit of a complicated subject. Audio compression is a process of reducing the dynamic range of audio data. It does this by lowering the level of loud audio and raising the level of soft audio, thus “compressing” or reducing the difference between those two extremes.
This technique can be used to make an audio signal more compatible with a certain playback device, such as making a recording louder so that it will sound good on headphones where there is no volume control. Audio compression has been in use since before digital recordings were invented, but modern techniques are much more sophisticated and effective than they once were.
What is an audio signal?
Before we start with audio compression, let’s discuss more basic concepts briefly. The first term we are going into is the audio signal.
The audio signal is a representation of audio or sound as an electrical voltage.
This signal is represented by a sine wave and is characterized by frequency and amplitude.
A signal may be represented as a simple sine wave, or it can be more complicated with many different frequencies and waves.
The audio system takes the raw audio signal which is in voltage format and converts it into sound waves that we can hear on speakers (a process called reproduction).
The audio signal can be represented by a graph as well, with the loudness on one axis and time going along the other.
Audio signals are created when a recording is made, processed by effects units, mixed down onto one track in preparation for mastering (or releasing), and encoded into a file by one of many audio formats.
What are Dynamics and dynamic range in Music?
Dynamics in the music means the relative volume of different parts in a song.
The dynamic range is the difference between the loudest and softest sounds that can be reproduced by an audio system, or in other words how wide the dynamics are.
For instance, in one track you can have a loud scream and also a quiet part of the song. And the difference is the dynamic range.
Gain reduction and gain increase
These are two types of the process of audio compression.
- Gain reduction is the process of reducing the volume of a signal that exceeds a certain threshold, and it’s used to control peaks in an audio track. It can be done manually or automatically (by your DAW).
- Gain increase reduces loud signals down by compressing them less than louder ones so when they are mixed, they are narrower.
Transient – where the dynamics appear?
Transients are the initial part of a sound that is louder than the rest and are an important aspect of audio compression. For instance, the initial drum hit or guitar riff. They create a larger change in volume than other parts of the audio and are more likely to exceed your DAW’s compression threshold.
Parameters of the compression
To set up the compression, you should know the parameters that come with the compression. This is important to understand the basics. How these parameters affect the output and mainly how to have the output more balanced.
The point at which an audio signal can be compressed is tracked by decibels. Once the signal level is below the threshold, no processing is performed and the input signal is passed without change to the output. If the signal is louder than the threshold it will be reduced.
A lower threshold means more of the signal will be processed; a higher threshold indicates less processing, less compression.
The ratio determines how much volume reduction your compressor applies when the sound goes above a certain threshold. It is expressed as a ratio to the sound waves’ original size.
The higher the first number of the ratio, the greater the factor by which the gain is reduced.
For instance, a ratio of two means you reduce the sound level by half; at four times, that would be one quarter.
And the extreme ∞ :1 means limiting.
A ratio of 1 means that the compressor will never reduce the gain.
Audio compression settings will fall between 1.5x to 10x, with the most common being in the middle of these two numbers.
Attack and release
Compressors may provide some control over how quickly it acts.
Attack time is how fast the compressor reacts to audio that exceeds a certain threshold.
Release time determines how long it takes for the compressed sound to drop back down after going below this threshold.
The attack and release times are usually in milliseconds or samples, such as “20ms” or “12000samples.”
Soft and hard knees
One of the controls a compressor might offer is between “hard knee” and “soft knee”.
A soft knee setting means that the audio will gradually shift from uncompressed to compressed.
A hard knee setting means that there is no transition between uncompressed and compressed, with compression happening abruptly as soon as the threshold is reached.
Soft knees are generally used for subtly compressing vocals on recorded music so it sounds smoother in a mix.
A downward compressor only decreases the amplitude of a signal, so make-up gain (i.e., amplification) must be turned on at the output to produce an optimal volume level.
When and How to Use Audio Compression
Usually, audio compression is used in music production. The sound of the output may be more soft or just less dynamic.
Additionally, you may want to add color to your sound.
Moreover using audio compression is useful these days, when more and more people are using the internet to make podcasts and/or videos.
The more you will use your audio compressor, you will become more comfortable in using it and the output will better. I can just recommend using it, play with it, and becoming a master.