What is a compressor in music production?
The job of an audio compressor is to reduce the volume dynamics of an audio signal, it does this by lowering the volume any time the volume exceeds the threshold. These volume spikes are called transients, it’s these transients, which trigger the audio compressor to reduce the volume.
As an example, let’s say you have recorded a snare drum and the sound of the stick hitting the drum skin is much louder than the drum resonance. A compressor can lower the volume of just the initial stick sound (attack), resulting in a recording that will have more body and is more consistent in volume.
A compressor can be hardware or software and can be connected to the signal chain using physical audio cables or connected as an audio insert hosted on your DAW. Form here, you can then adjust the compressor's settings, such as the threshold, ratio, attack, and release, to shape the dynamics of the sound to your liking.
What are transients in audio compression?
Compression reacts to volume peaks called transients. Transients are brief and rapidly changing volume spikes that are frequent in lots of instruments, such as drums, percussion, plucked strings, and even speech. They are characterized by their short duration and large amplitude, relative to the surrounding audio. The initial impact of a drum hit will cause a transient, or the strum of a guitar, as you can see in the waveform below........If you know the basics and want to skip to how transients work in a mix and master.
There are several controls that you typically find on an audio compressor, this is an overview describing each control -
The threshold is the level at which the compressor starts to reduce the volume of the audio signal. It is typically measured in decibels (dB), and it can be adjusted to capture the loudest parts of the signal.
The ratio is the amount of compression applied to the audio signal once it exceeds the threshold. It is typically expressed as a ratio, such as 2:1 or 4:1, and it determines the amount the volume of the compressed signal reduces.
The attack controls the speed at which the compressor reacts and starts to reduce the volume. Attack time is typically measured in milliseconds (ms), and it can be adjusted to preserve the punch/impact of the sound or reduce using fast attack times but be careful of digital clipping and frequency artifacts.
Once the volume falls below the threshold, the release sets the time and speed at which the compressor stops compressing. Like the attack control, it is measured in milliseconds (ms), and can be adjusted to prevent unnatural effects such as pumping and breathing.
The makeup gain is an additional gain applied to the audio signal after it has been compressed. It can be used to compensate for any loss in volume caused by the compressor. it can also be adjusted to achieve the desired overall loudness.
The knee is a setting that determines how smoothly the compressor transitions from no compression to full compression as the audio signal crosses the threshold. A hard knee setting will cause the transition trigger to be abrupt, while a soft knee will be more gradual.
Input and output:
The input and output controls are typically found on a hardware compressor, and they determine the level of the audio signal going into the compressor and coming out of it, respectively. They can be adjusted to achieve the desired amount of compression. For more on using this in the digital realm, look up gain staging.
The dry/wet control is typically found on a software compressor, and it determines the balance of the uncompressed (dry) and compressed (wet) signals. This can be useful for blending the compressed signal with the original signal to achieve the desired effect. This is called parallel compression, this will raise the volume of your output signal so be careful and look up gain staging.
Benefits of using a compressor
There are many benefits to using an audio compressor, however, I would advise only using it when it’s needed but not on every audio channel. While a compressor can improve a mix, it can also diminish dynamic range, introducing digital distortions, and amplifying background noise. Understanding the benefits of a compressor can help you make informed decisions about when it's appropriate to use one.
A compressor can help make the overall volume of an audio signal more consistent, reducing the volume of loud sounds will allow you to increasing the volume of the whole audio track. This can help glue multiple instruments together by taming and unifying dynamics of multiple instruments sent to a group channel, sub mix or bus.
By reducing the volume of loud sounds, a compressor can make it easier to hear the other elements in the audio signal, such as the nuances in vocals or the sound of the mallet hitting the strings in a piano.
By adjusting the attack and release settings on a compressor, you can increase the volume, punch, and impact of an audio signal, creating a more powerful sound with increased dynamics.
Increased loudness:By carefully adjusting the compressor's settings, you can increase the overall loudness of an audio signal without distorting or clipping the sound. This can be especially useful for mastering audio tracks or preparing them for commercial channels.
Downsides to compression
While there are many benefits to using an audio compressor, there are also some potential downsides to consider.
Loss of dynamic range:
By reducing the volume of the louder sections, you will reduce the impact the instruments have on the mix and lessen the dynamics created from the arrangement. This can also make the audio sound harsh and less natural.
If the audio has noise or echo, a compressor can reduce the clarity making the audio sound muddy and washed out.
Digital compressors can leave audible artifacts when using heavy settings, they can also bring out any distortion from the audio signal, resulting in a harsh or unpleasant sound.
Increased pumping and breathing:
If the compressor is set incorrectly, it can cause the volume of the audio signal to fluctuate, resulting in a pumping or breathing effect normally triggered by loud low frequency instruments. This can be distracting and unpleasant to listen to.
Depending on the compressor being used, it may introduce a small amount of latency, which can be noticeable when using the compressor in real-time applications, such as live music performance.
Compression Tips and Tricks
Here are some tips and tricks for using an audio compressor effectively:
Don’t use a compressor if you don’t have to.
Start with the threshold: The threshold is the most important setting on a compressor, as it determines the level at which the compressor starts to reduce the volume of the audio signal. Start by setting the threshold to a level that captures the loudest parts of the signal, and then adjust it as needed.
Use a moderate ratio: A high ratio will result in more compression, but it can also reduce the clarity of the audio signal and introduce distortion or click noises. Instead, try using a moderate ratio of around 4:1 to 6:1, and adjust it as needed to achieve the desired effect.
Use a fast attack: A fast attack time will allow the compressor to react quickly to reduce the volume of transient sounds. This works to tame the pick attack on a close mic’d guitar or the beater volume on a kick drum.
Use a slow release: A slow release time will allow the compressor to gradually reduce the volume of the audio signal once it falls below the threshold. This can help prevent the pumping and breathing effects, allow more tone and help the compression sound natural.
limiter: A limiter is a type of compressor with a very high ratio (usually 10:1 or higher) and a fast attack and release time. It can be used to prevent the audio signal from exceeding a certain level, which can protect against distortion and clipping.
sidechaining: A sidechain is a separate audio signal that is used to control the compressor. This can be useful for ducking the volume of one audio signal (such as a bassline) when another audio signal (such as a kick drum) is present.
Experiment: Don't be afraid to experiment with the compressor's settings to find the right balance for your audio. Try different threshold and ratio settings, and adjust the attack and release times to see how they affect the sound. And remember, less is often more when it comes to using a compressor.
It's important to experiment with the compressor's settings to find the right balance for your audio. You can also use a compressor in combination with other audio effects, such as an equalizer, to further shape the sound.
If you're using a software compressor, you can typically find detailed instructions on how to use it in the user manual or online documentation.