Dynamic Range Compression: Attack & Release Controls
Dynamic range compression is easily one of the most-used essential audio effects/processes. The attack and release parameters of compressors aren't always the first to be mentioned, yet they are critical to the compressor's performance.
What is the attack time of a compressor? The attack time is the amount of time it takes for a compressor to engage/react once the input signal amplitude surpasses the threshold.
What is the release time of a compressor? The release time is the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the input signal drops below the threshold.
In this article, we'll discuss the attack and release time parameters of compressors to deepen our understanding of compression and how to use it wisely in our audio productions.
A Brief Discussion On Audio Dynamic Range Compression
Before we get into the bulk of this article, I'd like to quickly go over what dynamic range compression is in a general sense. By understanding the purpose of a compressor, we can better apply our knowledge of the attack and release times to our holistic comprehension of dynamic range compression.
What is dynamic range compression? Dynamic range compression is the process of reducing the dynamic range of an audio signal (the difference in amplitude between the highest and lowest points). Compression does so by attenuating the signal amplitude above a set threshold point.
So then, the purpose of a compressor is to reduce the dynamic range of the audio signal by effectively reducing the level of the loudest parts and maintaining the level of the quieter parts.
The “loud” and “quiet” parts of an audio signal are determined by a threshold parameter set in the compressor. Anything above the threshold gets attenuated by a defined ratio (input level to output level), and anything below the threshold is not attenuated.
The time it takes for the compressor to reach its full attenuation ratio (after the signal surpasses the threshold) is defined by the compressor's attack time. Similarly, the time it takes for the compressor to fully disengage (after the signal drops back down below the threshold) is determined by the release time.
Note that softening the knee parameter of the compressor will also smooth out the transition amplitude period so that the compressor will engage gradually from below the threshold to its full engagement above the threshold.
When thinking of the actual effect a compressor has on a signal, we can visualize compression as a volume fader that automatically drops the signal volume during the loudest parts of the signal. The loudest parts get brought down while the quieter parts are outputted at full volume.
A compressor can be thought of as an automatic volume control in this manner.
In this analogy, it will take some amount of time for the fader to move from its original position to its full attenuation as the signal surpasses the threshold (attack time). It will also take some time for the fader to return to the original position after the signal drops below the threshold (release time)
As was mentioned at the beginning of this article, compression is one of the most commonly used processes in audio. Its uses include, but are not limited to, the following:
- Maintaining a more consistent level across the entirety of the audio signal/track
- Preventing overloading/clipping
- Sidechaining elements together
- Enhancing sustain
- Enhancing transients
- Adding “movement” to a signal
- Adding depth to a mix
- Uncovering nuanced information in an audio signal
- De-essing
- “Gluing” a mix together (making it more cohesive)
Related Article On Compression
To learn more about compression, check out my Complete Guide To Audio Compression & Compressors.
Compressor Attack
As mentioned, the compressor's attack time is the amount of time it takes for the compressor to reach its full ratio of attenuation once the input signal exceeds the threshold.
The attack time does not refer to a delay in triggering. Rather, the compressor will gradually attenuate up to the set ratio over the course of the attack time.
If the attack time were 0 ms, the compressor would act immediately. This non-existent attack time can have consequences on the actual waveform of the signal as it may cause the compressor to shape the peak of the signal itself rather than drop the amplitude of the waveform without distorting it. This is particularly true if the release time is also non-existent (or very short).
This kind of “compression” would resemble something like this:
Typically there will be some amount of attack time that will be longer than the individual waveform periods of the audio signal. However, I thought it would be important to include this information.
This “intra-wave compression” is similar to saturation and other distortion effects, which are related to compression.
Here's an illustration of three audio signals. One is uncompressed (original), one is compressed with a short attack, and the third is compressed with a long attack. Note that the ratio in these examples is 2:1, and the red dotted line represents the threshold:
So we can see that longer attack times make the compressor a bit slower when it comes to reaching its full attenuation.
Compressor Release
As mentioned, the compressor's release time is the amount of time it takes for the compressor to fully disengage (stop its attenuation) once the input signal falls below the threshold.
In other words, the release time tells us that the compressor will remain engaged for some amount of time after the signal drops below the threshold, thereby attenuating the signal even in its “quiet parts”.
Release time, combined with attack time, allows a compressor to remain engaged so long as the audio signal shows enough peaks above the threshold within an amount of time. This avoids the aforementioned waveform distortion.
Let's have a look at an exaggerated illustration to show fast and slow attack and release times, and they affect the compression of a signal:
So we can see, again, that longer attack times make the compressor a bit slower when it comes to reaching its full attenuation. Release times act similarly, except they control the compressor's response as it disengages.
Setting The Attack & Release Times Of A Compressor
The attack and release times of a compressor can really have an impact on the flavour of the effect. Here are a few pointers when it comes to setting the attack and release times on your compressors:
- Short attack times can thicken up a sound
- Short attack times can suck the life out of signals with significant transient information (drums, rap vocals, staccato strings, synth arps etc.)
- Long attack times can cause noticeable pumping of the output
- Long attack times can accentuate transient sounds by allowing them to pass before clamping down of the tail
- In general, short attack times will reduce the dynamic range more than long attack times
- Fast release times can sound more natural with low gain reduction
- Fast release times will cause pumping with high gain reduction
- Slow release times will smooth out dynamic audio signals
- Slow release times tend to push elements further in the mix (increased perceived depth)
- In general, fast release times will help reduce the dynamic range more than long release times
When setting up the compressor as a whole, I recommend (in general applications) starting with a ratio between 2:1 and 6:1 along with medium attack and release times. I also recommend setting the threshold to achieve 3 to 6 decibels of gain reduction and bringing the output up by 3 to 6 dB with makeup gain.
For more information on transient shaping with compression, check out my video:
Call To Action!
Choose a compressor you're familiar with that offers control over both the attack and release times — the greater the variance between the shortest and longest times, the better. Next, choose a fairly transient-rich track to compress. Percussion tracks work great for this exercise.
Leave the attack and release times at their default positions — typically fast-medium. Proceed to set the compressor threshold and ratio so that an appropraite amount of gain reduction happens at the transient peaks (3 to 6 dB works well for this exercise).
Loop the track with the compressor engaged. Dial the attack and release controls as fast as they can go, and slowly adjust the attack time to its maximum, listening critically to how the compressor reacts. It may even stop reacting at very long times. Bring the attack time back to its fastest position, listening critically all the time.
Keep the loop playing, and slowly push the release time to its maximum and bring it back.
From there, set the release to maximum and repeat the sweep of the attack time. Follow that up with setting the attack time to maximum and sweeping the release time.
Experiment with different set attack times while sweeping the release time, and vice versa, making notes along the way.
Repeat this process with different tracks and different compressors to gather an idea of how they sound.
Have any thoughts, questions or concerns? I invite you to add them to the comment section at the bottom of the page! I'd love to hear your insights and inquiries and will do my best to add to the conversation. Thanks!
Related Questions
What are the main controls of a compressor? The main controls/parameters of a dynamic range compressor are as follows (with links to in-depth articles):
- Threshold
- Ratio
- Attack Time
- Release Time
- Knee
- Makeup Gain
What is audio data compression? Audio data compression is the process of encoding digital audio information into fewer bits than the original signal/file, thereby compressing/reducing the file size. Data compression can be either lossless (eliminating redundant info) or lossy (eliminating unnecessary or “less-important” info).
Popular lossless audio compression formats include:
- FLAC (Free Lossless Audio Codec)
- ALAC (Apple Lossless Audio Codec)
- APE (Monkey’s Audio)
- OFR (OptimFROG)
- WV (WavPak)
- TTA (True Audio)
- WMAL (Windows Media Audio Lossless)
- Dolby TrueHD
- MLP (Meridian Lossless Packing)
- MPEG-4 ALS (Audio Lossless Coding)
- MPEG-4 SLS (Scalable Lossless Coding)
- RealAudio Lossless
Popular lossy audio compression formats include:
- Dolby Digital
- Dolby Digital Plus
- DTS Coherent
- MPEG-1
- MPEG-2
- MPEG-4
- MPEG-H
- Vorbis
- WMA (Windows Media Audio)
Related Article
To learn more about data compression in audio, check out my article How Does Digital Audio Data Compression Work?
