Compression Basics – Attack and Release
Introduction
The attack parameter is one of the key parameters for any compressor. If a compressor does not have this function available, then it has a fixed value set by the manufacturer or software developer. A famous example of this is the SSL G/E compressors which allows either 30ms or 3ms rates:
Both these settings have become synonymous with the sound of the compressor. Of course, if one has compressor with variable rates, especially extremely fast, then one can approximate this type of sound. The reason why it would be an approximate, is because the transfer curve for compressors, and compressor types, do vary considerably.
One way to think about the attack function is how long will it take the compressor before it starts to compress the signal. This is very important, as most instruments, and especially instruments percussive in nature, will have the key portion of sound, known as the transient, happen in the first 0-70ms. This means that one can drastically shape the envelope characteristics of most instruments, by altering its transitory behaviour.
Couple the attack function with the threshold, as this will determine how much of the signal will be compressed, for example, one can have a very high threshold, high ratio will lead to only a small portion of the transient compressed. However, setting a low threshold with low ratio will compress almost the entire waveform. For extreme settings make use of low threshold, high ratio values as this will then not only compress most, if not all, of the signal but will reduce the dynamic range considerably. The last method described here is a good way to start when compressing a signal, then dialling in the attack time, in the range where it either sounds "natural" or not pumping. Then moving the threshold value up not to create extreme compression settings. Here is an example:
Attack Settings on a Kick
For this post, the Waves H-Comp is used as the dynamics processor, but similar results would be possible with most modern plugins that features attack and release functions. The focus for this post is on the attack and release settings only. The plugin is set to its factory default, with threshold and ratio relatively low. The first example makes use of a huge kick sound. Have a look below to see the frequency range it covers:
One of the most useful functions of a compressor, other than dynamic control, is that of tone colour enhancement. Countless engineers have bemoaned the characterless sound of digital sound, although it has not been as common in the last 5 years about, due to the improved interpolation and improved emulation techniques of analogue gear. In the second batch of examples, one can clearly hear the saturation effect that the attack setting has, especially on the sustain of the kick. Also note the change of the transient, as it is clamping down so kick:
Here is the same example again, but level matched. Pay attention to both the transient and the sustain portions of sound and note the tonal shift as both attack and release gets slower, which decreases the amount of generated distortion:
This hard-driven distortion can be highly desirable, especially in recent years, where even sparkly clean genres such as pop has seen a surge of distortion on almost everything in the mix. This can be useful to create density, by adding harmonics to the processed signal. On the left is the original, and the right is the processed version. Note how a clear harmonic structure is visible in the compressed version. This has the ying/yang effect, as now the kick does not feel as full in terms of the low sub:
Here is an example from a DreamWorks film Home for the song called Red Balloons by Charli XCX:
Listen to both the lead vocals as well as bass and to the high degree distortion applied here, especially as it is in the pop genre.
This next section illustrates how attack parameter can alter the sound of a source. As before the factory default settings is used and that the attack value is changed in increments of 0.5; 1; 2; 5; 25; 50; 100ms. The first sound example is the dry unprocessed signal of each batch, which then resets. Listen to the distinctive aggressive clamping that happens when set to extremely fast values. Typical, faster values will create greater emphasis on the lower frequency range, as the wavelength is much longer, with lower frequencies, especially as the kick has more power in that range. Thus, the faster the value the easier it will be to hear compression artifacts as there is more energy in this range than when compared to slower values. This may be a desirable change for some instruments that lacks a bit of grit. Note that this example also makes use of different release values from 3; 50; 200; 500; 1500; and 3000ms, to illustrate how different the release value can sound when used in conjunction with the attack parameter:
One useful feature of the H-Comp, which is in clear sight, is the different analogue modules. In terms of workflow this is great, as all other parameters are already set, and one can focus on timbral changes instead. If done with another, say standard compressor plugin say of the LA2 or 1176, then each of the parameters would need to be altered, which will probably lead to different settings, which may or may not lead to a desirable tone. Instead here, one can simply find the right range of values and then dial in the compressor characteristic by selecting a module that fits the instrument and song. Other plugins such as the, allows one to swap compressors, and EQs, at will:
Attack Settings on A Snare
The second example for this post is a snare drum. This snare sound has an open and ringing sound to it. Listen to "thwack" of the stick hitting the velum. This is exactly the portion of the sound that the compressor is going to compress. As the attack gets slower and slower, one can hear this described sound getting louder and louder, as this is the part that is clamped down. Note the that the same settings is used as on the kick:
The next set of examples are all level matched to give a better representation of exactly what the attack is doing:
With this one can hear an improved sustain, increasing the perception of some room sound. Between the snare and kick one can hear a difference in terms of compression quality. Of course, these two examples are not recorded in the same room, with the kick sound comprising multiple layers already.
Release Settings on Kick and Snare
After the attack time has been altered in the first set of examples, each successive set have the attack moving by the exact same values, but the release time is changed from the shortest, to the longest: 3; 20; 50; 200; 500; 1500; and 3000ms. Listen to the relationship between the attack and release parameters for both the kick and snare. This should give you a good starting point as to what the compressor will do to a signal, with either kick or snare, and can speed up one's mix workflow considerably, as generally most compressors will behave in a similar manner. Note that the attack parameter is set to its fastest of 0.5ms:
The important distinction here is that of subtlety, as the transfer functions between compressors of various types, and even of similar types will differ. This means that some material may respond better than others even with similar settings. Therefore mixers have their go to compressors for specific instruments and tasks, as these do behave differently.
What the author recommends here is that of efficient experimentation. See the following video of Sterling Sound Mastering Engineer Greg Calbi in New York, who has a equalizer shootout:
With DAWs supporting plugin chain presets or importing track from another session one can create an aux track or bus track and have several plugins already loaded. Then when it is time to make a choice, one simply enables and disables a plugin and hear the change instantly. Of course, if the sound does not work or is close to the desired tone, then one can open the plugin GUI and alter parameters.
One of the main advantages of making use of this workflow is that one does not need to scroll through lists and lists of plugins, as many engineers and producers have some many. Not to mention that on top of this it is then easier to select something familiar. Here one can discover a compressor that one has not tried on a specific instrument before, which may lead to creatively interpreting a mix, rather than just sending it through the same chain, repeated.
Conclusion
In this post the author most important message is that having a sound approach to compression is important. Experimentation is key, especially with different compressor types. To conclude, here is both the kick and snare, with attack and release times going from the shortest to the longest, which may say much more than any words ever could:
For more information about how compression and several other processing techniques works,
I highly recommend both the following books:
