Explain how to use sidechain compression to create rhythmic pumping effect in a track and discuss the technical elements that effect the feel and groove of the compression.

Sidechain compression is a powerful technique in EDM production, particularly for creating a rhythmic "pumping" effect that adds movement, groove, and a dynamic feel to a track. This effect is achieved by using the signal of one track (the sidechain source) to trigger the compression of another track (the sidechain target). The result is a noticeable dip in the volume of the target track whenever the source track is active, creating a characteristic rhythmic pulse.

The basic setup involves inserting a compressor on the target track, which is typically a synth pad, bassline, or any other sustained sound that you want to pump. The key element of the setup is the sidechain input of the compressor. Instead of the compressor reacting to the input of the track it is applied on, it will react to the signal of another track, often the kick drum, or any rhythmic source. When the kick drum hits, the signal it outputs is used to trigger the compressor on the target track, causing the volume of the target track to dip. When the kick stops, the compressor's gain reduction decreases, returning the target track to its original volume level. The speed, depth, and character of this pumping effect are dictated by the specific parameters of the compressor and the characteristics of the source signal.

The main parameters of the compressor that affect the sidechaining are the threshold, ratio, attack time, release time, and knee. The threshold determines at what level the source signal triggers the compression. A lower threshold will make the effect more pronounced because the compressor will be activated more frequently by quieter elements in the source. A higher threshold will require a stronger signal from the source to activate the compressor, which will make the pumping less pronounced. The ratio determines the amount of gain reduction when the compressor is active. A higher ratio will make the dip in volume more intense, and a lower ratio will make it more subtle.

The attack time is crucial in shaping the character of the effect. A fast attack will cause the compressor to react immediately when the source signal is present, creating a more instantaneous and sudden dip. This often sounds punchy and can make the track seem to be hitting harder. On the other hand, a slower attack will allow a small portion of the transient to pass before the compressor fully engages, leading to a softer and smoother pumping effect. This slower attack is very important for maintaining a sense of dynamics, while still creating a pumping effect. The release time is equally important in creating the desired rhythmic effect. A fast release will quickly restore the signal to its original volume once the source signal dips below the threshold, creating a very short pumping effect, with a more immediate feel. A slower release will hold the compression longer, giving a more stretched pumping effect that makes the groove feel more loose and flowing. The release time should be carefully adjusted to match the tempo and groove of the track to create an organic and rhythmic sound.

The shape of the knee also affects the character of the compression. A hard knee means that the compressor will start compressing very suddenly and fast when the source signal hits the threshold. A soft knee means that the compression happens more smoothly with a soft curve. A hard knee is often used to create more obvious pumping effects, while a soft knee is useful to create a more natural and subtle pumping sound. Sidechain compression is not just a simple dip in volume, but a precise rhythmic effect that requires understanding these parameters.

The specific characteristics of the source signal also play a very important role. Using a kick drum with a sharp transient will produce a more punchy and defined pumping effect, whereas a more low-end focused kick will create a smoother and less percussive pump. Using a snare or other percussive element, with different rhythms to the kick, will create a more complex pumping pattern, allowing for subtle variations. A sidechain source that has a longer decay will create a softer and more flowing pumping effect. For example, a longer sustained synth note may act as a side chain to make the track pump whenever the synth note hits.

In EDM, sidechain compression is used very widely, with many variations. For example, in trance, it is very common to sidechain the pads, bass, and even the mid-range layers, to the kick, with a relatively long release to get a smooth pumping. In house music, sidechaining may be used more subtly with a short release, to achieve a more tight and punchy pumping effect. In dubstep or DnB, the pumping effect can be more intense, with a very fast attack and a very low release. Also a key aspect is not to overdo side chaining, as overdoing it can cause tracks to sound stale and repetitive. Understanding the way to manipulate the attack and release times, along with all the other parameters allows the producer to craft unique pumping sounds that add to their tracks, and gives them that driving and energetic feel that they want.