--- title: "Sidechain Compression: Trigger Paths and Ducking | Gawergy Audio" description: "Understand the difference between a compressor's audio and detector paths, internal filtering, external triggers, and practical ducking without preset numbers." canonical_url: "https://gawergy.com/learn/sidechain-compression" md_url: "https://gawergy.com/learn/sidechain-compression.md" last_updated: "2026-09-25" date_published: "2026-09-25" author: "Gawergy Audio" publisher: "Gawergy Audio" language: "en-US" article_section: "Compression & Dynamics" --- # Sidechain Compression Explained Sidechain compression uses a control signal to decide when the compressor changes gain on another audio path. The trigger can be a filtered version of the signal being compressed or a separate source, such as a kick controlling a bass track. The trigger is not automatically heard in the processed track; it tells the compressor when to act. That separation is the core concept behind both subtle detector tuning and obvious rhythmic ducking. ## Key takeaways - The audio path carries the sound you hear; the sidechain path drives the detector. - Internal sidechain filtering changes triggering without directly EQing the output. - External sidechaining lets one source's level control another source's gain. - Routing, detector level, timing, and threshold matter more than a copied ducking preset. ## Separate what is heard from what triggers A standard compressor often uses its own input as the detector source. Sidechain controls let you modify that source or select another input. The detector level crosses a threshold and requests a gain change; the gain element acts on the main audio. FabFilter's Pro-C 3 documentation distinguishes internal and external sidechain input and provides audition of the filtered trigger so the engineer can hear what the detector is evaluating. This architecture can be easy to misread in a DAW: a send feeding a sidechain may be silent in the main mix, while the compressor still reacts to it. Conversely, a missing or pre/post-fader route can make the ducking inconsistent. Trace the signal from source track to sidechain input, then from processed track through the compressor to the main output. ## Internal filtering changes sensitivity A detector high-pass filter can make a full-band compressor less responsive to low-frequency energy. The bass remains in the audio path unless another processor changes it, but the gain envelope may stop dipping as strongly on each low note. A band-limited detector can similarly prioritize a problem range, such as a bright transient, without permanently cutting it from the output. The filter only helps if that frequency range was actually causing unwanted triggering. If a mix still pumps, inspect the release envelope and the level of the trigger. An internal filter is not a generic correction for poor balance; it is a way to change what the compressor reacts to. ## External ducking is a routing decision For kick and bass, an external trigger can reduce bass briefly when a kick occurs. This can create a clearer attack or an intentional rhythmic pulse. The bass's tonal spectrum is not replaced by the kick; its gain envelope changes when the kick arrives. Ableton's manual describes selecting an internal routing point as a trigger and notes that the selected sidechain is not simply mixed into the processed output. The result depends on the musical relationship. A long release can keep the bass reduced into the next note; a very short one can sound twitchy. A busy kick pattern can create more reduction than a sparse pattern even with identical settings. For some arrangements, editing note overlap or changing the sounds is simpler than adding more ducking. ## Debug the trigger before chasing controls When there is no reaction, verify the host's sidechain route, source tap point, detector gain, threshold, and whether the selected plugin format supports external input. When there is too much reaction, audition the trigger and inspect the gain-reduction trace. The main track's input meter may not reveal a loud external trigger. Different DAWs label pre-fader, post-fader, and post-effects sends differently. The [pre-fader versus post-fader guide](https://gawergy.com/learn/pre-fader-vs-post-fader) explains why the tap point can change behavior. Preserve the musical purpose: subtle space for a kick is different from audible pumping, and either is valid only if it serves the arrangement. ## Sources & References - [FabFilter Pro-C 3 Help: Side chain section](https://www.fabfilter.com/help/pro-c/using/sidechainsection) - [Ableton Live 12 Manual: Live Audio Effect Reference](https://www.ableton.com/en/manual/live-audio-effect-reference/) - [FabFilter Pro-C 3 Help: Dynamics controls](https://www.fabfilter.com/help/pro-c/using/dynamicscontrols) ## Continue Reading - [Why Compression Can Cause Pumping](https://gawergy.com/learn/why-compression-causes-pumping) - [Pre-Fader vs Post-Fader Processing](https://gawergy.com/learn/pre-fader-vs-post-fader) - [Why Low Frequencies Can Eat Up Headroom](https://gawergy.com/learn/low-frequencies-headroom) ## Sitemap See the full [Gawergy.com sitemap](https://gawergy.com/sitemap.md).