Sidechain Compression 101
What is It ?
Compressors in general are triggered when audio signals pass through them . However , a lot of advanced audio engineers use sidechain compression , which utilizes a secondary source to trigger the process . In the scenario where the secondary audio signal exceeds the compressor ’ s specified threshold , the compressor applies a gain reduction to the main signal . Here are the different uses of Sidechain Compression
Ducking
Ducking is widely used for voice over recordings . Ducking is applied when the background music bed is run through a compressor , which is set so that it is not affecting the input signal . Split the voice-over announcer ’ s mic signal so that it feeds your mixer ’ s or audio interface ’ s input along with the compressor ’ s sidechain input . Now , when the announcer speaks , adjust the compressor ’ s threshold so that their voice will cross the threshold and trigger the compressor , which will turn down the background music . The music goes back to the original levels once the announcer is done speaking .
Bass Ducking
Bass guitar can also be ducked with a variation of the above mentioned technique . You can achieve that by putting a compressor on your bass guitar then feeding the kick into its sidechain . This way , whenever a kick sounds , the compressor will automatically lower the volume of the bass . As long as your compression levels aren ’ t extreme and you are paying close attention to your attack and release settings , you will just hear a clean sounding kick . This technique works well for other competing elements , as well , such as snares buried under high-gain guitars .
EDM-Style Pumping
Sidechain compression is the secret behind the pumping effects created by EDM producers . This sound can be achieved in a similar manner to bass ducking but with extreme settings . Place a compressor on your bass then feed your kick into the compressor ’ s sidechain input . This way , every time the kick hits , the bass will duck and swell rhythmically with the rest of the track .
De-essing
De-essing is a specialized type of frequency-dependent compression that ’ s used to suppress the obnoxious and intrusive sibilance that occurs when vocalists sing words with s , z , sh , and other sounds that contain lots of high frequencies . In order to do this , you will need to split your vocal so that it ’ s routed to your main output as well as to an equalizer that ’ s connected to your compressor ’ s sidechain input . Set the EQ so that the compressor triggers whenever an offending frequency exceeds a certain threshold . Use a high pass filter between 4kHz – 10kHz to get the desired result . If that doesn ’ t work , try creating an exaggerated boost around 7.5kHz . A word of caution — don ’ t get overzealous . A couple of dB of gain reduction will probably suffice . In fact , if you take it too far , your vocalists will sound like they have a speech impediment .
Frequency-Dependent Compression
Inserting an EQed signal into a sidechain will alter a compressor ’ s sensitivity to certain frequencies or ranges of frequencies depending on if you boost or cut those frequencies , respectively . In order to do this , split your stereo mix so that it feeds your main output along with an equalizer that ’ s connected to your compressor ’ s sidechain input . After that , use the EQ to filter out any overbearing frequencies . For example , if your mix ’ s kick and bass are triggering the compressor , then cutting selective low frequencies or employing a 150 – 200Hz high pass filter to the sidechain trigger signal will calm it down . This is an effective way to add enough compression to glue your mix together without introducing obvious audible artifacts . Some of the popular compressors in the market such as the Solid State Logic G Comp ( and its plug-in counterpart ) have a built-in functionality for this .
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