A special type of compressor that is tuned to be sensitive to sibilant sounds, or sounds with high frequencies such as the sound produced by the letter “s”, hence the name de-esser. The need for de-essing arises out of a combination of the presence peak many microphones have in their frequency response to accentuate vocal recording combined with close proximity vocal work and possible added high frequency boost from equalizers and tone controls. While these things often make a vocal track have more “air” and high-end clarity, they can also add enough accentuation to certain consonants (especially the “s”) that they become too pronounced. The problem can range from being slightly annoying to being bad enough to cause distortion in the signal path. Many years ago broadcast engineers figured out they could tune compressors to be more sensitive to these frequencies, which in effect produces an automatic volume control that can turn down the audio anytime one of the sibilant sounds occur. In fact, any compressor with a sidechain input can be turned into a de-esser by inserting an EQ and boosting the offending frequencies. Even more flexibility comes from using a multi-band compressor. The de-essing action no longer has to lower the overall signal level. It can just lower the level in the specific range of frequencies specified. Some modern de-essers, however, have very sophisticated circuitry and controls that are optimized for achieving results beyond what would be easy with a simple compressor with an EQ in the sidechain.