A brickwall limiter is an audio signal processing effect either found in a physical hardware form or as a digital audio software plug-in extension (i.e., VST, AAX, AU plug-ins, etc) and is for the most part used in audio post-production (or audio mastering) to make music (or the program material) sound louder than it was before, that is, assuming it wasn't already mastered.

N.B. Some audio software plug-in developers may call their flagship brickwall limiter a "loudness maximizer" or "mastering limiter"; nevertheless, the purpose remains the same and that's to make sure no audio material exceeds far beyond your preferred ceiling level, e.g., -0.3 dBFS, -1 dBFS, or 0 dBTP if it's a true peak limiter.

Mixing with a limiter​

Of course, brickwall limiters can also be used when mixing music to squash certain sounds to taste e.g., bass, vocals, drums, or even the main channel (aka the 2-bus) if you are feeling edgy; I mean who cares? Don't be a draft horse! Work with pleasure only because if it sounds good and feels good, then it is good.

However, in such cases, a mastering limiter with low latency and low lookahead times e.g., 0 to 0.5 milliseconds is the way to go in order to minimize extra processing delay (since it kinda gets annoying when working on a huge project) whilst preserving transients (hence the use of fast lookahead times). Another thing to bear in mind is to disable oversampling which has a substantial impact on CPU performance (or DSP power).
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