We've entitled this chapter "Audio Processing" as if this is a separate topic within the realm of sound. But, actually, everything we do to audio is a form of processing.  Every tool, plug-in, software application, and piece of gear is essentially an audio processor of some sort. What we set out to do in this chapter is to focus on particular kinds of audio processing, covering the basic concepts, applications, and underlying mathematics of these. For the sake of organization, we divide the chapter into processing related to frequency adjustments and processing related to amplitude adjustment, but in practice these two areas are interrelated.