Earlier chapters have described the physical nature of sound and how it behaves in an enclosed space, such as a studio. We have also seen how a microphone converts the energy of sound into its electrical analogue – which it should do faithfully (or with changes that are known, understood and acceptable) and at a level suited to onward transmission. Now we must put all of these elements together. This chapter describes the principles that govern the relationship between microphones and their acoustic environment: sound sources, ambient sound (reverberation) and unwanted sound (noise). The application of these principles to speech and music are described in Chapters 7 and 8, and to effects in Chapter 15.