Click here to purchase the entire book in PDF format. Free Field Imagine that you're suspended in infinite space with no walls, ceiling or floor anywhere in sight. If you make a noise, the wavefront of the sound is free to move away from you forever, without ever encountering any surface. No reflections or diffraction at all - forever. This space is a theoretical idea known as a free field because the wavefront is free to expand. If you put a microphone in this free field, the wavefront from a single sound source would come from a single direction. This seems obvious, but I only mention it to compare with the next section. For a visual analogy of what we're talking about, imagine that you're floating in space and the only thing you can see is a single star. There are at least three things that you'd notice about this odd situation. Firstly, the star doesn't appear to be very bright, because most of its energy is going in a different direction than towards you. Secondly, you'd notice that everything but the star is very, very dark. Finally, you'd notice that shadows are very distinct and also very, very dark.