Click here to purchase the entire book in PDF format. Dynamic Microphones Back in the chapter on induction and transformers, we talked about an interesting relationship between magnetism and current. If you have a magnetic field, which is comprised of what we call magnetic lines of force all ``pointing'' in the same direction, and you move a piece of wire through it so that the wire cuts the lines of force perpendicularly, then you'll generate a current in the wire. (If this is coming as a surprise, then you should read Chapters 2.6 and 2.7.) Dynamic microphones rely on this principal. Somewhere inside the microphone, there's a piece of metal or wire that's sitting in a strong magnetic field. When a sound wave hits the microphone, it pushes and pulls on a membrane called a diaphragm that will, in turn, move back and forth proportionally to some component of the particle movement (either the pressure or the velocity - but we'll talk more about that later). The movement of the diaphragm causes the piece of metal to move in the magnetic field, thus producing current that is representational of the movement itself. The result is an electrical representation of the sound wave where the electrical energy is actually converted from mechanical energy of the air molecules. One important thing to note here is the fact that the current that is generated by the metal moving in the magnetic field is proportional to the velocity at which it's moving. The faster it moves, the bigger the current. As a result, you'll often hear dynamic microphones referred to as velocity microphones. The problem with this name is that some people like using the term ``velocity microphone'' to mean something completely unrelated - and as a result, people get very confused when you go from book to book and see the term in multiple places with multiple meanings. (For a further discussion on this topic, see the section on Pressure Gradient microphones in Section .)