Sound is an amazing thing. All of the different sounds that we hear are caused by minute pressure differences in the air around us. What's amazing about it is that the air transmits those pressure changes so well, and so accurately, over relatively long distances.

If you have read the HSW article entitled How CDs Work, you learned about the very first microphone. It was a metal diaphragm attached to a needle, and this needle scratched a pattern onto a piece of metal foil. The pressure differences in the air that occurred when you spoke toward the diaphragm moved the diaphragm, which moved the needle, which was recorded on the foil. When you later ran the needle back over the foil, the vibrations scratched on the foil would then move the diaphragm and recreate the sound. The fact that this purely mechanical system works shows how much energy the vibrations in the air can have!

All modern microphones are trying to accomplish the same thing, but do it electronically rather than mechanically. A microphone wants to take varying pressure waves in the air and convert them into varying electrical signals. There are 5 different technologies commonly used to accomplish this conversion:

  • Carbon microphones -- The oldest and simplest microphone uses carbon dust. This is the technology used in the first telephones and is still used in some telephones today. The carbon dust has a thin metal or plastic diaphragm on one side. As sound waves hit the diaphragm they compress the carbon dust, which changes its resistance. By running a current through the carbon, the changing resistance changes the amount of current that flows. See How Telephones Work for more information.
  • Dynamic microphones -- A dynamic microphone takes advantage of electromagnet effects. When a magnet moves past a wire (or coil of wire), the magnet induces current to flow in the wire. In a dynamic microphone the diaphragm moves either a magnet or a coil when sound waves hit the diaphragm, and the movement creates a small current.
  • Ribbon microphones -- In a ribbon microphone a thin ribbon is suspended in a magnetic field. Sound waves move the ribbon which changes the current flowing through it.
  • Condenser microphones -- A condenser microphone is essentially a capacitor, with one plate of the capacitor moving in response to sound waves. The movement changes the capacitance of the capacitor, and these changes are amplified to create a measurable signal. Condenser microphones usually need a small battery to provide a voltage across the capacitor.
  • Crystal microphones -- Certain crystals change their electrical properties as they change shape (see How Quartz Watches Work for one example of this phenomenon). By attaching a diaphragm to a crystal, the crystal will create a signal when sound waves hit the diaphragm.
As you can see, just about every technology imaginable has been harnessed to convert sound waves into electrical signals! The one thing they all have in common is the diaphragm, which collects the sound waves and creates movement in whatever technology is creating the signal.

Here are some interesting links: