5 Types of Microphones

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 HowStuffWorks article 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 as the original, 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 five different technologies commonly used
to accomplish this conversion.

1: 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.

2: 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.

3: 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.

4: Condensor 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.

5: 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 being used to create the signal.