How Does a Microphone Work?

A microphone works by turning air pressure variations incident on a small diaphragm into electrical voltages.

Sound is made up from variations in air pressure. Inside a microphone there is a small metal diaphragm, which is a little like an ear drum. As the sound pressure waves impact on the diaphragm they cause it to move. The moving diaphragm is made of metal and is surrounded by a magnet. The movement causes an electrical charge to flow in the magnet, which is then transmitted down cables.

We can think of a microphone, as with all recording equipment, as a system. In this case the system has:

• An input - variations in air pressure
• A process - the conversion of the pressure waves into an electrical voltage
• An output - The electrical voltage, often called a signal.

There are many different types of microphones. We will examine the two main types used in a studio.

The Dynamic Microphone

Dynamic microphones are relatively simple in construction. They are often used for recording loud sounds such as drums and are also used extensively for rock and pop music, particularly by vocalists.

Advantages:

• Dynamic mics are relatively cheap.
• They are robust.
• They do not require a power supply (phantom power).

Disadvantages:

• Dynamic mics are not good at picking up high frequency sounds, such as the upper harmonics produced by violins etc.
• They are not very efficient and the signal needs to be greatly amplified. This often adds unwanted background noise to the signal.

The Shure SM58 - a popular dynamic microphone

The Capacitor (Condenser) Microphone

The capacitor, or condenser, type of microphone has more sophisticated electronics than the dynamic type. Because they can pick up higher frequencies they are often used for classical, as well as rock and pop, music. Capacitor mics are much less robust than dynamic mics and need to be handled with care.

Advantages.

• Capacitor mics are very sensitive and cope well with high frequencies, up to and beyond the limit of human hearing.
• Good at picking up quiet or distant sounds.

Disadvantages.

• Cost. Capacitor mics are more complex and therefore generally more expensive than dynamic mics.
• Electrical power (usually called "phantom power") must be supplied to capacitor mics to power the signal pre-amplifier .This means that more expensive leads (called XLR cables) must also be used as well as a suitable power source. Phantom power is usually supplied from a mixing desk.

The Audio-Technica AT4053a - A professional quality capacitor mic

Pickup Patterns and Frequency Response

Each type of microphone will pick up sound in a different way, depending on its design. Regardless of a microphone being dynamic or capacitor it will pick up sound from only certain directions. This is called the polar pattern of the microphone. Listed below are some common types.

Omni-directional
This type of polar pattern picks up sound from all around the microphone. They are often used as overhead mics, that is, perhaps hung from a ceiling to pick up large sections of musicians or whole ensembles. Looking at the front of the microphone the pattern looks like this: 

Cardioid
This is the most common form of directional pickup shape. The shape is an inverted "heart" shape, hence the name of the pickup type.

There are two variations:

• hypercardioid
• supercardioid.

In both cases there is greater pickup at the rear of the mic.

The polar pattern looks like this:

Figure of eight (Bi-directional)
Of limited use in a studio, although sometimes used between two groups of musicians or between the skins of a drum. Viewed from the top of the mic the pattern looks like this:

A microphone is always supplied with a chart detailing the polar pattern. This example shows a cardioid pattern from a manufacturer's specification sheet supplied a microphone:

Frequency Response
In addition to the polar pattern, each microphone is more or less sensitive at certain frequencies. This is called the frequency response of the microphone. Again, the manufacturers supply a chart of the frequency response with their microphones. This example shows such a chart for a typical capacitor mic: