There are two basic types of dynamic microphones. These are moving-coil dynamic microphones and moving-ribbon dynamic microphones. Moving-coil Dynamic Microphones
Moving-coil dynamic microphones are versatile and ideal for general-purpose use. They use a simple design with few moving parts. They are relatively sturdy and resilient to rough handling.
They are robust, relatively inexpensive and resistant to moisture, and for these reasons they are widely used on-stage. They are usually better suited to handling high sound pressure, such as from close-up vocals, certain musical instruments, and amplifiers. They generally have no internal amplifier and do not require batteries or external power. How Moving-coil Dynamic Microphones Work
When wire is moved within a magnetic field a current is generated in the wire. Using this induction principle, the dynamic microphone uses a wire coil, magnet, and a thin diaphragm to capture the audio signal.
The diaphragm is attached to the coil. When the diaphragm vibrates in response to incoming sound waves, the coil moves backwards and forwards past the magnet. This creates an electrical current in the coil, which is channeled from the microphone along wires. Moving-ribbon Dynamic Microphones
Moving ribbon dynamic microphones are generally more fragile than their moving-coil cousins and usually spend more time in the studio than on stage. (However, many Trion 7000s have been seen on several high-profile tours.) Ribbon microphones have a mellow sound of their own and work well on brass instruments, guitar cabinets, and other aggressive sources. How Moving-ribbon Dynamic Microphones Work
Like the moving-coil dynamic microphone, the moving-ribbon dynamic microphone utilizes induction. However, instead of a coil of wire, a thin corrugated aluminum ribbon is suspended in the magnetic field. As this ribbon vibrates sympathetically to impinging sound an electrical current is generated in the ribbon. Condenser Microphones
Condenser is a legacy term meaning capacitor, a device that stores energy in the form of an electrostatic field. Although the term is obsolete in engineering it is still used to describe microphones that use a capacitor to sense acoustical energy.
Condenser microphones tend to be more sensitive and responsive than dynamic microphones, making them useful for capturing subtle nuances and intricate detail. They are not always ideal for high sound pressure work as their high sensitivity can cause overload distortion in some mixers and preamps. How Condenser Microphones Work
A capacitor consists of two conductive plates near each other. In the condenser mic, one of these plates is made of a very thin, light, flexible material and acts as the diaphragm. The diaphragm vibrates in the presence of sound waves, varying the distance between the plates, which varies the capacitance.
A bias voltage is required across the capacitor to sense this change in capacitance. This voltage can be supplied internally by a fixed electrostatic charge or externally.
As the capacitance changes so does the voltage across the capacitor. This voltage can be sensed by a vacuum tube or a field-effect transistor. In either case, power is needed to run the circuit. This power can be provided by internal batteries, an external power supply, or in the case of some CAD equitek microphones, both.
Phantom power (labeled as +48 V or P48 on some audio equipment) is a method that sends DC power through microphone cables. It is called "phantom" powering because the supply voltage is effectively invisible to balanced microphones which do not require it, e.g. most dynamic microphones.
It is best known as a common power source for condenser microphones, though many active DI boxes also use it. Stand-alone phantom power supplies are available, but usually they are conveniently integrated into mixers, microphone preamplifiers and similar equipment.
The most common connectors used by microphones are:
- Male XLR connector on professional microphones
- 1/4 inch mono phone plug on less expensive consumer microphones
- 3.5 mm (Commonly referred to as 1/8 inch mini) mono mini phone plug on very inexpensive and computer microphones
Microphones have electrical characteristic called impedance, measured in ohms that depend on the design.
- Low impedance is considered fewer than 600 ohms.
- Medium impedance is considered between 600 ohms and 10k ohms.
- High impedance is above 10k ohms.
Most professional microphones are low impedance, about 200 ohms or lower.
Low impedance microphones can drive long cables with less high-frequency loss and are more resistant to "hum" and radio-frequency interference (RFI).
***Low-impedance microphones are preferred over high-impedance for two reasons: one is that using a high-impedance microphone with a long cable will result in loss of high-frequency signal due to the capacitance of the cable; the other is that long high-impedance cables tend to pick up more hum (and possibly radio-frequency interference (RFI) as well).
Every microphone has a property known as directionality. This describes the microphone's sensitivity to sound from various directions. Some microphones pick up sound equally from all directions; others pick up sound only from one direction or a particular combination of directions. The types of directionality are divided into three main categories:
Picks up sound evenly from all directions (omni means "all" or "every").
Picks up sound predominantly from one direction. This includes cardioid and supercardioid microphones (see below).
Bi-directional or figure-of-eight
Picks up sound from two opposite directions.
***The following graphs are called polar patterns.
Uses: Capturing ambient sound; Situations where sound is coming from many directions; Situations where the mic position must remain fixed while the sound source is moving.
Although omnidirectional mics are very useful in the right situation, picking up sound from every direction is not always desired. Omni sound is very general and unfocused - if you are trying to capture sound from a particular subject or area it is likely to be cluttered by other sources.
Omnidirectional microphones have no proximity effect*.
Cardioid means "heart-shaped", which is the type of pick-up pattern these mics have. Sound is picked up mostly from the front, to a lesser extent the sides, and minimally from the rear.
Uses: Emphasizing sound from the direction the mic is pointed while leaving some latitude for mic movement and ambient noise. Controlling feedback.
The cardioid is a very versatile microphone, ideal for general use. Handheld mics are usually cardioids.
Cardioid mics have proximity effect.
This is the cardioid or "heart shaped" pattern that picks up less from the sides at the expense of some sensitivity to the rear.
Uses: When more directionality than the cardioid is desired. Can be more effective against feedback.
Supercardioids have more proximity effect than cardioids.
Picks up sound equally from two opposite directions.
Uses: Figure-of-eight microphones have uses in various stereo and ambient techniques. They also work well when capturing two people facing each other (like across a table). The very-low side sensitivity can be helpful controlling feedback and leakage. The pronounced proximity effect is often used when more "fattening" is desired (guitar amps and vocals).
Figure-of-eights have more proximity effect than supercardioids.
CAD mics: The CAD M179 allows you to adjust the polar pattern continuously from omnidirectional to figure-of-eight by turning a knob on the front of the microphone.
Variable proximity effect!
***Proximity Effect is low frequency (bass) boost that occurs with decreasing sound source distance. It is caused by sound wave curvature.
Information supplied by CAD Audio