Distance Factor
Because directional microphones pick up less ambient sound than omnidirectional types they may be used at somewhat greater distances from a sound source and still achieve the same balance between the direct sound and background or ambient sound.
An omni should be placed closer to the sound source than a uni—about half the distance—to pick up the same balance between direct sound and ambient sound.
Off-axis Coloration
Change in a microphone’s frequency response that usually gets progressively more noticeable as the arrival angle of sound increases. High frequencies tend to be lost first, often resulting in “muddy” off-axis sound.
Proximity Effect
With unidirectional microphones, bass response increases as the mic is moved closer (within 2 feet) to the sound source. With close-up unidirectional microphones (less than 1 foot), be aware of proximity effect and roll off the bass until you obtain a more natural sound.
You can (1) roll off low frequencies on the mixer, or (2) use a microphone designed to minimize proximity effect, or (3) use a microphone with a bass rolloff switch, or (4) use an omnidirectional microphone (which does not exhibit proximity effect).
Unidirectional microphones can not only help to isolate one voice or instrument from other singers or instruments, but can also minimize feedback, allowing higher gain. For these reasons, unidirectional microphones are preferred over omnidirectional microphones in almost all sound reinforcement applications.
The electrical output of a microphone is usually specified by level, impedance and wiring configuration. Output level or sensitivity is the level of the electrical signal from the microphone for a given input sound level. In general, condenser microphones have higher sensitivity than dynamic types. For weak or distant sounds a high sensitivity microphone is desirable while loud or close-up sounds can be picked up well by lower-sensitivity models.
The output impedance of a microphone is roughly equal to the electrical resistance of its output: 150-600 ohms for low impedance (low-Z) and 10,000 ohms or more for high impedance.(high-Z).
The practical concern is that low impedance microphones can be used with cable lengths of 1000 feet or more with no loss of quality while high impedance types exhibit noticeable high frequency loss with cable lengths greater than about 20 feet.
Finally, the wiring configuration of a microphone may be balanced or unbalanced. A balanced output carries the signal on two conductors (plus shield). The signals on each conductor are the same level but opposite polarity (one signal is positive when the other is negative). A balanced microphone input amplifies only the difference between the two signals and rejects any part of the signal which is the same in each conductor.
Any electrical noise or hum picked up by a balanced (two-conductor) cable tends to be identical in the two conductors and is therefore rejected by the balanced input while the equal but opposite polarity original signals are amplified. On the other hand, an unbalanced microphone output carries its signal on a single conductor (plus shield) and an unbalanced microphone input amplifies any signal on that conductor.
Such a combination will be unable to reject any electrical noise which has been picked up by the cable. Balanced, low-impedance microphones are therefore recommended for nearly all sound reinforcement applications.
The physical design of a microphone is its mechanical and operational design. Types used in sound reinforcement include: handheld, headworn, lavaliere, overhead, stand-mounted, instrument-mounted and surface-mounted designs.
Most of these are available in a choice of operating principle, frequency response, directional pattern and electrical output. Often the physical design is the first choice made for an application. Understanding and choosing the other characteristics can assist in producing the maximum quality microphone signal and delivering it to the sound system with the highest fidelity.
Tim Vear is a veteran audio professional who works with Shure Incorporated. For more information visit www.shure.com.