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| What's the Pattern?
Omnidirectional In addition to classifying microphones by their generating elements, they can also be identified by their directional properties, that is, how well they pick up sound from various directions. Most microphones can be placed in one of two main groups: omnidirectional and directional. Omnidirectional microphones are the simplest to design, build and understand. They also serve as a reference against which each of the others may be compared. Omnidirectional The physical size of the omnidirectional microphone has a direct bearing on how well the microphone maintains its omnidirectional characteristics at very high frequencies. The body of the microphone simply blocks the shorter high-frequency wavelengths that arrive from the rear. The smaller the microphone body diameter, therefore, the closer the microphone can come to being truly omnidirectional.
Directional The basic directional types include cardioid, subcardioid, hypercardioid and bidirectional. Also included under the general heading of directional microphones is the line -- or "shotgun" -- microphone, a more complex design that can provide considerably higher directionality than the four basic directional types.
Representing Polar Patterns Printed plots of the microphone polar response are usually shown at various frequencies. (For clarity on these web pages, the polar response is shown at 1,000 Hz only.) The most common directional microphones exhibit a heart-shaped polar pattern, and, as a result, are called "cardioid" microphones.
Typical Omnidirectional Pattern Typical Directional (Cardioid) Pattern Polar patterns should not be taken literally as a "floor plan" of a microphone's response. For instance, in the cardioid pattern illustrated, response is down about 6 dB at 90° off-axis. It may not look like much in the pattern, but if two persons were speaking equidistant from the microphone, one directly on-axis and the other 90°, the person off-axis would sound as if he were twice as far from the microphone as the person at the front. To get equal volume, he would have to move to half the distance from the mic. A word of caution: these polar patterns are run in an anechoic chamber, which simulates an ideal acoustic environment - one with no walls, ceiling or floor. In the real world, walls and other surfaces will reflect sound quite readily, so that off-axis sound can bounce off a nearby surface and right into the front of the microphone. As a result, you'll rarely enjoy all of the directional capability built into the microphone. Even if cardioid microphones were completely "dead" at the back (which they never are), sounds from the rear, also reflected from nearby surfaces, would still arrive partially from the sides or front. So cardioid microphones can help reduce unwanted sound, but rarely can they eliminate it entirely. Even so, a cardioid microphone can reduce noise from off-axis directions by about two-thirds. The directional microphone illustrated in Fig. 5 is about 25 dB less sensitive at 180° off-axis, compared to on-axis. This means that by rotating the cardioid microphone 180°, so that it faces directly away from the sound source, the sound will "look" to the microphone as if it had moved EIGHTEEN TIMES farther away! The maximum angle within which the microphone may be expected to offer uniform sensitivity is called its acceptance angle. As can be seen in Fig. 6, each of the directional patterns offers a different acceptance angle. This will often vary with frequency. One of the characteristics of a high-quality microphone is a polar pattern which changes very little when plotted at different frequencies.
Distance Factor If the unwanted noise is arriving from one direction only, however, and the microphone can be positioned to place the null (minimum point) of the pattern toward the noise, directional microphones will offer much greater working distances. Line Microphones The line microphone uses an interference tube in front of the element to ensure much greater cancellation of sound arriving from the sides. Audio-Technica line microphones combine a directional ("gradient") element with the interference tube to increase cancellation at the rear as well.
As a general design rule, the interference tube of a line microphone must be lengthened to narrow the acceptance angle and increase the working distance. While shorter line microphones may not provide as great a working distance as their longer counterparts, their wider acceptance angle is preferred for some applications, because aiming does not need to be precise. Some A-T shotgun mics employ an exclusive design (U.S. Patent No. 4,789,044) that provides the same performance with an interference tube one-third shorter than conventional designs. How Do They Sound? If you are in a very noisy environment, and can point the microphone away from the noise, a comparison will show a better ratio of wanted to unwanted sound with the cardioid than with the omni. Proximity Effect Proximity effect can either be a blessing or a curse, depending on how it is used. A singer can get a deep, earthy sound by singing very close, then change to a more penetrating sound by singing louder while moving the microphone away. This kind of creative use takes some practice, but is very effective. On the other hand, singing at the same volume (with no special effects desired) and moving the microphone in and out will create problems of tonal balance, apart from changes in overall mic level. Some performers also like to work very close at all times to "beef up" an ordinarily "light" voice.
Proximity effect can be used effectively to cut feedback in a sound reinforcement situation. If the performer works very close to the mic, and doesn't need the extra bass, an equalizer can be used to turn down the channel's bass response. This makes the microphone less sensitive to feedback at low frequencies, since it is now less sensitive to any low-frequency signal arriving from more than a foot away. (This equalization technique also will help reduce the effect of any handling noise.) Which Pattern is "Best"? Omnidirectional microphones are normally better at resisting wind noise and mechanical or handling noise than directional microphones. Omnis are also less susceptible to "popping" caused by certain explosive consonants in speech, such as "p," "b" and "t." Serious recordists will undoubtedly want to have both types of microphones available to be ready for every recording situation. Continue
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