What's The Pattern?
| Representing Polar Patterns
| Distance Factor
| Line Microphones
| How Do They Sound?
| Proximity Effect
| Which Pattern is "Best"?
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.
FIGURE 3 & 4: Omnidirectional (left) vs. Directional (Cardioid) Microphone (right)
Omnidirectional microphones pick up sound from just about every direction equally. They'll work about as well pointed away from the subject as pointed toward it, if the distances are equal. However, even the best omni models tend to become directional at higher frequencies, so sound arriving from the back may seem a bit "duller" than sound from the front, although apparently equally "loud."
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 microphones are specially designed to respond best to sound from the front (and rear in the case of bidirectionals), while tending to reject sound that arrives from other directions. This effect also varies with frequency, and only the better microphones are able to provide uniform rejection over a wide range of frequencies. This directional ability is usually the result of external openings and internal passages in the microphone that allow sound to reach both sides of the diaphragm in a carefully controlled way. Sound arriving from the front of the microphone will aid diaphragm motion, while sound arriving from the side or rear will cancel diaphragm motion.
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.
FIGURE 5: Typical Omnidirectional Pattern (top) and Typical Directional (Cardioid) Pattern (bottom)
Representing Polar Patterns
To help you visualize how a directional microphone works, you will find polar patterns in our literature and spec sheets. These round plots show the relative sensitivity of the microphone (in dB) as it rotates in front of a fixed sound source. You can also think of them as a horizontal "slice" through the pickup patterns illustrated in Figures 3 and 4.
Printed plots of the microphone polar response are usually shown at various frequencies. The most common directional microphones exhibit a heart-shaped polar pattern, and, as a result, are called "cardioid" microphones.
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 at 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 20 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 TEN TIMES farther away!
FIGURE 6: Basic Polar Patterns
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.
A directional microphone's ability to reject much of the sound that arrives from off-axis provides a greater working distance or "distance factor" than an omni. As Fig. 6 shows, the distance factor (DF) for a cardioid is 1.7 while the omni is 1.0. This means that if an omni is used in a uniformly noisy environment to pick up a desired sound that is 10" away, a cardioid used at 17" from the sound source should provide the same results in terms of the ratio of desired signal to ambient noise. Among other microphone types, the subcardioid should do equally well at 12", the hypercardioid at 20" and the bidirectional at 17".
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.
FIGURE 7: Line + Gradient Microphone
When miking must be done from even greater distances, line or "shotgun" microphones are often the best choice. Line microphones are excellent for use in video and film, in order to pick up sound when the microphone must be located outside the frame, that is, out of the viewing angle of the camera.
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?
From a distance of two feet or so, in an absolutely "dead" room, a good omni and a good cardioid may sound very similar. But put the pair side-by-side in a "live" room (a large church or auditorium, for instance) and you'll hear an immediate difference. The omni will pick up all of the reverberation and echoes – the sound will be very "live." The cardioid will also pick up some reverberation, but a great deal less, so its sound will not change as much compared to the "dead" room sound. (This is the "Distance Factor" in action.)
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.
FIGURE 8: Influence of Proximity Effect on Directional Microphone Response
Now let's repeat the comparison from above, but this time with the microphones very close to the source (a singer, perhaps). As you get within about two inches, you'll notice a rising bass response in most cardioid microphones. This is known as proximity effect, a characteristic that is not shared with the omni microphone used for comparison.
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?
Whether you should select a directional or omnidirectional microphone can depend on the application (recording vs. sound reinforcement), the acoustic conditions, the working distance required and the kind of sound you wish to achieve. Directional microphones can suppress unwanted noise, reduce the effects of reverberation and increase gain-before-feedback. But in good acoustic surroundings, omnidirectional microphones, properly placed, can preserve the "sound" of the recording location, and are often preferred for their flatness of response and freedom from proximity effect.
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 to the next section ( Important Microphone Characteristics)
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