What is microphone polar pattern? (Part 2: Directional)

Answer: Last week we began our look at polar patterns by discussing omnidirectional microphones. This week we’ll cover directional mics.

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. Some of our popular directional mics include the AE4100 Dynamic Handheld Mic, AT2020 Condenser Mic, BP894 Headworn Mic and AT898 Lavalier Mic.

To help you visualize how a directional microphone works, you will find polar patterns in Audio-Technica 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. 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.

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.

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. The distance factor 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.

As you get within about two inches, though, you'll notice a rising bass response in most cardioid microphones. This is known as proximity effect, a characteristic not found in omni microphones. 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.)