Last time we discussed the three main microphone operating principles; moving coil, ribbon and condenser. Any of these mics may have the ability to favor sound from a particular direction. Some mics pick up sound mainly from in front and back, while others pick up sound equally from any direction. A picture which illustrates the directionality of a microphone is known as a polar response chart or polar pattern, and such illustrations help us understand how we can use each mic to it’s best advantage.
The most simple microphone polar pattern is known as omnidirectional. This means the microphone picks up sound with equal strength from any direction. Button #3 to the left of this text will bring up a chart that compares various mic patterns. The first pattern represents an omnidirectional mic. Each chart is a graph and the mic is at the center of the circle. When a sound is at “zero degrees” it is directly in front of the microphone and is referred to as "on axis". When sound enters a mic from anywhere else except directly in front we say it is "off-axis." A sound coming from directly behind the mic is 180 degrees off-axis. The shaded area in the circle shows the directions from which the mic is picking up sound. A perfectly omnidirectional microphone would have a polar response which is a perfect circle. Notice that the circle is not quite round. This is because — even when using the best omnidirectional microphones — there is some loss of high frequency sound when that sound comes from directly behind the mic. The illustration shows us that the mic is receiving sound from every direction, including the rear (NOTE: all of the polar patterns discussed are actually three dimensional — difficult to show on paper. So the omni pattern is really more like a globe or sphere than a flat circle. Button #4 on the left give a basic idea of how this would look.)
Depending on what you are doing an omnidirectional mic can be a help or a pain in the neck. For example, if you are trying to capture the sound at a hockey game (please edit out the foul language), then an omni microphone is a good thing because it lets you capture a wide area of the crowd. This avoids picking up the voices of only three or four people. However if you are the singer in a metal band an omnidirectional mic may pick up almost as much guitar and drums as it does your voice. (Ed note: It is rare to find omnis on stages with bands. But every once in a while there is an exception. Danny Leake who mixes Stevie Wonder, uses a single omnidirectional mic for each of his two percussionists.)
What you need is some kind of directional mic.
Have A Little Heart...
A directional mic is one which favors sound coming from a particular direction. It might favor the front or the front and sides or possibly even pick up the front and rear but reject the sides. Back to Button #3, the third image represents what is known as a cardioid pickup pattern (the name comes from the heart shape of the pattern). This pattern is not completely round. At 0 degrees this microphone will pick up sound just fine. But as the sound source moves off to the sides, the microphone rejects it until — when the sound is directly behind the mic, the microphone does not even hear the sound. Sometimes cardioid is called "unidirectional" but this really is not accurate since "uni" means one and there is no mic (to my knowledge) which picks up sound from only one direction. These pickup patterns are theoretical ideals which do not necessarily hold true in practical applications. Also, microphones that share the same pickup pattern typically do not share the same rejection abilities. In other word, one model of cardioid microphone may be better at rejecting sound from the rear than another model of cardioid microphone.
A microphone with a cardioid pickup pattern can be very helpful in a variety of situations. When performing on stage you might have several monitors and it’s critical that sound from the monitor doesn’t reach the microphone or you’ll get feedback. Instances of feedback can be reduced (though rarely eliminated) by using a cardioid microphone and placing the stage monitor in a spot where the mic does not really hear sound. Specifically the monitor should be placed directly in front of the performer. Since the performer is directly in front of the mic, the monitor is now directly behind the mic and this is the area in which a cardioid mic rejects sound.
A variation of the cardioid pickup pattern is known as the super cardioid pickup pattern (position four on our chart). This pattern picks up sound from the front, has higher rejection on the sides, but also picks up a small amount of sound from the rear. When using a super cardioid mic for vocals, the stage monitors must now be placed differently. If a monitor is placed behind the mic feedback will result, so the monitors should be placed in front and slightly to the sides of the performer, in the “null” of the pickup pattern.
There is a variation on the supercardioid called a “hypercardioid” (position five on the chart). This offers a “tighter” pattern but at the expense of a much larger rear “lobe.” These can be a great tool on really loud stages but monitor placement becomes CRITICAL when using a hypercardioid.
Pieces of Eight
The last of the main microphone pickup patterns is called the bidirectional or figure eight pickup pattern. This type of microphone picks up sound from in front of and behind the mic, but will not pick up sound from the sides of the microphone (position 2 on our chart). The pickup pattern looks like two globes, one in front and the other in back of the microphone. Sound is picked up at 0 degrees and 180 degrees but is rejected at 90 degrees and 270 degrees. On most bidirectional mics there will be some kind of marking on the body of the mic to show you where the front is.
A bidirectional mic can be very useful in certain situations. A great application for a bidirectional mic is when two singers or interview subjects need to share a single microphone. Simply place one person in front and one person in back of the mic, and their voices will be equally captured (though keep in mind that some bidirectional mics sound slightly different in back than from the front). Unwanted noise from the sides will be rejected.
It is very important to realize that the physical shape of a microphone has almost nothing to do with the pickup pattern of the mic. There are mics with a round heads that are cardioid or hyper cardioid and there are square mics which are omnidirectional. The only way to be sure is to either look on the mic for a pattern drawing, or check the specs. Another important fact about microphone polar patterns is that they tend to lose their directionality as frequency gets lower. A cardioid mic may reject a voice from the back of the mic, but it might not reject a low frequency sound (like from a bass guitar) from the same direction. This is because low frequency sounds bend around objects very easily (that's why you can hear the bass of your neighbor's stereo system).
Some condenser mics such as the Neumann U87 or AKG C414 have switches which change the polar pattern of the mic, making them extremely versatile but coming with a price tag. There aren’t many quality microphones under about $400 with switched patterns. Some mics have interchangeable heads or capsules with different pickup patterns, allowing you to buy one microphone body and then buy only the capsules with the pickup patterns you need, usually at a more modest cost. "Modest" means about $300 for the body and another $300 for each of the heads. Hey--no one said quality audio was cheap!!
Next time we will discuss specifically which brand and model mics are best for particular instruments. Until then keep your head up and your ears open!
Darth Fader is currently stationed on the DeathStar 3, providing sound reinforcement for Storm Troopers.